A nice article about dynamic nuclear polarization courtesy of Chemical and Engineering News.
http://pubs.acs.org/cen/coverstory/86/8643cover.html
Monday, October 27, 2008
Friday, October 24, 2008
J. Am. Chem. Soc., 130 (43), 14060–14061, 2008.
Dynamics of Ligand Binding from 13C NMR Relaxation Dispersion at Natural Abundance
John S. Zintsmaster, Brian D. Wilson, and Jeffrey W. Peng*
Abstract:
We show that Carr−Purcell−Meiboom−Gill (CPMG) 13Cα NMR relaxation dispersion measurements are a viable means for profiling μs−ms ligand dynamics involved in receptor binding. Critically, the dispersion is at natural 13C abundance; this matches typical pharmaceutical research settings in which ligand isotope-labeling is often impractical. The dispersion reveals ligand 13Cα nuclei that experience μs−ms modulation of their chemical shifts due to binding. 13Cα shifts are dominated by local torsion angles φ, ψ, χ1; hence, these experiments identify flexible torsion angles that may assist complex formation. Since the experiments detect the ligand, they are viable even in the absence of a receptor structure. The μs−ms dynamic information gained helps establish flexibility−activity relationships. We apply these experiments to study the binding of a phospho-peptide substrate ligand to the peptidyl-prolyl isomerase Pin1.
John S. Zintsmaster, Brian D. Wilson, and Jeffrey W. Peng*
Abstract:
We show that Carr−Purcell−Meiboom−Gill (CPMG) 13Cα NMR relaxation dispersion measurements are a viable means for profiling μs−ms ligand dynamics involved in receptor binding. Critically, the dispersion is at natural 13C abundance; this matches typical pharmaceutical research settings in which ligand isotope-labeling is often impractical. The dispersion reveals ligand 13Cα nuclei that experience μs−ms modulation of their chemical shifts due to binding. 13Cα shifts are dominated by local torsion angles φ, ψ, χ1; hence, these experiments identify flexible torsion angles that may assist complex formation. Since the experiments detect the ligand, they are viable even in the absence of a receptor structure. The μs−ms dynamic information gained helps establish flexibility−activity relationships. We apply these experiments to study the binding of a phospho-peptide substrate ligand to the peptidyl-prolyl isomerase Pin1.
Tuesday, October 21, 2008
Physical Review Letters - Up to Vol 101 No 16
93Nb NMR Spin Echo Spectroscopy in Single Crystal NbSe3
S. Suh, W. G. Clark, P. Monceau, R. E. Thorne, and S. E. Brown
Phys. Rev. Lett. 101, 136407 (2008)
We report electric field induced phase displacements of the charge density wave (CDW) in a single crystal of NbSe3 using 93Nb NMR spin-echo spectroscopy. CDW polarizations in the pinned state induced by unipolar and bipolar pulses are linear and reversible up to at least E=(0.96)ET. The polarizations have a broad distribution extending up to phase angles of order 60° for electric fields close to threshold. No evidence for polarizations in excess of a CDW wavelength or for a divergence in polarization near ET are observed. The results are consistent with elastic depinning models, provided that the critical regime expected in large systems is not observable.
Boroxol Rings in Liquid and Vitreous B2O3 from First Principles
Guillaume Ferlat, Thibault Charpentier, Ari Paavo Seitsonen, Akira Takada, Michele Lazzeri, Laurent Cormier, Georges Calas, and Francesco Mauri
Phys. Rev. Lett. 101, 065504 (2008)
We investigate the structural and vibrational properties of glassy B2O3 using first-principles molecular dynamics simulations. In particular, we determine the boroxol rings fraction f for which there is still no consensus in the literature. Two numerical models containing either a low or a high level of boroxol rings are tested against a gamut of experimental probes (static structure factor, Raman, 11B and 17O NMR data). We show that only the boroxol-rich model (f=75%) can reproduce the full set of observables. Total-energy calculations show that at the glass density, boroxol-rich structures are favored by about 6 kcal/(mol boroxol). Finally, the liquid state is explored in the 2000–4000 K range and a reduction of f to 10%–20% is obtained.
S. Suh, W. G. Clark, P. Monceau, R. E. Thorne, and S. E. Brown
Phys. Rev. Lett. 101, 136407 (2008)
We report electric field induced phase displacements of the charge density wave (CDW) in a single crystal of NbSe3 using 93Nb NMR spin-echo spectroscopy. CDW polarizations in the pinned state induced by unipolar and bipolar pulses are linear and reversible up to at least E=(0.96)ET. The polarizations have a broad distribution extending up to phase angles of order 60° for electric fields close to threshold. No evidence for polarizations in excess of a CDW wavelength or for a divergence in polarization near ET are observed. The results are consistent with elastic depinning models, provided that the critical regime expected in large systems is not observable.
Boroxol Rings in Liquid and Vitreous B2O3 from First Principles
Guillaume Ferlat, Thibault Charpentier, Ari Paavo Seitsonen, Akira Takada, Michele Lazzeri, Laurent Cormier, Georges Calas, and Francesco Mauri
Phys. Rev. Lett. 101, 065504 (2008)
We investigate the structural and vibrational properties of glassy B2O3 using first-principles molecular dynamics simulations. In particular, we determine the boroxol rings fraction f for which there is still no consensus in the literature. Two numerical models containing either a low or a high level of boroxol rings are tested against a gamut of experimental probes (static structure factor, Raman, 11B and 17O NMR data). We show that only the boroxol-rich model (f=75%) can reproduce the full set of observables. Total-energy calculations show that at the glass density, boroxol-rich structures are favored by about 6 kcal/(mol boroxol). Finally, the liquid state is explored in the 2000–4000 K range and a reduction of f to 10%–20% is obtained.
Physical Review B - Up to Vol 78 No 12
Nuclear magnetic resonance linewidth and spin diffusion in 29Si isotopically controlled silicon
Hiroshi Hayashi, Kohei M. Itoh and Leonid S. Vlasenko
Phys. Rev. B 78, 153201 (2008)
A nuclear magnetic resonance (NMR) study was performed with n-type silicon single crystals containing 29Si isotope abundance f ranges from 1.2% to 99.2%. The nuclear spin diffusion coefficient D has been determined from the linewidth of significantly enhanced 29Si NMR signals utilizing a developed dynamic nuclear polarization (DNP) method. The 29Si NMR linewidth depends linearly on f, at least when f<10%, and approaches f1/2 dependence when f>50%. The estimated 29Si nuclear spin diffusion time Tsd between phosphorus atoms used for DNP is more than ten times shorter than the nuclear polarization time T of 29Si nuclei around phosphorus. Therefore, the regime of “rapid spin diffusion” is realized in the DNP experiments.
Magnetization and Cu nuclear magnetic resonance study of Sr0.9La0.1Cu1−xNixO2
G. V. M. Williams, S. Richter, J. Haase, C. U. Jung, Hye-Gyong Lee, and Sung-Ik Lee
Phys. Rev. B 78, 104522 (2008)
We report the results from a Cu nuclear magnetic resonance (NMR) and magnetization study of the electron-doped high-temperature superconducting cuprate (HTSC) Sr0.9La0.1Cu1−xNixO2. This compound shows a large suppression of superconductivity by Ni [Tc/x=18 K/%(Ni)] that is comparable to that observed in underdoped and hole-doped HTSCs. We find small effective moments of 0.06µB/Cu for the pure sample and 0.76µB/Ni for the 1% Ni sample in the paramagnetic regime. The partial substitution of Ni for Cu causes the Cu NMR linewidth to increase and there is a further reduction in the Cu NMR intensity with decreasing temperature over and above that observed in the pure compound. This can be interpreted in terms of Ni inducing additional spin disorder and wipeout of the Cu NMR intensity for Cu sites near the Ni impurities.
19F NMR investigation of the iron pnictide superconductor LaFeAsO0.89F0.11
K. Ahilan, F. L. Ning, T. Imai, A. S. Sefat, R. Jin, M. A. McGuire, B. C. Sales, and D. Mandrus
Phys. Rev. B 78, 100501(R) (2008)
We report on 19F nuclear magnetic resonance (NMR) investigation of the high-temperature superconductor LaFeAsO0.89F0.11 (Tc~28 K). We demonstrate that low-frequency spin fluctuations exhibit pseudogap behavior above Tc. We also deduce the London penetration depth from NMR line broadening below Tc.
Atomic-scale measurement of ultraslow Li motions in glassy LiAlSi2O6 by two-time 6Li spin-alignment echo NMR correlation spectroscopy
M. Wilkening, A. Kuhn, and P. Heitjans
Phys. Rev. B 78, 054303 (2008)
6Li spin-alignment echo (SAE) nuclear-magnetic-resonance (NMR) spectroscopy is used to monitor single-particle two-time correlation functions in LiAlSi2O6 glass. The method, here applied in the temperature range from 300 to 400 K, is sensitive to ultraslow Li hopping processes with rates (1/SAE) down to 10 jumps/s. The use of a sample with natural 6Li abundance allowed the measurement of pure NMR spin-alignment echoes which are damped with increasing mixing time exclusively by slow Li jumps, i.e., free of influences arising from, e.g., interfering spin-diffusion effects. The considerably stretched correlation functions reveal the presence of a broad distribution of jump rates. The results are comprehensively compared with those recently obtained from both 7Li SAE and 7Li spin-lattice relaxation NMR as well as from dc conductivity measurements. Interestingly, the activation energy of the latter, which are sensitive to long-range Li transport parameters, is in good agreement with that microscopically probed by 6Li SAE NMR, here.
Hiroshi Hayashi, Kohei M. Itoh and Leonid S. Vlasenko
Phys. Rev. B 78, 153201 (2008)
A nuclear magnetic resonance (NMR) study was performed with n-type silicon single crystals containing 29Si isotope abundance f ranges from 1.2% to 99.2%. The nuclear spin diffusion coefficient D has been determined from the linewidth of significantly enhanced 29Si NMR signals utilizing a developed dynamic nuclear polarization (DNP) method. The 29Si NMR linewidth depends linearly on f, at least when f<10%, and approaches f1/2 dependence when f>50%. The estimated 29Si nuclear spin diffusion time Tsd between phosphorus atoms used for DNP is more than ten times shorter than the nuclear polarization time T of 29Si nuclei around phosphorus. Therefore, the regime of “rapid spin diffusion” is realized in the DNP experiments.
