Thursday, July 31, 2008

Journal of Magnetic Resonance - Vol 193 Issue 2

Direct 13C-detection for carbonyl relaxation studies of protein dynamics
Gabriela Pasat, John S. Zintsmaster and Jeffrey W. Peng
We describe a method that uses direct 13C-detection for measuring rotating-frame carbonyl (13CO) relaxation rates to describe protein functional dynamics. Key advantages of method include the following: (i) unique access to 13CO groups that lack a scalar-coupled 15N–1H group; (ii) insensitivity to 15N/1H exchange-broadening that can derail 1H-detected 15N and HNCO methods; (iii) avoidance of artifacts caused by incomplete water suppression. We demonstrate the approach for both backbone and side-chain 13CO groups. Accuracy of the 13C-detected results is supported by their agreement with those obtained from established HNCO-based approaches. Critically, we show that the 13C-detection approach provides access to the 13CO groups of functionally important residues that are invisible via 1H-detected HNCO methods because of exchange-broadening. Hence, the 13C-based method fills gaps inherent in canonical 1H-detected relaxation experiments, and thus provides a novel complementary tool for NMR studies of biomolecular flexibility.

29Si NMR in solid state with CPMG acquisition under MAS
J.W. Wiench, V.S.-Y. Lin and M. Pruski
A remarkable enhancement of sensitivity can be often achieved in 29Si solid-state NMR by applying the well-known Carr–Purcell–Meiboom–Gill (CPMG) train of rotor-synchronized π pulses during the detection of silicon magnetization. Here, several one- and two-dimensional (1D and 2D) techniques are used to demonstrate the capabilities of this approach. Examples include 1D 29Si{X} CPMAS spectra and 2D 29Si{X} HETCOR spectra of mesoporous silicas, zeolites and minerals, where X = 1H or 27Al. Data processing methods, experimental strategies and sensitivity limits are discussed and illustrated by experiments. The mechanisms of transverse dephasing of 29Si nuclei in solids are analyzed. Fast magic angle spinning, at rates between 25 and 40 kHz, is instrumental in achieving the highest sensitivity gain in some of these experiments. In the case of 29Si–29Si double-quantum techniques, CPMG detection can be exploited to measure homonuclear J-couplings.

Solid-state NMR adiabatic TOBSY sequences provide enhanced sensitivity for multidimensional high-resolution magic-angle-spinning 1H MR spectroscopy
Ovidiu C. Andronesi, Dionyssios Mintzopoulos, Jochem Struppe, Peter M. Black and A. Aria Tzika
We propose a solid-state NMR method that maximizes the advantages of high-resolution magic-angle-spinning (HRMAS) applied to intact biopsies when compared to more conventional liquid-state NMR approaches. Theoretical treatment, numerical simulations and experimental results on intact human brain biopsies are presented. Experimentally, it is proven that an optimized adiabatic TOBSY (TOtal through Bond correlation SpectroscopY) solid-state NMR pulse sequence for two-dimensional 1H–1H homonuclear scalar-coupling longitudinal isotropic mixing provides a 20%–50% improvement in signal-to-noise ratio relative to its liquid-state analogue TOCSY (TOtal Correlation SpectroscopY). For this purpose we have refined the symmetry-based 13C TOBSY pulse sequence for 1H MRS use and compared it to MLEV-16 TOCSY sequence. Both sequences were rotor-synchronized and implemented using WURST-8 adiabatic inversion pulses. As discussed theoretically and shown in simulations, the improved magnetization-transfer comes from actively removing residual dipolar couplings from the average Hamiltonian. Importantly, the solid-state NMR techniques are tailored to perform measurements at low temperatures where sample degradation is reduced. This is the first demonstration of such a concept for HRMAS metabolic profiling of disease processes, including cancer, from biopsies requiring reduced sample degradation for further genomic analysis.

Enhanced resolution in proton solid-state NMR with very-fast MAS experiments
Jean-Paul Amoureux, Bingwen Hu and Julien Trébosc
We present a new smooth amplitude-modulated (SAM) method that allows to observe highly resolved 1H spectra in solid-state NMR. The method, which works mainly at fast or ultra-fast MAS speed (νR > 25 kHz) is complementary to previous methods, such as DUMBO, FSLG/PMLG or symmetry-based sequences. The method is very robust and efficient and does not present line-shape distortions or fake peaks. The main limitation of the method is that it requires a modern console with fast electronics that must be able to define the cosine line-shape in a smooth way, without any transient. However, this limitation mainly occurs at ultra-fast MAS where the rotation period is very short.

A frequency-selective REDOR experiment for an SI2 spin system
Eugene Mihaliuk and Terry Gullion
A frequency-selective REDOR experiment is described for SI2 spin systems. The experiment causes the net dipolar dephasing of the S spin to evolve only under the influence of one of the I spins. The experiment is based on a single pair of appropriately phased 90° I-spin pulses, and the I spin causing the S-spin dipolar dephasing is determined by the relative phases between the two 90° pulses. The experiment is demonstrated on a sample of 15N2-l-asparagine.

3D 1H–13C–14N correlation solid-state NMR spectrum
Renée Siegel, Julien Trébosc, Jean-Paul Amoureux, and Zhehong Gan
Nitrogen-14 (spin I = 1) has always been a nucleus difficult to observe in solid-state NMR and until recently its observation was restricted to one-dimensional (1D) spectra. We present here the first 3D 1H–13C–14N NMR correlation spectrum. This spectrum was acquired on a test sample l-histidine·HCl·H2O using a recently developed technique, which consists in indirectly observing 14N nuclei via dipolar recoupling with an HMQC-type experiment.

Friday, July 25, 2008

ASAP J. Am. Chem. Soc., ASAP Article, 10.