Magnetization and Cu nuclear magnetic resonance study of Sr0.9La0.1Cu1−xNixO2
G. V. M. Williams, S. Richter, J. Haase, C. U. Jung, Hye-Gyong Lee, and Sung-Ik Lee
Phys. Rev. B 78, 104522 (2008)
We report the results from a Cu nuclear magnetic resonance (NMR) and magnetization study of the electron-doped high-temperature superconducting cuprate (HTSC) Sr0.9La0.1Cu1−xNixO2. This compound shows a large suppression of superconductivity by Ni [Tc/x=18 K/%(Ni)] that is comparable to that observed in underdoped and hole-doped HTSCs. We find small effective moments of 0.06µB/Cu for the pure sample and 0.76µB/Ni for the 1% Ni sample in the paramagnetic regime. The partial substitution of Ni for Cu causes the Cu NMR linewidth to increase and there is a further reduction in the Cu NMR intensity with decreasing temperature over and above that observed in the pure compound. This can be interpreted in terms of Ni inducing additional spin disorder and wipeout of the Cu NMR intensity for Cu sites near the Ni impurities.
19F NMR investigation of the iron pnictide superconductor LaFeAsO0.89F0.11
K. Ahilan, F. L. Ning, T. Imai, A. S. Sefat, R. Jin, M. A. McGuire, B. C. Sales, and D. Mandrus
Phys. Rev. B 78, 100501(R) (2008)
We report on 19F nuclear magnetic resonance (NMR) investigation of the high-temperature superconductor LaFeAsO0.89F0.11 (Tc~28 K). We demonstrate that low-frequency spin fluctuations exhibit pseudogap behavior above Tc. We also deduce the London penetration depth from NMR line broadening below Tc.
Atomic-scale measurement of ultraslow Li motions in glassy LiAlSi2O6 by two-time 6Li spin-alignment echo NMR correlation spectroscopy
M. Wilkening, A. Kuhn, and P. Heitjans
Phys. Rev. B 78, 054303 (2008)
6Li spin-alignment echo (SAE) nuclear-magnetic-resonance (NMR) spectroscopy is used to monitor single-particle two-time correlation functions in LiAlSi2O6 glass. The method, here applied in the temperature range from 300 to 400 K, is sensitive to ultraslow Li hopping processes with rates (1/SAE) down to 10 jumps/s. The use of a sample with natural 6Li abundance allowed the measurement of pure NMR spin-alignment echoes which are damped with increasing mixing time exclusively by slow Li jumps, i.e., free of influences arising from, e.g., interfering spin-diffusion effects. The considerably stretched correlation functions reveal the presence of a broad distribution of jump rates. The results are comprehensively compared with those recently obtained from both 7Li SAE and 7Li spin-lattice relaxation NMR as well as from dc conductivity measurements. Interestingly, the activation energy of the latter, which are sensitive to long-range Li transport parameters, is in good agreement with that microscopically probed by 6Li SAE NMR, here.
Journal of Magnetic Resonance - Vol 195 Issue 1
Solid state water motions revealed by deuterium relaxation in 2H2O-synthesized kanemite and 2H2O-hydrated Na+-Zeolite A
Bernie O’Hare, Michael W. Grutzeck, Seong H. Kim, David B. Asay and Alan J. Benesi
Deuterium NMR relaxation experiments, low temperature deuterium NMR lineshape analysis, and FTIR spectra are consistent with a new model for solid state jump dynamics of water in 2H2O-synthesized kanemite and 2H2O-hydrated Na+-Zeolite A. Exchange occurs between two populations of water: one in which water molecules are directly coordinated to sodium ions and experience C2 symmetry jumps of their OH bonds, and a population of interstitial water molecules outside the sodium ion coordination sphere that experience tetrahedral jumps of their OH bonds. For both samples the C2 jump rate is much faster than the tetrahedral jump rate. 2H NMR relaxation experiments match well with the fast exchange regime of the model over a wide range of temperatures, including room temperature and above.
Coherence selection in double CP MAS NMR spectroscopy
Jen-Hsien Yang, Fang-Chieh Chou and Der-Lii M. Tzou
Applications of double cross-polarization (CP) magic-angle spinning (MAS) NMR spectroscopy, via 1H/15N and then 15N/13C coherence transfers, for 13C coherence selection are demonstrated on a 15N/13C-labeled N-acetyl-glucosamine (GlcNAc) compound. The 15N/13C coherence transfer is very sensitive to the settings of the experimental parameters. To resolve explicitly these parameter dependences, we have systematically monitored the 13C{15N/1H} signal as a function of the rf field strength and the MAS frequency. The data reveal that the zero-quantum coherence transfer, with which the 13C effective rf field is larger than that of the 15N by the spinning frequency, would give better signal sensitivity. We demonstrate in one- and two-dimensional double CP experiments that spectral editing can be achieved by tailoring the experimental parameters, such as the rf field strengths and/or the MAS frequency.
Bernie O’Hare, Michael W. Grutzeck, Seong H. Kim, David B. Asay and Alan J. Benesi
Deuterium NMR relaxation experiments, low temperature deuterium NMR lineshape analysis, and FTIR spectra are consistent with a new model for solid state jump dynamics of water in 2H2O-synthesized kanemite and 2H2O-hydrated Na+-Zeolite A. Exchange occurs between two populations of water: one in which water molecules are directly coordinated to sodium ions and experience C2 symmetry jumps of their OH bonds, and a population of interstitial water molecules outside the sodium ion coordination sphere that experience tetrahedral jumps of their OH bonds. For both samples the C2 jump rate is much faster than the tetrahedral jump rate. 2H NMR relaxation experiments match well with the fast exchange regime of the model over a wide range of temperatures, including room temperature and above.
Coherence selection in double CP MAS NMR spectroscopy
Jen-Hsien Yang, Fang-Chieh Chou and Der-Lii M. Tzou
Applications of double cross-polarization (CP) magic-angle spinning (MAS) NMR spectroscopy, via 1H/15N and then 15N/13C coherence transfers, for 13C coherence selection are demonstrated on a 15N/13C-labeled N-acetyl-glucosamine (GlcNAc) compound. The 15N/13C coherence transfer is very sensitive to the settings of the experimental parameters. To resolve explicitly these parameter dependences, we have systematically monitored the 13C{15N/1H} signal as a function of the rf field strength and the MAS frequency. The data reveal that the zero-quantum coherence transfer, with which the 13C effective rf field is larger than that of the 15N by the spinning frequency, would give better signal sensitivity. We demonstrate in one- and two-dimensional double CP experiments that spectral editing can be achieved by tailoring the experimental parameters, such as the rf field strengths and/or the MAS frequency.
Journal of Magnetic Resonance - Vol 194 Issue 2
The utility of phase alternated pulses for the measurement of dipolar couplings in 2D-SLF experiments
Bibhuti B. Das, N. Sinha and K.V. Ramanathan
The measurement of hetero-nuclear dipolar coupling using two-dimensional separated local field (SLF-2D) NMR experiments is a powerful technique for the determination of the structure and dynamics of molecules in the solid state and in liquid crystals. However, the experiment is sensitive to a number of factors such as the Hartmann–Hahn match condition, proton frequency off-set and rf heating. It is shown here that by the use of phase alternated pulses during spin-exchange the effect of rf mismatch on the dipolar coupling measurement can be compensated over a wide range of off-sets. Phase alternation together with time and amplitude modulation has also been considered and incorporated into a pulse scheme that combines spin exchange with homonuclear spin decoupling based on magic sandwich sequence and named as SAMPI4. Such time and amplitude averaged nutation experiments use relatively low rf power and generate less sample heating. One of these schemes has been applied on liquid crystal samples and is observed to perform well and yield spectra with high resolution.
Practical aspects of Lee–Goldburg based CRAMPS techniques for high-resolution 1H NMR spectroscopy in solids: Implementation and applications
Cristina Coelho, João Rocha, P.K. Madhu and Luís Mafra
Elucidating the local environment of the hydrogen atoms is an important problem in materials science. Because 1H spectra in solid-state nuclear magnetic resonance (NMR) suffer from low resolution due to homogeneous broadening, even under magic-angle spinning (MAS), information of chemical interest may only be obtained using certain high-resolution 1H MAS techniques. 1H Lee–Goldburg (LG) CRAMPS (Combined Rotation And Multiple-Pulse Spectroscopy) methods are particularly well suited for studying inorganic–organic hybrid materials, rich in 1H nuclei. However, setting up CRAMPS experiments is time-consuming and not entirely trivial, facts that have discouraged their widespread use by materials scientists. To change this status quo, here we describe and discuss some important aspects of the experimental implementation of CRAMPS techniques based on LG decoupling schemes, such as FSLG (Frequency Switched), and windowed and windowless PMLG (Phase Modulated). In particular, we discuss the influence on the quality of the 1H NMR spectra of the different parameters at play, for example LG (Lee–Goldburg) pulses, radio-frequency (rf) phase, frequency switching, and pulse imperfections, using glycine and adamantane as model compounds. The efficiency and robustness of the different LG-decoupling schemes is then illustrated on the following materials: organo-phosphorus ligand, N-(phosphonomethyl)iminodiacetic acid [H4pmida] [I], and inorganic–organic hybrid materials (C4H12N2)[Ge2(pmida)2OH2]·4H2O [II] and (C2H5NH3)[Ti(H1.5PO4)(PO4)]2·H2O [III].
Double-acquisition: Utilization of discarded coherences in a 2D separation experiment using the States method
Masashi Fukuchia, Munehiro Inukai, Kazuyuki Takeda and K. Takegoshi
We propose a new data-acquisition scheme for 2D separation experiments to save the spectrometer time by 1/2. This scheme, referred to as a double-acquisition scheme, is applicable to most of separation experiments with the hypercomplex time-domain data-acquisition scheme (the States method) for data collection.
Bibhuti B. Das, N. Sinha and K.V. Ramanathan
The measurement of hetero-nuclear dipolar coupling using two-dimensional separated local field (SLF-2D) NMR experiments is a powerful technique for the determination of the structure and dynamics of molecules in the solid state and in liquid crystals. However, the experiment is sensitive to a number of factors such as the Hartmann–Hahn match condition, proton frequency off-set and rf heating. It is shown here that by the use of phase alternated pulses during spin-exchange the effect of rf mismatch on the dipolar coupling measurement can be compensated over a wide range of off-sets. Phase alternation together with time and amplitude modulation has also been considered and incorporated into a pulse scheme that combines spin exchange with homonuclear spin decoupling based on magic sandwich sequence and named as SAMPI4. Such time and amplitude averaged nutation experiments use relatively low rf power and generate less sample heating. One of these schemes has been applied on liquid crystal samples and is observed to perform well and yield spectra with high resolution.
Practical aspects of Lee–Goldburg based CRAMPS techniques for high-resolution 1H NMR spectroscopy in solids: Implementation and applications
Cristina Coelho, João Rocha, P.K. Madhu and Luís Mafra
Elucidating the local environment of the hydrogen atoms is an important problem in materials science. Because 1H spectra in solid-state nuclear magnetic resonance (NMR) suffer from low resolution due to homogeneous broadening, even under magic-angle spinning (MAS), information of chemical interest may only be obtained using certain high-resolution 1H MAS techniques. 1H Lee–Goldburg (LG) CRAMPS (Combined Rotation And Multiple-Pulse Spectroscopy) methods are particularly well suited for studying inorganic–organic hybrid materials, rich in 1H nuclei. However, setting up CRAMPS experiments is time-consuming and not entirely trivial, facts that have discouraged their widespread use by materials scientists. To change this status quo, here we describe and discuss some important aspects of the experimental implementation of CRAMPS techniques based on LG decoupling schemes, such as FSLG (Frequency Switched), and windowed and windowless PMLG (Phase Modulated). In particular, we discuss the influence on the quality of the 1H NMR spectra of the different parameters at play, for example LG (Lee–Goldburg) pulses, radio-frequency (rf) phase, frequency switching, and pulse imperfections, using glycine and adamantane as model compounds. The efficiency and robustness of the different LG-decoupling schemes is then illustrated on the following materials: organo-phosphorus ligand, N-(phosphonomethyl)iminodiacetic acid [H4pmida] [I], and inorganic–organic hybrid materials (C4H12N2)[Ge2(pmida)2OH2]·4H2O [II] and (C2H5NH3)[Ti(H1.5PO4)(PO4)]2·H2O [III].
Double-acquisition: Utilization of discarded coherences in a 2D separation experiment using the States method
Masashi Fukuchia, Munehiro Inukai, Kazuyuki Takeda and K. Takegoshi
We propose a new data-acquisition scheme for 2D separation experiments to save the spectrometer time by 1/2. This scheme, referred to as a double-acquisition scheme, is applicable to most of separation experiments with the hypercomplex time-domain data-acquisition scheme (the States method) for data collection.
Friday, October 10, 2008
ASAP J. Am. Chem. Soc., ASAP Article, 10.1021/ja801789t
Raftlike Mixtures of Sphingomyelin and Cholesterol Investigated by Solid-State 2H NMR Spectroscopy
Tim Bartels, Ravi S. Lankalapalli, Robert Bittman, Klaus Beyer, and Michael F. Brown*
Abstract:
Sphingomyelin is a lipid that is abundant in the nervous systems of mammals, where it is associated with putative microdomains in cellular membranes and undergoes alterations due to aging or neurodegeneration. We investigated the effect of varying the concentration of cholesterol in binary and ternary mixtures with N-palmitoylsphingomyelin (PSM) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) using deuterium nuclear magnetic resonance (2H NMR) spectroscopy in both macroscopically aligned and unoriented multilamellar dispersions. In our experiments, we used PSM and POPC perdeuterated on the N-acyl and sn-1 acyl chains, respectively. By measuring solid-state 2H NMR spectra of the two lipids separately in mixtures with the same compositions as a function of cholesterol mole fraction and temperature, we obtained clear evidence for the coexistence of two liquid-crystalline domains in distinct regions of the phase diagram. According to our analysis of the first moments M1 and the observed 2H NMR spectra, one of the domains appears to be a liquid-ordered phase. We applied a mean-torque potential model as an additional tool to calculate the average hydrocarbon thickness, the area per lipid, and structural parameters such as chain extension and thermal expansion coefficient in order to further define the two coexisting phases. Our data imply that phase separation takes place in raftlike ternary PSM/POPC/cholesterol mixtures over a broad temperature range but vanishes at cholesterol concentrations equal to or greater than a mole fraction of 0.33. Cholesterol interacts preferentially with sphingomyelin only at smaller mole fractions, above which a homogeneous liquid-ordered phase is present. The reasons for these phase separation phenomena seem to be differences in the effects of cholesterol on the configurational order of the palmitoyl chains in PSM-d31 and POPC-d31 and a difference in the affinity of cholesterol for sphingomyelin observed at low temperatures. Hydrophobic matching explains the occurrence of raftlike domains in cellular membranes at intermediate cholesterol concentrations but not saturating amounts of cholesterol.
Tim Bartels, Ravi S. Lankalapalli, Robert Bittman, Klaus Beyer, and Michael F. Brown*
Abstract:
Sphingomyelin is a lipid that is abundant in the nervous systems of mammals, where it is associated with putative microdomains in cellular membranes and undergoes alterations due to aging or neurodegeneration. We investigated the effect of varying the concentration of cholesterol in binary and ternary mixtures with N-palmitoylsphingomyelin (PSM) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) using deuterium nuclear magnetic resonance (2H NMR) spectroscopy in both macroscopically aligned and unoriented multilamellar dispersions. In our experiments, we used PSM and POPC perdeuterated on the N-acyl and sn-1 acyl chains, respectively. By measuring solid-state 2H NMR spectra of the two lipids separately in mixtures with the same compositions as a function of cholesterol mole fraction and temperature, we obtained clear evidence for the coexistence of two liquid-crystalline domains in distinct regions of the phase diagram. According to our analysis of the first moments M1 and the observed 2H NMR spectra, one of the domains appears to be a liquid-ordered phase. We applied a mean-torque potential model as an additional tool to calculate the average hydrocarbon thickness, the area per lipid, and structural parameters such as chain extension and thermal expansion coefficient in order to further define the two coexisting phases. Our data imply that phase separation takes place in raftlike ternary PSM/POPC/cholesterol mixtures over a broad temperature range but vanishes at cholesterol concentrations equal to or greater than a mole fraction of 0.33. Cholesterol interacts preferentially with sphingomyelin only at smaller mole fractions, above which a homogeneous liquid-ordered phase is present. The reasons for these phase separation phenomena seem to be differences in the effects of cholesterol on the configurational order of the palmitoyl chains in PSM-d31 and POPC-d31 and a difference in the affinity of cholesterol for sphingomyelin observed at low temperatures. Hydrophobic matching explains the occurrence of raftlike domains in cellular membranes at intermediate cholesterol concentrations but not saturating amounts of cholesterol.
ASAP J. Am. Chem. Soc., ASAP Article, 10.1021/ja805839y
Dynamics of Ligand Binding from 13C NMR Relaxation Dispersion at Natural Abundance
John S. Zintsmaster, Brian D. Wilson, and Jeffrey W. Peng*
Abstract:
We show that Carr−Purcell−Meiboom−Gill (CPMG) 13Cα NMR relaxation dispersion measurements are a viable means for profiling μs−ms ligand dynamics involved in receptor binding. Critically, the dispersion is at natural 13C abundance; this matches typical pharmaceutical research settings in which ligand isotope-labeling is often impractical. The dispersion reveals ligand 13Cα nuclei that experience μs−ms modulation of their chemical shifts due to binding. 13Cα shifts are dominated by local torsion angles φ, ψ, χ1; hence, these experiments identify flexible torsion angles that may assist complex formation. Since the experiments detect the ligand, they are viable even in the absence of a receptor structure. The μs−ms dynamic information gained helps establish flexibility−activity relationships. We apply these experiments to study the binding of a phospho-peptide substrate ligand to the peptidyl-prolyl isomerase Pin1.
John S. Zintsmaster, Brian D. Wilson, and Jeffrey W. Peng*
Abstract:
We show that Carr−Purcell−Meiboom−Gill (CPMG) 13Cα NMR relaxation dispersion measurements are a viable means for profiling μs−ms ligand dynamics involved in receptor binding. Critically, the dispersion is at natural 13C abundance; this matches typical pharmaceutical research settings in which ligand isotope-labeling is often impractical. The dispersion reveals ligand 13Cα nuclei that experience μs−ms modulation of their chemical shifts due to binding. 13Cα shifts are dominated by local torsion angles φ, ψ, χ1; hence, these experiments identify flexible torsion angles that may assist complex formation. Since the experiments detect the ligand, they are viable even in the absence of a receptor structure. The μs−ms dynamic information gained helps establish flexibility−activity relationships. We apply these experiments to study the binding of a phospho-peptide substrate ligand to the peptidyl-prolyl isomerase Pin1.
Wednesday, October 08, 2008
J. Am. Chem. Soc., 130 (41), 13664–13672, 2008.
Nanochannels of Two Distinct Cross-Sections in a Porous Al-Based Coordination Polymer
Angiolina Comotti, Silvia Bracco, Piero Sozzani, Satoshi Horike, Ryotaro Matsuda, Jinxi Chen, Masaki Takata, Yoshiki Kubota, and Susumu Kitagawa
Abstract:
A new aluminum naphthalenedicarboxylate Al(OH)(1,4-NDC)·2H2O compound has been synthesized. The crystal structure exhibits a three-dimensional framework composed of infinite chains of corner-sharing octahedral Al(OH)2O4 with 1,4-naphthanedicarboxylate ligands forming two types of channels with squared-shape cross-section. The large channels present a cross-section of 7.7 × 7.7 Å2, while the small channels are about 3.0 × 3.0 Å2. When water molecules are removed, no structural transformation occurs, generating a robust structure with permanent porosity and remarkable thermal stability. 2D 1H−13C heteronuclear correlation NMR measurements, together with the application of Lee-Goldburg homonuclear decoupling, were applied, for the first time, to porous coordination polymers revealing the spatial relationships between the 1H and 13C spin-active nuclei of the framework. To demonstrate the open pore structure and the easy accessibility of the nanochannels to the gas phase, highly sensitive hyperpolarized (HP) xenon NMR, under extreme xenon dilution, has been applied. Xenon can diffuse selectively into the large nanochannels, while the small ones show no substantial uptake of xenon due to severe restrictions along the channels that prevent the diffusion. Two-dimensional exchange experiments showed the exchange time to be as short as 15 ms. Through variable-temperature HP 129Xe NMR experiments we were able to achieve an unprecedented description of the large nanochannel space and surface, a physisorption energy of 10 kJ mol−1, and the chemical shift value of xenon probing the internal surfaces. The large pore channels are straight, parallel, and independent, allowing one-dimensional anisotropic diffusion of gases and vapors. Their walls are composed of the naphthalene moieties that create an unique environment for selective sorption. These results prompted us to measure the storage capacity toward methanol, acetone, benzene, and carbon dioxide. The selective adsorption of methanol and acetone vs that of water, together with the permanent porosity and high thermal stability, makes this compound a suitable matrix for separation and purification.