Separating Chemical Shift and Quadrupolar Anisotropies via Multiple-Quantum NMR Spectroscopy

Jason T. Ash, Nicole M. Trease, and Philip J. Grandinetti*

Chemical shift anisotropy (CSA) has been an invaluable probe of structure and dynamics for a variety of systems in NMR spectroscopy. Unfortunately, the presence of strong quadrupolar couplings has severely limited the ability to measure CSA in nuclei with spins I > 1/2. Here we show that these two interactions can be refocused at different times in a 2D multiple-quantum NMR experiment on polycrystalline samples. Combining this experiment with appropriate affine transformations allows these interactions to be cleanly separated into orthogonal dimensions. The 1D projection onto each axis can be fit to extract the respective principal tensor components. These components can then be used to fit the 2D spectrum for the relative orientation between the CSA and quadrupolar-coupling tensors. The necessary affine transformation parameters are given for all possible I values. Illustrative examples of spectra and analyses are given for 63Cu in K3[Cu(CN)4], 59Co in K3[Co(CN)6], and 87Rb in RbCrO4.

ASAP J. Am. Chem. Soc., ASAP Article, 10.

Evidence for Dynamics on a 100 ns Time Scale from Single- and Double-Quantum Nitrogen-14 NMR in Solid Peptides

Simone Cavadini,† Anuji Abraham,*† Simone Ulzega,† and Geoffrey Bodenhausen†‡

The indirect detection of 14N spectra via protons in the manner of heteronuclear multiple-quantum correlation (HMQC) allows one to obtain single- (SQ) and double-quantum (DQ) 14N spectra in solids. A comparison of the SQ and DQ line widths as a function of temperature with simulations reveals motions in the tripeptide AAG with rates on the order of 107 s−1 at 49 °C.

ASAP J. Am. Chem. Soc., ASAP Article, 10.1021/ja802210u

Nitrogen-14 Solid-State NMR Spectroscopy of Aligned Phospholipid Bilayers to Probe Peptide−Lipid Interaction and Oligomerization of Membrane Associated Peptides

Ayyalusamy Ramamoorthy,* Dong-Kuk Lee,† Jose S. Santos, and Katherine A. Henzler-Wildman‡

Characterization of the oligomerization of membrane-associated peptides is important to understand the folding and function of biomolecules like antimicrobial peptides, fusion peptides, amyloid peptides, toxins, and ion channels. However, this has been considered to be very difficult, because the amphipathic properties of the constituents of the cell membrane pose tremendous challenges to most commonly used biophysical techniques. In this study, we present the application of a simple 14N solid-state NMR spectroscopy of aligned model membranes containing a phosphatidyl choline lipid to investigate the oligomerization of membrane-associated peptides. Since the near-symmetric nature of the choline headgroup of a phosphocholine lipid considerably reduces the 14N quadrupole coupling, there are significant practical advantages in using 14N solid-state NMR experiments to probe the interaction of peptide or protein with the surface of model membranes. Experimental results for several membrane-associated peptides are presented in this paper. Our results suggest that the experimentally measured 14N quadrupole splitting of the lipid depends on the peptide-induced changes in the electrostatic potential of the lipid bilayer surface and therefore on the nature of the peptide, peptide-membrane interaction, and peptide−peptide interaction. It is inferred that the membrane orientation and oligomerization of the membrane-associated peptides can be measured using 14N solid-state NMR spectroscopy.

ASAP J. Am. Chem. Soc., ASAP Article, 10.1021/ja8036492

Molecular Structure from a Single NMR Experiment

E̅riks Kupče† and Ray Freeman*‡

A procedure is described for determining the structure of a small molecule from a single NMR experiment. Several standard NMR sequences are combined so that the essential structural information is obtained in just one pass. Two-dimensional 13C−13C correlations (“INADEQUATE”), single- and multiple-bond 13C−1H correlations, and the conventional 13C spectrum are recorded in parallel, making use of separate receiver channels for acquisition of 13C and 1H signals. The natural-abundance 13C−13C correlation measurements employ a high-sensitivity cryogenically cooled probe, optimized for 13C detection. An extension of this “all-in-one” sequence with three parallel receivers permits the corresponding natural-abundance 15N spectra to be included.

ASAP Chem. Mater., ASAP Article, 10.1021/cm8012683

High-Field 17O MAS NMR Investigation of Phosphonic Acid Monolayers on Titania

Florence Brodard-Severac,† Gilles Guerrero,† Jocelyne Maquet,‡ Pierre Florian,§ Christel Gervais,*‡ and P. Hubert Mutin*†

High-field 17O MAS NMR was used to investigate the binding of self-assembled monolayers of 17O-enriched phosphonic acids deposited on a titania anatase support. The spectra were recorded at two different magnetic fields (9.4 and 17.6 T), to improve the reliability of the simulations of the different resonances. The spectra recorded at 17.6 T offer an excellent resolution between the different oxygen sites, PO, P−O−H, and P−O−Ti, thus greatly facilitating their quantification. The data reported here give direct evidence of the extensive formation of Ti−O−P bonds in the surface modification of titania by phosphonic acids. The presence of residual PO and P−O−H sites indicates the presence of several different binding modes in phosphonic acid monolayers. The chemical shift of P−O−Ti sites is consistent with bridging (as opposed to chelating) modes.