Angiolina Comotti, Silvia Bracco, Piero Sozzani, Satoshi Horike, Ryotaro Matsuda, Jinxi Chen, Masaki Takata, Yoshiki Kubota, and Susumu Kitagawa
Abstract:
A new aluminum naphthalenedicarboxylate Al(OH)(1,4-NDC)·2H2O compound has been synthesized. The crystal structure exhibits a three-dimensional framework composed of infinite chains of corner-sharing octahedral Al(OH)2O4 with 1,4-naphthanedicarboxylate ligands forming two types of channels with squared-shape cross-section. The large channels present a cross-section of 7.7 × 7.7 Å2, while the small channels are about 3.0 × 3.0 Å2. When water molecules are removed, no structural transformation occurs, generating a robust structure with permanent porosity and remarkable thermal stability. 2D 1H−13C heteronuclear correlation NMR measurements, together with the application of Lee-Goldburg homonuclear decoupling, were applied, for the first time, to porous coordination polymers revealing the spatial relationships between the 1H and 13C spin-active nuclei of the framework. To demonstrate the open pore structure and the easy accessibility of the nanochannels to the gas phase, highly sensitive hyperpolarized (HP) xenon NMR, under extreme xenon dilution, has been applied. Xenon can diffuse selectively into the large nanochannels, while the small ones show no substantial uptake of xenon due to severe restrictions along the channels that prevent the diffusion. Two-dimensional exchange experiments showed the exchange time to be as short as 15 ms. Through variable-temperature HP 129Xe NMR experiments we were able to achieve an unprecedented description of the large nanochannel space and surface, a physisorption energy of 10 kJ mol−1, and the chemical shift value of xenon probing the internal surfaces. The large pore channels are straight, parallel, and independent, allowing one-dimensional anisotropic diffusion of gases and vapors. Their walls are composed of the naphthalene moieties that create an unique environment for selective sorption. These results prompted us to measure the storage capacity toward methanol, acetone, benzene, and carbon dioxide. The selective adsorption of methanol and acetone vs that of water, together with the permanent porosity and high thermal stability, makes this compound a suitable matrix for separation and purification.
Friday, October 03, 2008
ASAP Cryst. Growth Des., ASAP Article, 10.
1H, 13C, and 15N Solid-State NMR Studies of Imidazole- and Morpholine-Based Model Compounds Possessing Halogen and Hydrogen Bonding Capabilities
Karim Bouchmella,† Sylvain G. Dutremez,*† Bruno Alonso,‡ Francesco Mauri,§ and Christel Gervais*#
Abstract:
The halogen and hydrogen bonding interactions present in solid 1-(2,3,3-triiodoallyl)imidazole (1), morpholinium iodide (2), the 1:1 cocrystal 1-(2,3,3-triiodoallyl)imidazole·morpholinium iodide (3), morpholine (4), imidazole (5), and 1-(3-iodopropargyl)imidazole (6) have been investigated by solid-state 1H, 13C, and 15N NMR spectroscopies. Comparison of the 15N CP MAS NMR spectrum of 3 with that of 2 indicates that protonated morpholine is present in solid 3, but this conclusion must be taken with caution as GIPAW calculations predict a 15N chemical shift for morpholine similar to that of the morpholinium cation. Conclusive evidence for the presence of a morpholinium cation in crystalline 3 was obtained by recording the static 15N NMR spectrum of this host−guest complex and comparing the morpholinium/morpholine part of the spectrum with the static spectra of 3 and 4 as obtained from ab initio calculations of NMR parameters based on the X-ray structures of these compounds. Concerning the imidazolyl group, 15N NMR spectroscopy has proven quite valuable to identify changes in the bonding situation of the C−NC nitrogen on passing from 1 to 3. In addition, slight differences are observed between the 15N chemical shifts of 1 and 6 that are ascribed to differences in halogen bond strengths between the two compounds. Attempts have also been made to study halogen bonding by 13C NMR spectroscopy, but this method did not provide exploitable results as signals corresponding to the sp and sp2 carbon atoms bonded to iodine could not be observed experimentally. 1H NMR spectroscopy is a powerful tool to study hydrogen bonding interactions of moderate energies such as +NH2···X (X = N, O, I). Indeed, we have found that the chemical shifts of the NH hydrogens were quite sensitive to the nature of X and to the N−H···X distance. This is demonstrated by the fact that the chemical shifts of the +NH2 protons of the morpholinium cation in 2 and 3 are noticeably different.
Karim Bouchmella,† Sylvain G. Dutremez,*† Bruno Alonso,‡ Francesco Mauri,§ and Christel Gervais*#
Abstract:
The halogen and hydrogen bonding interactions present in solid 1-(2,3,3-triiodoallyl)imidazole (1), morpholinium iodide (2), the 1:1 cocrystal 1-(2,3,3-triiodoallyl)imidazole·morpholinium iodide (3), morpholine (4), imidazole (5), and 1-(3-iodopropargyl)imidazole (6) have been investigated by solid-state 1H, 13C, and 15N NMR spectroscopies. Comparison of the 15N CP MAS NMR spectrum of 3 with that of 2 indicates that protonated morpholine is present in solid 3, but this conclusion must be taken with caution as GIPAW calculations predict a 15N chemical shift for morpholine similar to that of the morpholinium cation. Conclusive evidence for the presence of a morpholinium cation in crystalline 3 was obtained by recording the static 15N NMR spectrum of this host−guest complex and comparing the morpholinium/morpholine part of the spectrum with the static spectra of 3 and 4 as obtained from ab initio calculations of NMR parameters based on the X-ray structures of these compounds. Concerning the imidazolyl group, 15N NMR spectroscopy has proven quite valuable to identify changes in the bonding situation of the C−NC nitrogen on passing from 1 to 3. In addition, slight differences are observed between the 15N chemical shifts of 1 and 6 that are ascribed to differences in halogen bond strengths between the two compounds. Attempts have also been made to study halogen bonding by 13C NMR spectroscopy, but this method did not provide exploitable results as signals corresponding to the sp and sp2 carbon atoms bonded to iodine could not be observed experimentally. 1H NMR spectroscopy is a powerful tool to study hydrogen bonding interactions of moderate energies such as +NH2···X (X = N, O, I). Indeed, we have found that the chemical shifts of the NH hydrogens were quite sensitive to the nature of X and to the N−H···X distance. This is demonstrated by the fact that the chemical shifts of the +NH2 protons of the morpholinium cation in 2 and 3 are noticeably different.
Wednesday, October 01, 2008
J. Am. Chem. Soc., 130 (40), 13425–13432, 2008.
In Situ Observation of the Internal Structure and Composition of Biomineralized Emiliania huxleyi Calcite by Solid-State NMR Spectroscopy
Ronen Gertman, Ira Ben Shir, Shifi Kababya, and Asher Schmidt*
Abstract:
Biomineralization, particularly the formation of calcium carbonate structures by organisms under ambient conditions, is of vast fundamental and applied interest. Organisms finely control all aspects of the formation of the biomaterials: composition, polymorph, morphology, and macroscopic properties. While in situ molecular-level characterization of the resulting biominerals is a formidable task, solid-state magic angle spinning NMR is one of the most powerful analytical techniques for this purpose. It is employed in this study to elucidate the structure and composition of biogenic calcite formed by Emiliania huxleyi, a unicellular alga distinguished by its exquisitely sculptured calcite cell coverings known as coccoliths. Strain 371 (CCMP) was grown and harvested from 15N- and 13C-enriched growth medium, with biosynthetic labeling to enhance the sensitivity of the NMR measurements. Crystalline and interfacial calcite environments were selectively probed using direct and indirect (cross-polarized) 13C excitation, respectively. Different crystalline environments, in particular structural defect sites at concentrations of up to 1.4% with P and N moieties incorporated, were identified using 13C rotational-echo double-resonance (REDOR) NMR. REDOR-derived geometrical constraints show that the P and N atoms at the defect sites are 3.2 and 2.3 (0.2) Å apart from a crystalline carbon carbonate. The phosphorus and nitrogen moieties within the biogenic calcite are identified as small, non-protonated moieties, attributed to inorganic ions such as PO43− and NO3−. The carbonates adjacent to these defects are chemically indistinguishable from bulk crystalline carbonates, yet their immediate environments experience reduced rigidity, as reflected by substantial T1(13CO32−) shortening. Interfacial carbonates, on the other hand, reside in structurally/chemically perturbed environments, as reflected by heterogeneous line broadening. This study is the first to directly unravel evidence on the incorporation of P/N moieties as structural defects within E. huxleyi biogenic calcite, and on the state of the adjacent crystalline carbonates.
Ronen Gertman, Ira Ben Shir, Shifi Kababya, and Asher Schmidt*
Abstract:
Biomineralization, particularly the formation of calcium carbonate structures by organisms under ambient conditions, is of vast fundamental and applied interest. Organisms finely control all aspects of the formation of the biomaterials: composition, polymorph, morphology, and macroscopic properties. While in situ molecular-level characterization of the resulting biominerals is a formidable task, solid-state magic angle spinning NMR is one of the most powerful analytical techniques for this purpose. It is employed in this study to elucidate the structure and composition of biogenic calcite formed by Emiliania huxleyi, a unicellular alga distinguished by its exquisitely sculptured calcite cell coverings known as coccoliths. Strain 371 (CCMP) was grown and harvested from 15N- and 13C-enriched growth medium, with biosynthetic labeling to enhance the sensitivity of the NMR measurements. Crystalline and interfacial calcite environments were selectively probed using direct and indirect (cross-polarized) 13C excitation, respectively. Different crystalline environments, in particular structural defect sites at concentrations of up to 1.4% with P and N moieties incorporated, were identified using 13C rotational-echo double-resonance (REDOR) NMR. REDOR-derived geometrical constraints show that the P and N atoms at the defect sites are 3.2 and 2.3 (0.2) Å apart from a crystalline carbon carbonate. The phosphorus and nitrogen moieties within the biogenic calcite are identified as small, non-protonated moieties, attributed to inorganic ions such as PO43− and NO3−. The carbonates adjacent to these defects are chemically indistinguishable from bulk crystalline carbonates, yet their immediate environments experience reduced rigidity, as reflected by substantial T1(13CO32−) shortening. Interfacial carbonates, on the other hand, reside in structurally/chemically perturbed environments, as reflected by heterogeneous line broadening. This study is the first to directly unravel evidence on the incorporation of P/N moieties as structural defects within E. huxleyi biogenic calcite, and on the state of the adjacent crystalline carbonates.
Friday, September 26, 2008
ASAP J. Am. Chem. Soc., ASAP Article, 10.1021/ja802903a
Vicinal Deuterium Perturbations on Hydrogen NMR Chemical Shifts in CyclohexanesDaniel J. O’Leary,† Damian G. Allis,‡ Bruce S. Hudson,‡ Shelly James,‡ Katherine B. Morgera,‡ and John E. Baldwin*‡
Abstract:
The substitution of a deuterium for a hydrogen is known to perturb the NMR chemical shift of a neighboring hydrogen atom. The magnitude of such a perturbation may depend on the specifics of bonding and stereochemical relationships within a molecule. For deuterium-labeled cyclohexanes held in a chair conformation at −80 °C or lower, all four possible perturbations of H by D as H−C−C−H is changed to D−C−C−H have been determined experimentally, and the variations seen, ranging from 6.9 to 10.4 ppb, have been calculated from theory and computational methods. The predominant physical origins of the NMR chemical shift perturbations in deuterium-labeled cyclohexanes have been identified and quantified. The trends defined by the Δδ perturbation values obtained through spectroscopic experiments and by theory agree satisfactorily. They do not match the variations typically observed in vicinal JH−H coupling constants as a function of dihedral angles.
Abstract:
The substitution of a deuterium for a hydrogen is known to perturb the NMR chemical shift of a neighboring hydrogen atom. The magnitude of such a perturbation may depend on the specifics of bonding and stereochemical relationships within a molecule. For deuterium-labeled cyclohexanes held in a chair conformation at −80 °C or lower, all four possible perturbations of H by D as H−C−C−H is changed to D−C−C−H have been determined experimentally, and the variations seen, ranging from 6.9 to 10.4 ppb, have been calculated from theory and computational methods. The predominant physical origins of the NMR chemical shift perturbations in deuterium-labeled cyclohexanes have been identified and quantified. The trends defined by the Δδ perturbation values obtained through spectroscopic experiments and by theory agree satisfactorily. They do not match the variations typically observed in vicinal JH−H coupling constants as a function of dihedral angles.
ASAP J. Am. Chem. Soc., ASAP Article, 10.1021/ja8041614
Synthesis and X-ray or NMR/DFT Structure Elucidation of Twenty-One New Trifluoromethyl Derivatives of Soluble Cage Isomers of C76, C78, C84, and C90
Ivan E. Kareev,*†§ Alexey A. Popov,*‡ Igor V. Kuvychko,¶ Natalia B. Shustova,¶ Sergey F. Lebedkin,§ Vyachevslav P. Bubnov,† Oren P. Anderson,¶ Konrad Seppelt,# Steven H. Strauss,*¶ and Olga V. Boltalina*¶
Abstract:
Adding 1% of the metallic elements cerium, lanthanum, and yttrium to graphite rod electrodes resulted in different amounts of the hollow higher fullerenes (HHFs) C76-D2(1), C78-C2v(2), and C78-C2v(3) in carbon-arc fullerene-containing soots. The reaction of trifluoroiodomethane with these and other soluble HHFs at 520−550 °C produced 21 new C76,78,84,90(CF3)n derivatives (n = 6, 8, 10, 12, 14). The reaction with C76-D2(1) produced an abundant isomer of C2-(C76-D2(1))(CF3)10 plus smaller amounts of an isomer of C1-(C76-D2(1))(CF3)6, two isomers of C1-(C76-D2(1))(CF3)8, four isomers of C1-(C76-D2(1))(CF3)10, and one isomer of C2-(C76-D2(1))(CF3)12. The reaction with a mixture of C78-D3(1), C78-C2v(2), and C78-C2v(3) produced the previously reported isomer C1-(C78-C2v(3))(CF3)12 (characterized by X-ray crystallography in this work) and the following new compounds: C2-(C78-C2v(3))(CF3)8; C2-(C78-D3(1))(CF3)10 and Cs-(C78-C2v(2))(CF3)10 (both characterized by X-ray crystallography in this work); C2-(C78-C2v(2))(CF3)10; and C1-C78(CF3)14 (cage isomer unknown). The reaction of a mixture of soluble higher fullerenes including C84 and C90 produced the new compounds C1-C84(CF3)10 (cage isomer unknown), C1-(C84-C2(11))(CF3)12 (X-ray structure reported recently), D2-(C84-D2(22))(CF3)12, C2-(C84-D2(22))(CF3)12, C1-C84(CF3)14 (cage isomer unknown), C1-(C90-C1(32))(CF3)12, and another isomer of C1-C90(CF3)12 (cage isomer unknown). All compounds were studied by mass spectrometry, 19F NMR spectroscopy, and DFT calculations. An analysis of the addition patterns of these compounds and three other HHF(X)n compounds with bulky X groups has led to the discovery of the following addition-pattern principle for HHFs: In general, the most pyramidal cage C(sp2) atoms in the parent HHF, which form the most electron-rich and therefore the most reactive cage C−C bonds as far as 1,2-additions are concerned, are not the cage C atoms to which bulky substituents are added. Instead, ribbons of edge-sharing p-C6(X)2 hexagons, with X groups on less pyramidal cage C atoms, are formed, and the otherwise “most reactive” fullerene double bonds remain intact.
Ivan E. Kareev,*†§ Alexey A. Popov,*‡ Igor V. Kuvychko,¶ Natalia B. Shustova,¶ Sergey F. Lebedkin,§ Vyachevslav P. Bubnov,† Oren P. Anderson,¶ Konrad Seppelt,# Steven H. Strauss,*¶ and Olga V. Boltalina*¶
Abstract:
Adding 1% of the metallic elements cerium, lanthanum, and yttrium to graphite rod electrodes resulted in different amounts of the hollow higher fullerenes (HHFs) C76-D2(1), C78-C2v(2), and C78-C2v(3) in carbon-arc fullerene-containing soots. The reaction of trifluoroiodomethane with these and other soluble HHFs at 520−550 °C produced 21 new C76,78,84,90(CF3)n derivatives (n = 6, 8, 10, 12, 14). The reaction with C76-D2(1) produced an abundant isomer of C2-(C76-D2(1))(CF3)10 plus smaller amounts of an isomer of C1-(C76-D2(1))(CF3)6, two isomers of C1-(C76-D2(1))(CF3)8, four isomers of C1-(C76-D2(1))(CF3)10, and one isomer of C2-(C76-D2(1))(CF3)12. The reaction with a mixture of C78-D3(1), C78-C2v(2), and C78-C2v(3) produced the previously reported isomer C1-(C78-C2v(3))(CF3)12 (characterized by X-ray crystallography in this work) and the following new compounds: C2-(C78-C2v(3))(CF3)8; C2-(C78-D3(1))(CF3)10 and Cs-(C78-C2v(2))(CF3)10 (both characterized by X-ray crystallography in this work); C2-(C78-C2v(2))(CF3)10; and C1-C78(CF3)14 (cage isomer unknown). The reaction of a mixture of soluble higher fullerenes including C84 and C90 produced the new compounds C1-C84(CF3)10 (cage isomer unknown), C1-(C84-C2(11))(CF3)12 (X-ray structure reported recently), D2-(C84-D2(22))(CF3)12, C2-(C84-D2(22))(CF3)12, C1-C84(CF3)14 (cage isomer unknown), C1-(C90-C1(32))(CF3)12, and another isomer of C1-C90(CF3)12 (cage isomer unknown). All compounds were studied by mass spectrometry, 19F NMR spectroscopy, and DFT calculations. An analysis of the addition patterns of these compounds and three other HHF(X)n compounds with bulky X groups has led to the discovery of the following addition-pattern principle for HHFs: In general, the most pyramidal cage C(sp2) atoms in the parent HHF, which form the most electron-rich and therefore the most reactive cage C−C bonds as far as 1,2-additions are concerned, are not the cage C atoms to which bulky substituents are added. Instead, ribbons of edge-sharing p-C6(X)2 hexagons, with X groups on less pyramidal cage C atoms, are formed, and the otherwise “most reactive” fullerene double bonds remain intact.
Thursday, September 25, 2008
ASAP J. Am. Chem. Soc., ASAP Article, 10.1021/ja802578z
Residual Dipolar Coupling Measurements of Transmembrane Proteins Using Aligned Low-q Bicelles and High-Resolution Magic Angle Spinning NMR Spectroscopy
Christian G. Canlas, Dejian Ma, Pei Tang, and Yan Xu*
Abstract:
Bicelles are a major medium form to produce weak alignment of soluble proteins for residual dipolar coupling (RDC) measurements. The obstacle to using the same type of bicelles for transmembrane proteins with solution-state NMR spectroscopy is the loss of signals due to the adhesion or penetration of the proteins into large bicelles, resulting in slow protein tumbling. In this study, weak alignment of the second and third transmembrane domains (TM23) of the human glycine receptor (GlyR) was achieved in low-q bicelles (q = DMPC/DHPC). Although protein-free bicelles with such low q would likely show isotropic properties, the insertion of TM23 induced weakly preferred orientations so that the RDC of the embedded protein can be measured. The extent of the alignment increased but the TM23 signal intensity decreased when q was varied from 0.19 to 0.60. A q of 0.50 was found to be an optimal compromise between alignment and the signal-to-noise ratio. In each pair of NMR experiments for RDC measurements, the same sample and pulse sequence were used, with one being performed at high-resolution magic-angle spinning to obtain pure J-couplings without RDC. A meaningful structure refinement in bicelles was possible by iteratively fitting the experimental RDCs to the back-calculated RDCs using the high-resolution NMR structure of GlyR TM23 in trifluoroethanol as the starting template. Combination of this method with the conventional high-resolution NMR in membrane mimicking mixtures of water and organic solvents offers an attractive way to derive structural information for membrane proteins in their native environment.