J. Am. Chem. Soc., 130 (30), 9871–9877, 2008.

Determining Secondary Structure in Spider Dragline Silk by Carbon−Carbon Correlation Solid-State NMR Spectroscopy

Gregory P. Holland,* Melinda S. Creager, Janelle E. Jenkins, Randolph V. Lewis, and Jeffery L. Yarger*

Two-dimensional (2D) 13C−13C NMR correlation spectra were collected on 13C-enriched dragline silk fibers produced from Nephila clavipes spiders. The 2D NMR spectra were acquired under fast magic-angle spinning (MAS) and dipolar-assisted rotational resonance (DARR) recoupling to enhance magnetization transfer between 13C spins. Spectra obtained with short (150 ms) recoupling periods were utilized to extract distinct chemical shifts for all carbon resonances of each labeled amino acid in the silk spectra, resulting in a complete resonance assignment. The NMR results presented here permit extraction of the precise chemical shift of the carbonyl environment for each 13C-labeled amino acid in spider silk for the first time. Spectra collected with longer recoupling periods (1 s) were implemented to detect intermolecular magnetization exchange between neighboring amino acids. This information is used to ascribe NMR resonances to the specific repetitive amino acid motifs prevalent in spider silk proteins. These results indicate that glycine and alanine are both present in two distinct structural environments: a disordered 31-helical conformation and an ordered β-sheet structure. The former can be ascribed to the Gly-Gly-Ala motif while the latter is assigned to the poly(Ala) and poly(Gly-Ala) domains.

ASAP J. Am. Chem. Soc., ASAP Article, 10.1021/ja801464g

Methyl Proton Contacts Obtained Using Heteronuclear Through-Bond Transfers in Solid-State NMR Spectroscopy

Antoine Loquet,† Ségolène Laage,‡ Carole Gardiennet,† Bénédicte Elena,‡ Lyndon Emsley,‡ Anja Böckmann,*† and Anne Lesage*‡

A two-dimensional proton-mediated carbon−carbon correlation experiment that relies on through-bond heteronuclear magnetization transfers is demonstrated in the context of solid-state NMR of proteins. This new experiment, dubbed J-CHHC by analogy to the previously developed dipolar CHHC techniques, is shown to provide selective and sensitive correlations in the methyl region of 2D spectra of crystalline organic compounds. The method is then demonstrated on a microcrystalline sample of the dimeric protein Crh (2 × 10.4 kDa). A total of 34 new proton−proton contacts involving side-chain methyl groups were observed in the J-CHHC spectrum, which had not been observed with the conventional experiment. The contacts were then used as additional distance restraints for the 3D structure determination of this microcrystalline protein. Upon addition of these new distance restraints, which are in large part located in the hydrophobic core of the protein, the root-mean-square deviation with respect to the X-ray structure of the backbone atom coordinates of the 10 best conformers of the new ensemble of structures is reduced from 1.8 to 1.1 Å.

Thursday, July 24, 2008

CMR - Concepts in Magnetic Resonance - up to Jul. 11, 2008

CMR - Concepts in Magnetic Resonance - up to Jul. 11, 2008

Highlights: NMR of chiral compounds; and some must reads, including an overview of NMR relaxation (good for newbies) and an excellent tutorial on CP/MAS by David Rovnyak.

Concepts in Magnetic Resonance Part A

Volume 32A Issue 3, Pages 201 - 218
Published Online: 1 May 2008
NMR studies of chiral organic compounds in non-isotropic phases
by Marek J. Potrzebowski, Agata Jeziorna, S[lstrok]awomir Ka[zacute]mierski

In this article, new applications and perspectives of one- and two-dimensional NMR spectroscopy for study of chiral organic compounds in the non-isotropic phases (solid state and liquid crystals) are presented. The review is organized into five sections. In the first part, theoretical background and short introduction to solid state NMR are shown. The second part presents how NMR isotropic chemical shift can be used for distinguishing of racemates and enantiomers. In the third section, the power of the ODESSA pulse sequence for investigation of racemates, enantiomers and establishing of enantiomeric excess are discussed. The fourth part shows the application of analysis of principal elements of chemical shift tensors obtained by means of 2D NMR techniques for searching of absolute configuration and conformational changes in the solid state. The final part presents recent achievements of chiral liquid crystals NMR methodology for study of chiral compounds. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part A 32A:201-218, 2008.


Concepts in Magnetic Resonance Part A
Volume 32A Issue 3, Pages 168 - 182
Published Online: 1 May 2008

Beyond excitation NMR relaxation
by Armando Ariza-Castolo

The understanding of NMR relaxation processes allows us to comprehend a variety of experiments ranging from the recording of spectra to the determination of images. A brief history of NMR spectroscopy, the state of the art of this technique, as well as its comparison with the coupling constants and chemical shifts measured in the condensed phase are presented in this paper. © 2008 Wiley Periodicals, Inc.Concepts Magn Reson Part A 32A: 168-182, 2008.


Concepts Magn Reson Part A 32A: 254-276, 2008.

Tutorial on analytic theory for cross-polarization in solid state NMR
by David Rovnyak

This tutorial aims to be a self-contained and explicit analysis of the basic cross polarization (CP) solid-state NMR experiment (SSNMR) in the isolated spin-pair approximation using standard quantum mechanical arguments. The general result of obtaining coherence transfer between two dipolar coupled spin-active nuclei while applying radio frequency fields to both nuclei is described, with emphasis on the origin of the well known Hartmann-Hahn matching conditions. No new theory is presented; rather several common analytical methods in SSNMR are demonstrated in the context of cross-polarization under static and magic-angle spinning (MAS) conditions. A background in NMR and quantum mechanics is assumed, however this work attempts to minimize the need for excursions into the literature. This article was written to aid a reader in advancing into more detailed descriptions of CP and dipolar recoupling in general. © 2008 Wiley Periodicals, Inc.