Christian G. Canlas, Dejian Ma, Pei Tang, and Yan Xu*
Abstract:
Bicelles are a major medium form to produce weak alignment of soluble proteins for residual dipolar coupling (RDC) measurements. The obstacle to using the same type of bicelles for transmembrane proteins with solution-state NMR spectroscopy is the loss of signals due to the adhesion or penetration of the proteins into large bicelles, resulting in slow protein tumbling. In this study, weak alignment of the second and third transmembrane domains (TM23) of the human glycine receptor (GlyR) was achieved in low-q bicelles (q = DMPC/DHPC). Although protein-free bicelles with such low q would likely show isotropic properties, the insertion of TM23 induced weakly preferred orientations so that the RDC of the embedded protein can be measured. The extent of the alignment increased but the TM23 signal intensity decreased when q was varied from 0.19 to 0.60. A q of 0.50 was found to be an optimal compromise between alignment and the signal-to-noise ratio. In each pair of NMR experiments for RDC measurements, the same sample and pulse sequence were used, with one being performed at high-resolution magic-angle spinning to obtain pure J-couplings without RDC. A meaningful structure refinement in bicelles was possible by iteratively fitting the experimental RDCs to the back-calculated RDCs using the high-resolution NMR structure of GlyR TM23 in trifluoroethanol as the starting template. Combination of this method with the conventional high-resolution NMR in membrane mimicking mixtures of water and organic solvents offers an attractive way to derive structural information for membrane proteins in their native environment.
Wednesday, September 24, 2008
J. Am. Chem. Soc., 130 (38), 12663–12670, 2008.
Density Functional Theory Calculations of Hydrogen-Bond-Mediated NMR J Coupling in the Solid State
Siân A. Joyce,† Jonathan R. Yates,*‡ Chris J. Pickard,§ and Steven P. Brown∥
Abstract:
A recently developed method for calculating NMR J coupling in solid-state systems is applied to calculate hydrogen-bond-mediated 2hJNN couplings across intra- or intermolecular N−H···N hydrogen bonds in two 6-aminofulvene-1-aldimine derivatives and the ribbon structure formed by a deoxyguanosine derivative. Excellent quantitative agreement is observed between the calculated solid-state J couplings and those previously determined experimentally in two recent spin-echo magic-angle-spinning NMR studies (Brown, S. P.; et al. Chem. Commun.2002, 1852−1853 and Pham, T. N.; et al. Phys. Chem. Chem. Phys. 2007, 9, 3416−3423). For the 6-aminofulvene-1-aldimines, the differences in 2hJNN couplings in pyrrole and triazole derivatives are reproduced, while for the guanosine ribbons, an increase in 2hJNN is correlated with a decrease in the N−H···N hydrogen-bond distance. J couplings are additionally calculated for isolated molecules of the 6-aminofulevene-1-aldimines extracted from the crystal with and without further geometry optimization. Importantly, it is shown that experimentally observed differences between J couplings determined by solution- and solid-state NMR are not solely due to differences in geometry; long-range electrostatic effects of the crystal lattice are shown to be significant also. J couplings that are yet to be experimentally measured are calculated. Notably, 2hJNO couplings across N−H···O hydrogen bonds are found to be of a similar magnitude to 2hJNN couplings, suggesting that their utilization and quantitative determination should be experimentally feasible.
Siân A. Joyce,† Jonathan R. Yates,*‡ Chris J. Pickard,§ and Steven P. Brown∥
Abstract:
A recently developed method for calculating NMR J coupling in solid-state systems is applied to calculate hydrogen-bond-mediated 2hJNN couplings across intra- or intermolecular N−H···N hydrogen bonds in two 6-aminofulvene-1-aldimine derivatives and the ribbon structure formed by a deoxyguanosine derivative. Excellent quantitative agreement is observed between the calculated solid-state J couplings and those previously determined experimentally in two recent spin-echo magic-angle-spinning NMR studies (Brown, S. P.; et al. Chem. Commun.2002, 1852−1853 and Pham, T. N.; et al. Phys. Chem. Chem. Phys. 2007, 9, 3416−3423). For the 6-aminofulvene-1-aldimines, the differences in 2hJNN couplings in pyrrole and triazole derivatives are reproduced, while for the guanosine ribbons, an increase in 2hJNN is correlated with a decrease in the N−H···N hydrogen-bond distance. J couplings are additionally calculated for isolated molecules of the 6-aminofulevene-1-aldimines extracted from the crystal with and without further geometry optimization. Importantly, it is shown that experimentally observed differences between J couplings determined by solution- and solid-state NMR are not solely due to differences in geometry; long-range electrostatic effects of the crystal lattice are shown to be significant also. J couplings that are yet to be experimentally measured are calculated. Notably, 2hJNO couplings across N−H···O hydrogen bonds are found to be of a similar magnitude to 2hJNN couplings, suggesting that their utilization and quantitative determination should be experimentally feasible.
J. Am. Chem. Soc., 130 (38), 12568–12569, 2008.
Native Conformation at Specific Residues in Recombinant Inclusion Body Protein in Whole Cells Determined with Solid-State NMR Spectroscopy
Jaime Curtis-Fisk, Ryan M. Spencer, and David P. Weliky*
Abstract:
Inclusion bodies are insoluble aggregates that are formed by bacteria to store excess recombinant protein produced during expression. The structure of the protein in inclusion bodies is poorly understood but it has been hypothesized that the protein may form misfolded β sheet aggregates. This paper presents an isotopic labeling and solid-state nuclear magnetic resonance approach to determine the secondary structure of individual residues within a recombinant influenza virus “FHA2” protein in inclusion bodies. The inclusion bodies were studied either in the context of the unlysed hydrated E. coli cells or in the hydrated pellet formed from centrifugation of the material insoluble in the cell lysate. The native structure of FHA2 is predominantly helical and native helical structure was also observed for several specific residues in the inclusion body FHA2. This approach will be applicable to structural analysis of many inclusion body proteins and should provide useful information for optimizing solubilization and purification protocols of these proteins.
Jaime Curtis-Fisk, Ryan M. Spencer, and David P. Weliky*
Abstract:
Inclusion bodies are insoluble aggregates that are formed by bacteria to store excess recombinant protein produced during expression. The structure of the protein in inclusion bodies is poorly understood but it has been hypothesized that the protein may form misfolded β sheet aggregates. This paper presents an isotopic labeling and solid-state nuclear magnetic resonance approach to determine the secondary structure of individual residues within a recombinant influenza virus “FHA2” protein in inclusion bodies. The inclusion bodies were studied either in the context of the unlysed hydrated E. coli cells or in the hydrated pellet formed from centrifugation of the material insoluble in the cell lysate. The native structure of FHA2 is predominantly helical and native helical structure was also observed for several specific residues in the inclusion body FHA2. This approach will be applicable to structural analysis of many inclusion body proteins and should provide useful information for optimizing solubilization and purification protocols of these proteins.
Friday, September 19, 2008
Chem. Mater., 20 (18), 5787–5795, 2008.
Characterization of Noncrystalline Nanomaterials: NMR of Zinc Phosphate as a Case Study
Marcus Roming,† Claus Feldmann,*† Yamini S. Avadhut,‡ and Jörn Schmedt auf der Günne*‡
Abstract:
Zinc phosphate nanoparticles are prepared via a polyol-mediated synthesis. The nanomaterial turns out to be nonagglomerated and very uniform in size and shape, in particular 20 nm in diameter. X-ray powder diffraction analysis and high-resolution transmission electron microscopy indicate as-prepared nanoparticles to be noncrystalline. To investigate the chemical composition (stoichiometry, material homogeneity, amount of ortho-/metaphosphate, water content, type of surface-allocated adsorbents, differentiation of surface/inner core), X-ray diffraction, NMR-spectroscopy, energy-dispersive X-ray analysis, infrared spectroscopy, and thermal analysis are performed. To validate the local structure and composition, we performed 1H, 13C, and 31P magic angle spinning nuclear magnetic resonance spectroscopy and multidimensional homo- and heteronuclear multiple-pulse solid-state NMR experiments. Moreover, 31P{1H} rotational echo double-resonance experiments for various spin topologies are analyzed analytically and numerically, in order to differentiate between homogeneous nanoparticles and core−shell nanoparticles. The analysis gives a length scale to homogeneity and for bulk materials allows us to differentiate between mono- and dihydrogen phosphates, and phosphate hydrates.
Marcus Roming,† Claus Feldmann,*† Yamini S. Avadhut,‡ and Jörn Schmedt auf der Günne*‡
Abstract:
Zinc phosphate nanoparticles are prepared via a polyol-mediated synthesis. The nanomaterial turns out to be nonagglomerated and very uniform in size and shape, in particular 20 nm in diameter. X-ray powder diffraction analysis and high-resolution transmission electron microscopy indicate as-prepared nanoparticles to be noncrystalline. To investigate the chemical composition (stoichiometry, material homogeneity, amount of ortho-/metaphosphate, water content, type of surface-allocated adsorbents, differentiation of surface/inner core), X-ray diffraction, NMR-spectroscopy, energy-dispersive X-ray analysis, infrared spectroscopy, and thermal analysis are performed. To validate the local structure and composition, we performed 1H, 13C, and 31P magic angle spinning nuclear magnetic resonance spectroscopy and multidimensional homo- and heteronuclear multiple-pulse solid-state NMR experiments. Moreover, 31P{1H} rotational echo double-resonance experiments for various spin topologies are analyzed analytically and numerically, in order to differentiate between homogeneous nanoparticles and core−shell nanoparticles. The analysis gives a length scale to homogeneity and for bulk materials allows us to differentiate between mono- and dihydrogen phosphates, and phosphate hydrates.
J. Am. Chem. Soc., 130 (38), 12712–12724, 2008.
Resolving the Structure of Ligands Bound to the Surface of Superparamagnetic Iron Oxide Nanoparticles by High-Resolution Magic-Angle Spinning NMR
Spectroscopy
Laura Polito, Miriam Colombo, Diego Monti, Sergio Melato, Enrico Caneva,* and Davide Prosperi*
Abstract:
A major challenge in magnetic nanoparticle synthesis and (bio)functionalization concerns the precise characterization of the nanoparticle surface ligands. We report the first analytical NMR investigation of organic ligands stably anchored on the surface of superparamagnetic nanoparticles (MNPs) through the development of a new experimental application of high-resolution magic-angle spinning (HRMAS). The conceptual advance here is that the HRMAS technique, already being used for MAS NMR analysis of gels and semisolid matrixes, enables the fine-structure-resolved characterization of even complex organic molecules bound to paramagnetic nanocrystals, such as nanosized iron oxides, by strongly decreasing the effects of paramagnetic disturbances. This method led to detail-rich, well-resolved 1H NMR spectra, often with highly structured first-order couplings, essential in the interpretation of the data. This HRMAS application was first evaluated and optimized using simple ligands widely used as surfactants in MNP synthesis and conjugation. Next, the methodology was assessed through the structure determination of complex molecular architectures, such as those involved in MNP3 and MNP4. The comparison with conventional probes evidences that HRMAS makes it possible to work with considerably higher concentrations, thus avoiding the loss of structural information. Consistent 2D homonuclear 1H−1H and 1H−13C heteronuclear single-quantum coherence correlation spectra were also obtained, providing reliable elements on proton signal assignments and carbon characterization and opening the way to 13C NMR determination. Notably, combining the experimental evidence from HRMAS 1H NMR and diffusion-ordered spectroscopy performed on the hybrid nanoparticle dispersion confirmed that the ligands were tightly bound to the particle surface when they were dispersed in a ligand-free solvent, while they rapidly exchanged when an excess of free ligand was present in solution. In addition to HRMAS NMR, matrix-assisted laser desorption ionization time-of-flight MS analysis of modified MNPs proved very valuable in ligand mass identification, thus giving a sound support to NMR characterization achievements.
Spectroscopy
Laura Polito, Miriam Colombo, Diego Monti, Sergio Melato, Enrico Caneva,* and Davide Prosperi*
Abstract:
A major challenge in magnetic nanoparticle synthesis and (bio)functionalization concerns the precise characterization of the nanoparticle surface ligands. We report the first analytical NMR investigation of organic ligands stably anchored on the surface of superparamagnetic nanoparticles (MNPs) through the development of a new experimental application of high-resolution magic-angle spinning (HRMAS). The conceptual advance here is that the HRMAS technique, already being used for MAS NMR analysis of gels and semisolid matrixes, enables the fine-structure-resolved characterization of even complex organic molecules bound to paramagnetic nanocrystals, such as nanosized iron oxides, by strongly decreasing the effects of paramagnetic disturbances. This method led to detail-rich, well-resolved 1H NMR spectra, often with highly structured first-order couplings, essential in the interpretation of the data. This HRMAS application was first evaluated and optimized using simple ligands widely used as surfactants in MNP synthesis and conjugation. Next, the methodology was assessed through the structure determination of complex molecular architectures, such as those involved in MNP3 and MNP4. The comparison with conventional probes evidences that HRMAS makes it possible to work with considerably higher concentrations, thus avoiding the loss of structural information. Consistent 2D homonuclear 1H−1H and 1H−13C heteronuclear single-quantum coherence correlation spectra were also obtained, providing reliable elements on proton signal assignments and carbon characterization and opening the way to 13C NMR determination. Notably, combining the experimental evidence from HRMAS 1H NMR and diffusion-ordered spectroscopy performed on the hybrid nanoparticle dispersion confirmed that the ligands were tightly bound to the particle surface when they were dispersed in a ligand-free solvent, while they rapidly exchanged when an excess of free ligand was present in solution. In addition to HRMAS NMR, matrix-assisted laser desorption ionization time-of-flight MS analysis of modified MNPs proved very valuable in ligand mass identification, thus giving a sound support to NMR characterization achievements.
J. Am. Chem. Soc., 130 (38), 12671–12679, 2008.
Residue Ionization in LpxC Directly Observed by 67Zn NMR Spectroscopy
Andrew S. Lipton,† Robert W. Heck,† Marcy Hernick,‡§ Carol A. Fierke,*‡ and Paul D. Ellis*†
Abstract:
The pH dependence of the solid-state 67Zn NMR lineshapes has been measured for both the wild type (WT) and the H265A mutant of Aquifex aeolicus LpxC, each in the absence of substrate (resting state). The 67Zn NMR spectrum of WT LpxC at pH 6 (prepared at 0 °C) contains two overlapping quadrupole lineshapes with Cq values of 10 and 12.9 MHz, while the spectrum measured for the sample prepared at a pH near 9 (at 0 °C) is dominated by the appearance of a third species with a Cq of 14.3 MHz. These findings are consistent with the two pKa values previously observed by the bell-shaped dependence of the LpxC-catalyzed reaction. On the basis of comparison of the experimental results with predictions from quantum mechanical/molecular mechanical (QM/MM) modeling, we suggest that pKa1 (low pH) represents the ionization of Glu78 and pKa2 (high pH) reflects the ionization of another active site residue located near the zinc ion, such as His265. These results are also consistent with water being bound to the Zn2+ ion throughout this pH range. The 67Zn NMR spectra of the H265A mutant appear to be pH independent, with a Cq of 9.55 MHz being sufficient to describe both low- and high-pH data. The QM/MM models of the H265A mutant suggest that over this pH range water is bound to the zinc ion while Glu78 is protonated.
Andrew S. Lipton,† Robert W. Heck,† Marcy Hernick,‡§ Carol A. Fierke,*‡ and Paul D. Ellis*†
Abstract:
The pH dependence of the solid-state 67Zn NMR lineshapes has been measured for both the wild type (WT) and the H265A mutant of Aquifex aeolicus LpxC, each in the absence of substrate (resting state). The 67Zn NMR spectrum of WT LpxC at pH 6 (prepared at 0 °C) contains two overlapping quadrupole lineshapes with Cq values of 10 and 12.9 MHz, while the spectrum measured for the sample prepared at a pH near 9 (at 0 °C) is dominated by the appearance of a third species with a Cq of 14.3 MHz. These findings are consistent with the two pKa values previously observed by the bell-shaped dependence of the LpxC-catalyzed reaction. On the basis of comparison of the experimental results with predictions from quantum mechanical/molecular mechanical (QM/MM) modeling, we suggest that pKa1 (low pH) represents the ionization of Glu78 and pKa2 (high pH) reflects the ionization of another active site residue located near the zinc ion, such as His265. These results are also consistent with water being bound to the Zn2+ ion throughout this pH range. The 67Zn NMR spectra of the H265A mutant appear to be pH independent, with a Cq of 9.55 MHz being sufficient to describe both low- and high-pH data. The QM/MM models of the H265A mutant suggest that over this pH range water is bound to the zinc ion while Glu78 is protonated.
Thursday, September 18, 2008
Solid-State NMR - Volume 34 Issues 1-2
Combined NMR and computational study for azide binding to human manganese superoxide dismutase
Th. Emmler, I. Ayala, D. Silverman, S. Hafner, A.S. Galstyan, E.W. Knapp and G. Buntkowsky
Human manganese superoxide dismutase (MnSOD) labeled with 3-fluorotyrosine (Tyf) was complexed with the 15N-labeled inhibitor azide ([15N3−]). The sample was characterized by solid-state NMR (SSNMR) spectroscopy (19F-MAS and 15N-CPMAS). Employing 19F-15N-REDOR spectroscopy, we determined the distances between the fluorine label in Tyrosine-34 and the three 15N-nuclei of the azide and the relative orientation of the azide in the binding pocket of the MnSOD. A distance of R1=4.85 Å between the 19F-label of Tyf34 and the nearest 15N of the azide and an azide–fluorotyrosine Tyf34 angle of 90° were determined. These geometry data are employed as input for molecular modeling of the location of the inhibitor in the active site of the enzyme. In the computations, several possible binding geometries of the azide near the Mn-complex were assumed. Only when the azide replaces the water ligand at the Mn-complex we obtained a geometry of the azide–Mn-complex, which is consistent with the present NMR data. This indicates that the water molecule ligating to the Mn-complex is removed and the azide is placed at this position. As a consequence the azide forms an H bond with Gln143 instead with Tyf34, in contrast to non-19F-labeled MnSOD, where the azide is hydrogen bonded to the hydroxy group of Tyr34.
A new application for an old concept: Constant time (CT)-REDOR for an accurate determination of second moments in multiple spin systems with strong heteronuclear dipolar couplings
Thomas Echelmeyer, Leo van Wüllen, and Sebastian Wegner
In this contribution we present a constant time version of the well known REDOR pulse sequence which enables us to determine the second moments in multiple spin systems with strong dipolar couplings. From the resulting dipolar evolution curves, accurate values for the second moments can be obtained without the need to incorporate the full information about the detailed spin geometry of the multiple spin systems into the simulation protocol.
Characterization of local environments in crystalline borophosphates using single and double resonance NMR
Devidas B. Raskar, Hellmut Eckert, Bastian Ewald and Rüdiger Kniep
11B and 31P magic-angle spinning as well as 11B{31P} and 31P{11B} rotational echo double resonance (REDOR) NMR have been applied to characterize the local environments in the crystalline borophosphates K3[BP3O9(OH)3], NH4[ZnBP2O8] and Rb3[B2P3O11(OH)2]. Dipolar second moment values extracted from the REDOR curves at short evolution times (ΔS/S00.2) are in reasonable agreement with those calculated from the internuclear distances in the corresponding crystal structures. In particular, the method is found to be useful for distinguishing between boron and phosphorus local environments with different numbers of B–O–P connectivities, making REDOR a well-suited tool for medium-range order investigations in glasses.
Lithium ionic jump motion in the fast solid ion conductor Li5La3Nb2O12
Barbara Koch and Michael Vogel
Using 7Li NMR line-shape analysis, spin–lattice relaxation measurements and stimulated-echo spectroscopy, we investigate the lithium ionic jump motion in the garnet Li5La3Nb2O12. Results for two samples are compared, which were annealed at (GR-850) and at (GR-900), respectively. All 7Li NMR data consistently show that two lithium species with distinguishable dynamical behaviors coexist in each of the samples. While the less mobile species is the majority component in GR-850, the more mobile species is the majority component in GR-900. 7Li NMR stimulated-echo spectroscopy provides straightforward access to the correlation functions describing the jumps of the respective majority component in both samples. From the temperature-dependent correlation times, we obtain activation energies of 56 and for GR-850 and GR-900, respectively. For both samples, the correlation functions substantially deviate from simple exponential behavior, indicating a high complexity of the lithium ionic motion in Li5La3Nb2O12.
13C CPMAS NMR and DFT calculations of anthocyanidins
M. Wolniak and I. Wawer
Anthocyanidins, red dyes from flower petals and fruits, are beneficial to human health. They attract considerable attention owing to their strong antioxidant and radical scavenging properties, however they are unstable in solution and available in small amounts only. 13C CP MAS NMR spectra were recorded to characterize solid-state conformation of nine anthocyanidins: apigenidin, pelargonidin, cyanidin, delphinidin, peonidin, malvidin robinetidin, luteolinidin and diosmetinidin chlorides. For some carbons, the solid-state chemical shifts were different from those obtained for solutions, indicating differences in conformation and intermolecular interactions. The principal elements of the 13C chemical shift tensor were measured for pelargonidin, cyanidin, delphinidin and malvidin chlorides using PASS-2D NMR technique. DFT GIAO calculations of shielding constants were performed for apigenidin and several geometric isomers of pelargonidin. Comparison of experimental 13C δii with the theoretical shielding parameters was helpful in predicting the most reliable geometry in the solid state. The cross-polarization parameters were obtained from variable-contact time experiments; TCH are longer and the values of T1ρH are shorter in the order: pelargonidin cyanidin delphinidin. It is probable that solid anthocyanidins become less ordered as the number of OH groups increases.