MRC: Up to July 21, 2008

MRC: Up to July 21, 2008 online

Highlights: 119Sn NMR tensors; 13C NMR of steroids; more from IUPAC on CS tensor conventions (will it ever end??); NMR of citrate in solids

Magnetic Resonance in Chemistry
Volume 46 Issue 7, Pages 690 - 692
Published Online: 28 Mar 2008

119Sn NMR chemical shift tensors in anhydrous and hydrated Si8O20(SnMe3)8 crystals
from Magnetic Resonance in Chemistry by Jian Jiao, Ming-Yung Lee, Craig E. Barnes, Edward W. Hagaman

119Sn chemical shift tensors of crystalline trialkyltin functionalized octameric spherosilicates, Si8O20(SnMe3)8, have been determined by fitting sideband intensities in solid-state magic angle spinning (MAS) NMR spectra. Tin chemical shift parameters are exquisitely sensitive to the presence of water of crystallization. Both hydrogen bonding and incipient oxygen-tin bonding from molecular water impact the local tin environment. Tin chemical shift tensors in the crystalline derivatives reflect the changes in geometry and coordination number at the tin centers. Chemical shift correlations on the crystalline derivatives, with known x-ray structures, are used to infer the tin coordination environment in an amorphous sample.


Magnetic Resonance in Chemistry
Volume 46 Issue 8, Pages 718 - 725

13C solid-state NMR analysis of steroid compounds
by Jen-Hsien Yang, Yu Ho, Der-Lii M. Tzou

13C CP/MAS solid-state NMR spectroscopy has been utilized to analyze six steroid compounds, namely testosterone (Tes), hydrocortisone (Cor), trans-dehydroandrosterone (Adr), prednisolone (Prd), prednisone (Pre) and estradiol (Est). Among them, Tes displays a doublet pattern for all residues, whereas Prd, Pre and Est, exhibit exclusively singlets. For Cor and Adr, the 13C spectra contain both doublet and singlet patterns. The 13C doublet signal, with splittings of 0.2-1.5 ppm, are ascribed to local differences in the ring conformations associated with polymorphism. We have assigned all of the 13C resonances to the different residues in these steroid compounds on the basis of solution NMR data. The C-7, C-8, C-10, C-15 and C-16 residues of Tes, Cor and Adr consistently give rise to singlets or doublets with splittings of less than 0.5 ppm, indicating similar local conformations. Accompanying hydration and dehydration processes, a reversible phase transformation between [delta]- and [alpha]-crystal forms has been observed in Tes, corresponding to singlet and doublet 13C patterns, respectively. To further characterize the ring conformations in the [alpha]-form, we have successfully extracted chemical shift tensor elements for the 13C doublets. It is demonstrated that 13C solid-state NMR spectroscopy provides a reliable and sensitive means of characterizing polymorphism in steroids.


Magnetic Resonance in Chemistry
Volume 46 Issue 6, Pages 582 - 598

Further Conventions for NMR Shielding and Chemical Shifts (IUPAC Recommendations 2008)
International Union of Pure and Applied Chemistry Physical and Biophysical Chemistry Division
Robin K. Harris 1 *, Edwin D. Becker 2, Sonia M. Cabral De Menezes 3, Pierre Granger 4, Roy E. Hoffman 5, Kurt W. Zilm 6,

nuclear magnetic resonance • recommendations • chemical shifts • conventions • IUPAC Physical and Biophysical Chemistry Division • shielding tensors.

IUPAC has published a number of recommendations regarding the reporting of nuclear magnetic resonance (NMR) data, especially chemical shifts. The most recent publication [Pure Appl. Chem. 73, 1795 (2001)] recommended that tetramethylsilane (TMS) serve as a universal reference for reporting the shifts of all nuclides, but it deferred recommendations for several aspects of this subject. This document first examines the extent to which the 1H shielding in TMS itself is subject to change by variation in temperature, concentration, and solvent. On the basis of recently published results, it has been established that the shielding of TMS in solution [along with that of sodium-3-(trimethylsilyl)propanesulfonate, DSS, often used as a reference for aqueous solutions] varies only slightly with temperature but is subject to solvent perturbations of a few tenths of a part per million (ppm). Recommendations are given for reporting chemical shifts under most routine experimental conditions and for quantifying effects of temperature and solvent variation, including the use of magnetic susceptibility corrections and of magic-angle spinning (MAS).
This document provides the first IUPAC recommendations for referencing and reporting chemical shifts in solids, based on high-resolution MAS studies. Procedures are given for relating 13C NMR chemical shifts in solids to the scales used for high-resolution studies in the liquid phase. The notation and terminology used for describing chemical shift and shielding tensors in solids are reviewed in some detail, and recommendations are given for best practice. © 2008 IUPAC. Reprinted with permission from Pure Appl. Chem. 2008; 80: 59. This article can be freely downloaded from and can be copied, provided acknowledgement to IUPAC is given.

Magnetic Resonance in Chemistry
Volume 46 Issue 5, Pages 408 - 417
Published Online: 28 Feb 2008

NMR spectroscopy of citrate in solids: cross-polarization kinetics in weakly coupled systems
from Magnetic Resonance in Chemistry by Jian Feng, Young J. Lee, James D. Kubicki, Richard J. Reeder, Brian L. Phillips

Solid-state NMR spectroscopy is a potentially powerful method for obtaining molecular level structural information crucial for understanding the specific relationship between calcite crystals and occluded organic molecules that are important in biomineralization and biomimetic materials. In this work, a method is developed based on cross-polarization/magic angle spinning (CP/MAS) NMR to measure the heteronuclear distances and obtain structural information for large intracrystalline citrate defects in a synthetic calcite/citrate composite. Using compounds with well-characterized crystal structures, Mg(II) citrate and Sr(II) citrate, a correlation is established between TIS, the CP time, and M2IS, the van Vleck heteronuclear dipolar second moment, which contains distance and structural information. This correlation is supported by peak assignments obtained from calculations of the 13C chemical shifts for crystalline Mg(II) citrate. On the basis of TIS-1 versus M2IS correlation, measurement of TIS for carbonate ions associated with citrate defects in a calcite(13C-enriched)/citrate coprecipitate yields an estimate for the distance between citrate and the nearest carbonate carbon that indicates close spatial proximity and provides useful constraints for future computational study. The applicability of TIS-1 versus M2IS correlations to other weakly coupled spin-1/2 systems is discussed in terms of the effects of 1H homonuclear dipolar coupling, using the CP kinetics of Zn(II) dihydroxybenzoate and kaolinite for comparison. The results suggest a limited range of correlation constants and indicate that quantitative information can be obtained from CP/MAS kinetics obtained under similar experimental conditions.