51V solid-state NMR investigations and DFT studies of model compounds for vanadium haloperoxidases
Annika Schweitzer, Torsten Gutmann, Maria Wächtler, Hergen Breitzke, Axel Buchholz, Winfried Plass and Gerd Buntkowsky
Three cis-dioxovanadium(V) complexes with similar N-salicylidenehydrazide ligands modeling hydrogen bonding interactions of vanadate relevant for vanadium haloperoxidases are studied by 51V solid-state NMR spectroscopy. Their parameters describing the quadrupolar and chemical shift anisotropy interactions (quadrupolar coupling constant CQ, asymmetry of the quadrupolar tensor ηQ, isotropic chemical shift δiso, chemical shift anisotropy δσ, asymmetry of the chemical shift tensor ησ and the Euler angles α, β and γ) are determined both experimentally and theoretically using DFT methods. A comparative study of different methods to determine the NMR parameters by numerical simulation of the spectra is presented. Detailed theoretical investigations on the DFT level using various basis sets and structural models show that by useful choice of the methodology, the calculated parameters agree to the experimental ones in a very good manner.
Spin-diffusion NMR at low field for the study of multiphase solids
M. Mauri, Y. Thomann, H. Schneider and K. Saalwächter
The use of spin-diffusion NMR for the measurement of domain sizes in multiphase materials is becoming increasingly popular, in particular for the study of heterogeneous polymers. Under conditions where T1 relaxation can be neglected, which is mostly the case at high field, analytical and approximate solutions to the evolution of spin diffusion are available. In order to extend the technique to more general conditions, we performed a comprehensive study of the diffusion of magnetization in a model copolymer at low field, where T1 tends to be of the same order of magnitude as the typical spin-diffusion time. In order to study the effects of T1 and to delineate the optimal T1 values for back correction prior to applying the initial-rate approximation, we developed a numerical simulation based on the diffusion equation and including longitudinal relaxation. We present and discuss the limits of simple correction strategies for initial-slope analysis based on apparent relaxation times from saturation-recovery experiments or the spin-diffusion experiments themselves. Our best strategy faithfully reproduces domain sizes obtained by both TEM investigations and full simultaneous fitting of spin-diffusion and saturation-recovery curves. Full fitting of such independent data sets not only yields correct domain sizes, but also the true longitudinal relaxation times, as well as spin-diffusion coefficients. Effects of interphases with distinct mobility on spin-diffusion curves, as well as practical hints concerning the reliable component decomposition of the detected low-resolution FID signal by help of different magnetization filters are also discussed in detail.
Th. Emmler, I. Ayala, D. Silverman, S. Hafner, A.S. Galstyan, E.W. Knapp and G. Buntkowsky
Human manganese superoxide dismutase (MnSOD) labeled with 3-fluorotyrosine (Tyf) was complexed with the 15N-labeled inhibitor azide ([15N3−]). The sample was characterized by solid-state NMR (SSNMR) spectroscopy (19F-MAS and 15N-CPMAS). Employing 19F-15N-REDOR spectroscopy, we determined the distances between the fluorine label in Tyrosine-34 and the three 15N-nuclei of the azide and the relative orientation of the azide in the binding pocket of the MnSOD. A distance of R1=4.85 Å between the 19F-label of Tyf34 and the nearest 15N of the azide and an azide–fluorotyrosine Tyf34 angle of 90° were determined. These geometry data are employed as input for molecular modeling of the location of the inhibitor in the active site of the enzyme. In the computations, several possible binding geometries of the azide near the Mn-complex were assumed. Only when the azide replaces the water ligand at the Mn-complex we obtained a geometry of the azide–Mn-complex, which is consistent with the present NMR data. This indicates that the water molecule ligating to the Mn-complex is removed and the azide is placed at this position. As a consequence the azide forms an H bond with Gln143 instead with Tyf34, in contrast to non-19F-labeled MnSOD, where the azide is hydrogen bonded to the hydroxy group of Tyr34.
A new application for an old concept: Constant time (CT)-REDOR for an accurate determination of second moments in multiple spin systems with strong heteronuclear dipolar couplings
Thomas Echelmeyer, Leo van Wüllen, and Sebastian Wegner
In this contribution we present a constant time version of the well known REDOR pulse sequence which enables us to determine the second moments in multiple spin systems with strong dipolar couplings. From the resulting dipolar evolution curves, accurate values for the second moments can be obtained without the need to incorporate the full information about the detailed spin geometry of the multiple spin systems into the simulation protocol.
Characterization of local environments in crystalline borophosphates using single and double resonance NMR
Devidas B. Raskar, Hellmut Eckert, Bastian Ewald and Rüdiger Kniep
11B and 31P magic-angle spinning as well as 11B{31P} and 31P{11B} rotational echo double resonance (REDOR) NMR have been applied to characterize the local environments in the crystalline borophosphates K3[BP3O9(OH)3], NH4[ZnBP2O8] and Rb3[B2P3O11(OH)2]. Dipolar second moment values extracted from the REDOR curves at short evolution times (ΔS/S00.2) are in reasonable agreement with those calculated from the internuclear distances in the corresponding crystal structures. In particular, the method is found to be useful for distinguishing between boron and phosphorus local environments with different numbers of B–O–P connectivities, making REDOR a well-suited tool for medium-range order investigations in glasses.
Lithium ionic jump motion in the fast solid ion conductor Li5La3Nb2O12
Barbara Koch and Michael Vogel
Using 7Li NMR line-shape analysis, spin–lattice relaxation measurements and stimulated-echo spectroscopy, we investigate the lithium ionic jump motion in the garnet Li5La3Nb2O12. Results for two samples are compared, which were annealed at (GR-850) and at (GR-900), respectively. All 7Li NMR data consistently show that two lithium species with distinguishable dynamical behaviors coexist in each of the samples. While the less mobile species is the majority component in GR-850, the more mobile species is the majority component in GR-900. 7Li NMR stimulated-echo spectroscopy provides straightforward access to the correlation functions describing the jumps of the respective majority component in both samples. From the temperature-dependent correlation times, we obtain activation energies of 56 and for GR-850 and GR-900, respectively. For both samples, the correlation functions substantially deviate from simple exponential behavior, indicating a high complexity of the lithium ionic motion in Li5La3Nb2O12.
13C CPMAS NMR and DFT calculations of anthocyanidins
M. Wolniak and I. Wawer
Anthocyanidins, red dyes from flower petals and fruits, are beneficial to human health. They attract considerable attention owing to their strong antioxidant and radical scavenging properties, however they are unstable in solution and available in small amounts only. 13C CP MAS NMR spectra were recorded to characterize solid-state conformation of nine anthocyanidins: apigenidin, pelargonidin, cyanidin, delphinidin, peonidin, malvidin robinetidin, luteolinidin and diosmetinidin chlorides. For some carbons, the solid-state chemical shifts were different from those obtained for solutions, indicating differences in conformation and intermolecular interactions. The principal elements of the 13C chemical shift tensor were measured for pelargonidin, cyanidin, delphinidin and malvidin chlorides using PASS-2D NMR technique. DFT GIAO calculations of shielding constants were performed for apigenidin and several geometric isomers of pelargonidin. Comparison of experimental 13C δii with the theoretical shielding parameters was helpful in predicting the most reliable geometry in the solid state. The cross-polarization parameters were obtained from variable-contact time experiments; TCH are longer and the values of T1ρH are shorter in the order: pelargonidin cyanidin delphinidin. It is probable that solid anthocyanidins become less ordered as the number of OH groups increases.
51V solid-state NMR investigations and DFT studies of model compounds for vanadium haloperoxidases
Annika Schweitzer, Torsten Gutmann, Maria Wächtler, Hergen Breitzke, Axel Buchholz, Winfried Plass and Gerd Buntkowsky
Three cis-dioxovanadium(V) complexes with similar N-salicylidenehydrazide ligands modeling hydrogen bonding interactions of vanadate relevant for vanadium haloperoxidases are studied by 51V solid-state NMR spectroscopy. Their parameters describing the quadrupolar and chemical shift anisotropy interactions (quadrupolar coupling constant CQ, asymmetry of the quadrupolar tensor ηQ, isotropic chemical shift δiso, chemical shift anisotropy δσ, asymmetry of the chemical shift tensor ησ and the Euler angles α, β and γ) are determined both experimentally and theoretically using DFT methods. A comparative study of different methods to determine the NMR parameters by numerical simulation of the spectra is presented. Detailed theoretical investigations on the DFT level using various basis sets and structural models show that by useful choice of the methodology, the calculated parameters agree to the experimental ones in a very good manner.
Spin-diffusion NMR at low field for the study of multiphase solids
M. Mauri, Y. Thomann, H. Schneider and K. Saalwächter
The use of spin-diffusion NMR for the measurement of domain sizes in multiphase materials is becoming increasingly popular, in particular for the study of heterogeneous polymers. Under conditions where T1 relaxation can be neglected, which is mostly the case at high field, analytical and approximate solutions to the evolution of spin diffusion are available. In order to extend the technique to more general conditions, we performed a comprehensive study of the diffusion of magnetization in a model copolymer at low field, where T1 tends to be of the same order of magnitude as the typical spin-diffusion time. In order to study the effects of T1 and to delineate the optimal T1 values for back correction prior to applying the initial-rate approximation, we developed a numerical simulation based on the diffusion equation and including longitudinal relaxation. We present and discuss the limits of simple correction strategies for initial-slope analysis based on apparent relaxation times from saturation-recovery experiments or the spin-diffusion experiments themselves. Our best strategy faithfully reproduces domain sizes obtained by both TEM investigations and full simultaneous fitting of spin-diffusion and saturation-recovery curves. Full fitting of such independent data sets not only yields correct domain sizes, but also the true longitudinal relaxation times, as well as spin-diffusion coefficients. Effects of interphases with distinct mobility on spin-diffusion curves, as well as practical hints concerning the reliable component decomposition of the detected low-resolution FID signal by help of different magnetization filters are also discussed in detail.
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