Wednesday, July 23, 2008

J. Am. Chem. Soc., 130 (29), 9332–9341, 2008.

Characterization of Mg2+ Binding to the DNA Repair Protein Apurinic/Apyrimidic Endonuclease 1 via Solid-State 25Mg NMR Spectroscopy

A. S. Lipton,† R. W. Heck,† S. Primak,† D. R. McNeill,‡ D. M. Wilson, III,*‡ and P. D. Ellis*†

Apurinic/apyrimidinic endonuclease 1 (APE1), a member of the divalent cation-dependent phosphoesterase superfamily of proteins that retain the conserved four-layered α/β-sandwich structural core, is an essential protein that functions as part of base excision repair to remove mutagenic and cytotoxic abasic sites from DNA. Using low-temperature solid-state 25Mg NMR spectroscopy and various mutants of APE1, we demonstrate that Mg2+ binds to APE1 and a functional APE1−substrate DNA complex with an overall stoichiometry of one Mg2+ per mole of APE1 as predicted by the X-ray work of Tainer and co-workers (Mol, C. D.; Kuo, C. F.; Thayer, M. M.; Cunningham, R. P.; Tainer, J. A. Nature 1995, 374, 381−386). However, the NMR spectra show that the single Mg2+ site is disordered. We discuss the probable reasons for the disorder at the Mg2+ binding site. The most likely source of this disorder is arrangement of the protein−ligands about the Mg2+ (cis and trans isomers). The existence of these isomers reinforces the notion of the plasticity of the metal binding site within APE1.

Monday, July 14, 2008

Joel's Journal Updates

NMR crystallography of p-tert-butylcalix[4]arene host-guest complexes using 1H complexation-induced chemical shifts
Darren H. Brouwer, Saman Alavi, and John A. Ripmeester

1H magic-angle spinning (MAS) NMR spectra of p-tert-butylcalix[4]arene inclusion compounds with toluene and pyridine show large complexation-induced shifts of the guest proton resonances arising from additional magnetic shielding caused by the aromatic rings of the cavities of the host calixarene lattice. In combination with ab initio calculations, these observations can be employed for NMR crystallography of host–guest complexes, providing important spatial information about the location of the guest molecules in the host cavities.

Deuteron quadrupole coupling in benzene: librational corrections using a temperature-dependent Einstein model, and summary. The symmetries of electric field gradients and conditions for eta = 1
Pekka Pyykko and Fatemeh Elmi

Librational corrections are added to previous single-crystal and polycrystalline measurements of the deuteron quadrupole coupling constant in benzene. The results are related to gas-phase and liquid-crystal measurements and to theoretical values. A temperature-dependent Einstein model is introduced for the purpose. The group-theoretical relations of the electric field-gradient (EFG) tensor to the nuclear site symmetry are discussed. The conditions for h = 1 for reorientational processes are identified.

Revealing complex formation in acetone–n-alkane mixtures by MAS PFG NMR diffusion measurement in nanoporous hosts
Moises Fernandez, André Pampel, Ryoji Takahashi, Satoshi Sato, Dieter Freude and Jörg Kärger

Magic-angle spinning pulsed field gradient nuclear magnetic resonance (MAS PFG NMR) was applied for selective self-diffusion measurements of acetone–n-alkane (C6 up to C9) mixtures in nanoporous silica gel. Two specimens of silica gel with mean pore sizes of about 4 and 10 nm are considered. In the smaller pores, the n-alkane diffusivities are by about one and the acetone diffusivities by about two orders of magnitude smaller than in the larger pores. In addition, the acetone diffusivities in the narrow-pore specimen exhibit a pronounced oscillation with increasing chain length of the solvent n-alkanes: the diffusivities of acetone dissolved in odd-carbon number n-alkanes exceed those of acetone dissolved in even-carbon number n-alkanes by about 50%! These findings reproduce the tendencies observed in previous macroscopic release studies (Phys. Chem. Chem. Phys. 2003, 5, 2476) and suggest the formation of acetone–n-alkane complex-like assemblages in the narrow-pore silica gel.

Direct DIVAM Experiment: A Spin Dynamics Analysis
Paul hazendonk, Philip Wormald, and Tony Montina

Domain selection in polymer NMR is limited to experiments specifically suited to each structural domain owing to its particular spin dynamics and relaxation properties. The DIVAM experiment can be tuned to select for signal from the domain of interest, making it possible to obtain signals specific to different domains using only one experiment. An early description of this sequence explains this tunability using a simple one-spin-relaxation model, thereby limiting the selection mechanism to incoherent processes and thus ignoring the coherent terms such as chemical shift anisotropy (CSA), dipolar coupling and offset terms. Experiments have shown that when the DIVAM sequence is applied directly to the nucleus of interest, referred to as direct DIVAM (DD), transient behavior is observed in the signal intensity on the sample spinning time scale. This indicates that the coherent terms are involved in the selection process; the exact role of these terms is explored in this work. SIMPSON simulations illustrate that the CSA and offset terms can play a dominant role in domain selection; however, the dipole term was relatively ineffective and required large values before substantial selection was predicted. Using a one-spin-relaxation model, which now includes a chemical shift evolution term, an analytical expression for the signal intensity was provided as a function of interpulse delay (?), excitation angle (?), relaxation time (T2), and offset frequency (??). These indicate that the selection behavior varies substantially with differing time scales and excitation angles. For small angles and long delay times DD behaves primarily as a relaxation filter, whereas for larger angles and short delay times the coherent terms take over dominated by the CSA interactions. The DD sequence can therefore be set to select on the basis of the transverse relaxation rate or the strength of the CSA interaction, depending on the excitation angle used.

A Technique for In Situ Monitoring of Crystallization from Solution by Solid-State 13C CPMAS NMR Spectroscopy
Hughes, C. E.; Harris, K. D. M.
JPCA(2008) ASAP.

We report a technique for carrying out in situ solid-state NMR studies of crystallization from solution, allowing the evolution of different solid state structures (polymorphs) produced during the crystallization process to be identified. The technique exploits selectivity in NMR properties (specifically, the efficiency of cross-polarization from 1H to 13C) between molecules in the solid and solution states, such that the first solid particles produced during the crystallization process are observed selectively, without detecting any signal from dissolved solute (or solvent) molecules. The application of the technique is demonstrated to reveal new insights concerning an isotope effect on the polymorphic outcome of crystallization of glycine from water. As revealed by this example, the in situ solid-state NMR approach reported here creates significant new opportunities for probing and understanding details of the evolution of solid state structures produced during crystallization from solution.

Wednesday, July 09, 2008

Physical Review B - Vol 77 No 21-24

Intrinsic origin of spin echoes in dipolar solids generated by strong pi pulses
Dale Li, Yanqun Dong, R. G. Ramos, J. D. Murray, K. MacLean, A. E. Dementyev, and S. E. Barrett
In spectroscopy, it is conventional to treat pulses much stronger than the linewidth as delta functions. In NMR, this assumption leads to the prediction that pi pulses do not refocus the dipolar coupling. However, NMR spin echo measurements in dipolar solids defy these conventional expectations when more than one pi pulse is used. Observed effects include a long tail in the CPMG echo train for short delays between pi pulses, an even-odd asymmetry in the echo amplitudes for long delays, an unusual fingerprint pattern for intermediate delays, and a strong sensitivity to pi-pulse phase. Experiments that set limits on possible extrinsic causes for the phenomena are reported. We find that the action of the system's internal Hamiltonian during any real pulse is sufficient to cause the effects. Exact numerical calculations, combined with average Hamiltonian theory, identify terms that are sensitive to parameters such as pulse phase, dipolar coupling, and system size. Visualization of the entire density matrix shows a unique flow of quantum coherence from nonobservable to observable channels when applying repeated pi pulses.

NMR study of the impurity induced ordered state in the doped Haldane chain compound SrNi1.93Mg0.07 V2O8
B. Pahari, K. Ghoshray, R. Sarkar, and A. Ghoshray
We report the effect of Mg (S=0) substitution at the Ni site of S=1 Haldane chain compound SrNi2V2O8 from magnetic susceptibility and 51V NMR studies. The magnetic-susceptibility results in presence of different external fields (H=0.1, 1.198, 3, and 7 T) in SrNi1.93Mg0.07V2O8 indicate a strongly field dependent nature of the antiferromagnetic (AF) ordering in this compound, with the ordering being suppressed at H3 T. A comparison of these results with those reported in isostructural compound PbNi1.76Mg0.24V2O8 indicates a stronger interchain exchange in the Sr compound. In order to probe the local magnetic properties of SrNi1.93Mg0.07V2O8 both in the AF ordered state and also when the ordering is suppressed, the 51V NMR studies were performed in presence of the magnetic fields of H=1.198 and 7.04 T. The resonance line shape in presence of both the fields remains almost unaltered in the temperature range of 50–300 K, as was reported in the pure system. However, the linewidth increased appreciably in the range 4T20 K in presence of both the fields. The temperature dependence of the first moment (M1) and the second moment (M2) of the NMR spectra indicates the effect of the development of three-dimensional (3D) correlations among the staggered spins below 20 K when H=1.198 T. Whereas, the behavior at H=7.04 T indicates the more dominance of the short-range correlations among the staggered spins within the chain in the same temperature range. The behavior of the spin-lattice relaxation rate and the dynamic susceptibility in the same temperature range further support these findings. Moreover it is seen that the intrachain exchange (J=106 K) and the spin gap (=25 K) for the uninterrupted chain remain almost unchanged after Mg substitution. Finally the present results suggest the coexistence of spin gapped phase of the uninterrupted chain together with the 3D correlated impurity induced staggered spins at H=1.198 T in SrNi1.93Mg0.07V2O8.

Physical Review Letters - Vol 100 No 25

Temperature Response of 129Xe Depolarization Transfer and Its Application for Ultrasensitive NMR Detection
Leif Schröder, Tyler Meldrum, Monica Smith, Thomas J. Lowery, David E. Wemmer and Alexander Pines
Trapping xenon in functionalized cryptophane cages makes the sensitivity of hyperpolarized (HP) 129Xe available for specific NMR detection of biomolecules. Here, we study the signal transfer onto a reservoir of unbound HP xenon by gating the residence time of the nuclei in the cage through the temperature-dependant exchange rate. Temperature changes larger than ~0.6 K are detectable as an altered reservoir signal. The temperature response is adjustable with lower concentrations of caged xenon providing more sensitivity at higher temperatures. Ultrasensitive detection of functionalized cryptophane at 310 K is demonstrated with a concentration of 10 nM, corresponding to a ~4000-fold sensitivity enhancement compared to conventional detection. This makes HPNMR capable of detecting such constructs in concentrations far below the detection limit of benchtop uv-visible light absorbance.

Thursday, July 03, 2008

J. Am. Chem. Soc., 130 (27), 8856–8864, 2008.

Asymmetric Insertion of Membrane Proteins in Lipid Bilayers by Solid-State NMR Paramagnetic Relaxation Enhancement: A Cell-Penetrating Peptide Example

Yongchao Su, Rajeswari Mani, and Mei Hong*

A novel solid-state NMR technique for identifying the asymmetric insertion depths of membrane proteins in lipid bilayers is introduced. By applying Mn2+ ions on the outer but not the inner leaflet of lipid bilayers, the sidedness of protein residues in the lipid bilayer can be determined through paramagnetic relaxation enhancement (PRE) effects. Protein-free lipid membranes with one-side Mn2+-bound surfaces exhibit significant residual 31P and lipid headgroup 13C intensities, in contrast to two-side Mn2+-bound membranes, where lipid headgroup signals are mostly suppressed. Applying this method to a cell-penetrating peptide, penetratin, we found that at low peptide concentrations, penetratin is distributed in both leaflets of the bilayer, in contrast to the prediction of the electroporation model, which predicts that penetratin binds to only the outer lipid leaflet at low peptide concentrations to cause an electric field that drives subsequent peptide translocation. The invalidation of the electroporation model suggests an alternative mechanism for intracellular import of penetratin, which may involve guanidinium−phosphate complexation between the peptide and the lipids.

J. Am. Chem. Soc., 130 (27), 8620–8632, 2008.

NMR Studies of Ultrafast Intramolecular Proton Tautomerism in Crystalline and Amorphous N,N′-Diphenyl-6-aminofulvene-1-aldimine: Solid-State, Kinetic Isotope, and Tunneling Effects

Juan Miguel Lopez del Amo, Uwe Langer, Verónica Torres, Gerd Buntkowsky, Hans-Martin Vieth, Marta Pérez-Torralba, Dionísia Sanz, Rosa María Claramunt, José Elguero, and Hans-Heinrich Limbach*

Using solid-state NMR spectroscopy, we have detected and characterized ultrafast intramolecular proton tautomerism in the N−H−N hydrogen bonds of solid N,N′-diphenyl-6-aminofulvene-1-aldimine (I) on the microsecond-to-picosecond time scale. 15N cross-polarization magic-angle-spinning NMR experiments using 1H decoupling performed on polycrystalline I-15N2 and the related compound N-phenyl-N′-(1,3,4-triazole)-6-aminofulvene-1-aldimine (II) provided information about the thermodynamics of the tautomeric processes. We found that II forms only a single tautomer but that the gas-phase degeneracy of the two tautomers of I is lifted by solid-state interactions. Rate constants, including H/D kinetic isotope effects (KIEs), on the microsecond-to-picosecond time scale were obtained by measuring and analyzing the longitudinal 15N and 2H relaxation times of I-15N2, I-15N2-d10, and I-15N2-d1 over a wide temperature range. In addition to the microcrystalline modification, a novel amorphous modification of I was found and studied. In this modification, proton transfer is much faster than in the crystalline form. For both modifications, we observed large H/D KIEs that were temperature-dependent at high temperatures and temperature-independent at low temperatures. These findings are interpreted in terms of a simple quasiclassical tunneling model proposed by Bell and modified by Limbach. We obtained evidence that a reorganization energy is necessary in order to compress the N−H−N hydrogen bond and achieve a molecular configuration in which the barrier for H transfer is reduced and tunneling or an over-barrier reaction can occur.

Solid-State Nuclear Magnetic Resonance - Vol 33 Issue 4

1H NMR signal broadening in spectra of alkane molecules adsorbed on MFI-type zeolites
Ekaterina E. Romanova, Cordula B. Krause, Alexander G. Stepanov, Ursula Wilczok, Wolfgang Schmidt, Jasper M. van Baten, Rajamani Krishna, André Pampel, Jörg Kärger and Dieter Freud
The anisotropic behavior of C1–C6 alkane molecules adsorbed in MFI zeolite was studied by 1H nuclear magnetic resonance (NMR) using single-pulse excitation, Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence, Hahn echo (HE) pulse sequence, and magic-angle spinning. The molecular order parameter was obtained by both static 2H NMR spectroscopy and molecular simulations. This yields an order parameter in the range of 0.28–0.42 for linear alkanes in MFI zeolite, whereas the parameter equals zero for FAU zeolite with a cubic symmetry. Thus, in the case of a zeolite with a non-cubic symmetry like MFI, the mobility of the molecules in one crystallite cannot fully average the dipolar interaction. As a consequence, transverse nuclear magnetization as revealed in the echo attenuation notably deviates from a mono-exponential decay. This information is of particular relevance for the performance of pulsed field gradient (PFG) NMR diffusion experiments, since the occurrence of non-exponential magnetization attenuation could be taken as an indication of the existence of different molecules or of molecules in different states of mobility.

Solid-state 17O NMR spectroscopy of a phospholemman transmembrane domain protein: Implications for the limits of detecting dilute 17O sites in biomaterials
Alan Wong, Andrew J. Beevers, Andreas Kukol, Ray Dupree and Mark E. Smith
The 17O-‘diluted’ glycine-14 sites in a phospholemman (PLM) transmembrane domain protein are characterized by solid-state 17O NMR spectroscopy. The PLM transmembrane domain is an -helical tetramer unit of four 28-residue peptides and is rigidly embedded in a bilayer where each -helix has an average tilt of 7.3° against the membrane normal. The PLM sample investigated here consists of a high lipid/peptide molar ratio (25:1) with one glycine residue in each helix enriched to <40% 17O; thus, this is a very dilute 17O-sample and is the most dilute 17O-membrane protein to date to be characterized by solid-state 17O NMR spectroscopy. Based on the spectral analysis of 17O magic angle spinning (MAS) at 14.1 and 18.8 T, the PLM transmembrane domain protein consists of multiple crystallographic gly14 sites, suggesting that the tetramer protein is an asymmetric unit with either C2- or C1-rotational symmetry along the bilayer normal.

Determination of 27Al–29Si connectivities in zeolites with 2D 27Al→29Si RAPT–CPMG–HETCOR NMR
Gordon J. Kennedy, Jerzy W. Wiench and Marek Pruski
The recently introduced concept of the combined use of rotor assisted population transfer (RAPT) and Carr–Purcell–Meiboom–Gill (CPMG) techniques to boost the sensitivity of cross polarization (CP) based NMR experiments is applied to a synthetic zeolite (ZSM-4). The sensitivity was increased by a factor of 4, which enabled acquisition of a high quality two-dimensional 27Al–29Si HETCOR (heteronuclear correlation) spectrum. By separating the resonances in two dimensions, through-space connectivities between spins were revealed and the effective resolution was improved in both dimensions, which allowed determination of the existing ambiguities in spectral assignments in this material. The spectra provided clear indication of random distribution of aluminum and silicon within the ZSM-4 network. Additionally, unexpected correlations were observed between different components of inhomogeneously broadened 29Si and 27Al lines, which are most likely due to differences in the second coordination sphere environments.

Probing amide bond nitrogens in solids using 14N NMR spectroscopy
Sasa Antonijevic and Nicholas Halpern-Manners
A novel two-dimensional nuclear magnetic resonance (NMR) experiment is proposed for indirect observation of 14N nuclei in various types of nitrogen-containing solids. In a method somewhat similar to the heteronuclear single-quantum correlation (HSQC) experiment widely used for protein structure determination in solutions, this technique correlates spin S=1/2 nuclei, e.g., 1H, 13C, with the 14N spin I=1 nucleus in solids. The present experiment, however, transfers coherence from neighboring 1H or 13C nuclei to 14N via a combination of J-couplings and residual dipolar splittings (RDS). Projections of the two-dimensional NMR spectra onto the 14N dimension yield powder patterns that reflect the 14N quadrupolar interaction, which can be used to study molecular structure and dynamics. Indirect detection of amide nitrogen-14 via 1H and 13C is shown experimentally on a model compound of N-acetyl-glycine.

Determination of the orientations for the 17O NMR tensors in a polycrystalline l-alanine hydrochloride
Kazuhiko Yamada, Tadashi Shimizu, Toshio Yamazaki and Shinobu Ohki
We report a solid-state 17O NMR study of the 17O electric-field-gradient (EFG) and chemical shielding (CS) tensors for the carboxyl oxygen in an l-alanine hydrochloride. Using [17O]– and [13C,17O]–l-alanine hydrochlorides, both the magnitudes and the orientations in the molecular frame of the 17O EFG and CS tensors could be determined by the analysis of the 17O magic-angle spinning (MAS) and stationary NMR spectra. For the carbonyl oxygen, the smallest EFG tensor component, VXX, and the largest EFG component, VZZ, roughly lies in the carboxyl molecular plane and the direction of VXX is parallel to the dipolar vector between 13C and 17O, that is, the direction of CO bond. The angles between the intermediate EFG component, VYY, and δ33 component, and between δ22 component and VZZ are found to be approximately 10° and 35°, respectively. We also present the results of the quantum chemical calculations for a theoretical hydrogen-bonding model, indicating that hydrogen-bonding strengths make it possible to vary both magnitudes and orientations of the carbonyl 17O EFG tensors in amino acid hydrochlorides.

Wednesday, July 02, 2008

J. Am. Chem. Soc., 130 (27), 8754–8761, 2008.

Enantiodiscrimination in Deuterium NMR Spectra of Flexible Chiral Molecules with Average Axial Symmetry Dissolved in Chiral Liquid Crystals: The Case of Tridioxyethylenetriphenylene

Philippe Lesot, Olivier Lafon, Herbert Zimmermann, and Zeev Luz

Flexible chiral molecules undergoing fast interconversion (on the NMR time scale) between different conformational enantiomers may yield “average” axial species with enantiotopically related sites. Contrary to the situation observed for rigid axial molecules, signals from these enantiotopic sites in NMR spectra recorded in chiral liquid-crystalline solvents can be resolved. In the present work, we studied the deuterium NMR spectra of tridioxyethylenetriphenylene (compound 4) statistically deuterated to 10% in the flexible side chains and dissolved in chiral and achiral lyotropic liquid crystals based on poly(γ-benzylglutamate). The fast chair−chair flipping of the side chains in 4 on average renders the molecule axially symmetric (D3h) with pairs of enantiotopic ethylene deuterons. These deuterons exhibit unusually large enantiodiscrimination. To explain this observation, we first describe how the average symmetry of flexible molecules can be derived from the symmetry of the “frozen” conformers and the nature of the averaging process. The procedure is then applied to 4 and used to analyze the NMR results. It is shown that the large enantiodiscrimination in the present case reflects a large difference in the orientational ordering of the conformational enantiomers participating in the interconversion processes as well as a large geometrical factor due to the special shape of the dioxyethylene side groups. 1H and 13C NMR spectra of 4 in the same lyotropic liquid crystalline solvent are analyzed to determine its ordering characteristics. Several related cases are also discussed.