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*‡

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*¶

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*
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∥

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*

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*‡

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

Laura Polito, Miriam Colombo, Diego Monti, Sergio Melato, Enrico Caneva,* and Davide Prosperi*

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*†

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.

Thursday, September 11, 2008

J. Am. Chem. Soc., 130 (36), 12007–12020, 2008.

Changes in Dynamics of SRE-RNA on Binding to the VTS1p-SAM Domain Studied by 13C NMR Relaxation
Florian C. Oberstrass, Frédéric H.-T. Allain, and Sapna Ravindranathan

RNA recognition by proteins is often accompanied by significant changes in RNA dynamics in addition to conformational changes. However, there are very few studies which characterize the changes in molecular motions in RNA on protein binding. We present a quantitative 13C NMR relaxation study of the changes in RNA dynamics in the pico−nanosecond time scale and micro−millisecond time scale resulting from interaction of the stem−loop SRE-RNA with the VTS1p-SAM domain. 13C relaxation rates of the protonated carbons of the nucleotide base and anomeric carbons have been analyzed by employing the model-free formalism, for a fully 13C/15N-labeled sample of the SRE-RNA in the free and protein-bound forms. In the free RNA, the nature of molecular motions are found to be distinctly different in the stem and the loop region. On binding to the protein, the nature of motions becomes more homogeneous throughout the RNA, with many residues showing increased flexibility at the aromatic carbon sites, while the anomeric carbon sites become more rigid. Surprisingly, we also observe indications of a slow collective motion of the RNA in the binding pocket of the protein. The observation of increased motions on binding is interesting in the context of growing evidence that binding does not always lead to motional restrictions and the resulting entropy gain could favor the free energy of association.

J. Am. Chem. Soc., 130 (36), 11892–11900, 2008.

13C-Labeled N-Acetyl-neuraminic Acid in Aqueous Solution: Detection and Quantification of Acyclic Keto, Keto Hydrate, and Enol Forms by 13C NMR Spectroscopy
Thomas Klepach, Ian Carmichael, and Anthony S. Serianni

Aqueous solutions of N-acetyl-neuraminic acid (Neu5Ac, 1) labeled with 13C at C1, C2, and/or C3 were analyzed by 13C NMR spectroscopy to detect and quantitfy the acyclic forms (keto, keto hydrate, enol) present at varying pHs. In addition to pyranoses, solutions contained the keto form, based on the detection of C2 signals at ~198 ppm (~0.7% at pH 2). Spectra of [2-13C] and [3-13C] isotopomers contained signals arising from labeled carbons at ~143 and ~120 ppm, respectively, which were attributed to enol forms. Solution studies of [1,2,3-13C3]1 substantiated the presence of enol (~0.5% at pH 2). Enol was not detected at pH > 6.0. A C2 signal observed at ~94 ppm was identified as C2 of the keto hydrate (~1.9% at pH 2), based partly on its abundance as a function of solution pH. Density functional theory (DFT) calculations were used to study the effect of enol and hydrate structure on JCH and JCC values involving C2 and C3 of these forms. Solvated DFT calculations showed that 2JC2,H3 in cis and trans enols have similar magnitudes but opposite signs, making this J-coupling potentially useful to distinguish enol configurations. Solvent deuterium exchange studies of 1 showed rapid incorporation of 2H from 2H2O at H3axial in the pyranoses at p2H 8.0, followed by slower exchange at H3equatorial. The acyclic keto form, which presumably participates in this reaction, must assume a pseudo-cyclic conformation in solution in order to account for the exchange selectivity. Weak 13C signals arising from labeled species were also observed consistently and reproducibly in aqueous solutions of 13C-labeled 1, possibly arising from products of lactonization or intermolecular esterification.

Tuesday, September 09, 2008

Journal of Magnetic Resonance - Vol 194 Issue 1

Exploring the limits of broadband excitation and inversion: II. Rf-power optimized pulses
Kyryl Kobzar, Thomas E. Skinner, Navin Khaneja, Steffen J. Glaser and Burkhard Luy
In [K. Kobzar, T.E. Skinner, N. Khaneja, S.J. Glaser, B. Luy, Exploring the limits of broadband excitation and inversion, J. Magn. Reson. 170 (2004) 236–243], optimal control theory was employed in a systematic study to establish physical limits for the minimum rf-amplitudes required in broadband excitation and inversion pulses. In a number of cases, however, experimental schemes are not limited by rf-amplitudes, but by the overall rf-power applied to a sample. We therefore conducted a second systematic study of excitation and inversion pulses of varying pulse durations with respect to bandwidth and rf-tolerances, but this time using a modified algorithm involving restricted rf-power. The resulting pulses display a variety of pulse shapes with highly modulated rf-amplitudes and generally show better performance than corresponding pulses with identical pulse length and rf-power, but limited rf-amplitude. A detailed description of pulse shapes and their performance is given for the so-called power-BEBOP and power-BIBOP pulses.

Improved J-compensated sequences based on short composite pulses
A.M. Torres, W.A. Bubb, D.J. Philp and P.W. Kuchel
Efficient J-compensated sequences that are shorter in duration and use less RF pulses have been created from short but very efficient composite 90° RF pulses. The improved J-compensation transforms in-phase into antiphase magnetization and can be incorporated in any pulse sequence that involves evolution of heteronuclear J-couplings. The compensated sequences were tested and incorporated into an HMBC sequence. J-compensated experiments referred to as HMBC-J45 + 90A and HMBC-J45 + 90B, were found to be effective over a wide range of J values.

Clean HMBC: Suppression of strong-coupling induced artifacts in HMBC spectra
Peter Würtz, Perttu Permi, Niels Chr. Nielsen and Ole W. Sørensen
A new experiment, clean HMBC, is introduced for suppression of strong-coupling induced artifacts in HMBC spectra. The culprits of these artifacts are an inherent shortcoming of low-pass J filters in the presence of strong coupling and the 1H π pulse in the middle of the evolution period aimed at suppressing evolution under heteronuclear J couplings and 1H chemical shifts. A π pulse causes coherence transfer in strongly coupled spin systems and, as is well known in e.g., homonuclear J spectra, this leads to peaks that would not be there in the absence of strong coupling. Similar artifacts occur in HMBC spectra, but they have apparently been overlooked, presumably because they have been assigned to inefficiency of low-pass J filters or not noticed because of a coarse digital resolution in the spectra. Clean HMBC is the HMBC technique of choice for molecules notorious for strong coupling among protons, such as carbohydrates, and the new technique is demonstrated on D-mannose. Finally, a fundamental difference between HMBC and H2BC explains why strong-coupling artifacts are much less of a problem in the latter type of spectra.

A structure refinement strategy for NMR crystallography: An improved crystal structure of silica-ZSM-12 zeolite from 29Si chemical shift tensors
Darren H. Brouwer
A strategy for performing crystal structure refinements with NMR chemical shift tensors is described in detail and implemented for the zeolite silica-ZSM-12 (framework type code MTW). The 29Si chemical shift tensors were determined from a slow magic-angle spinning spectrum obtained at an ultrahigh magnetic field of 21.1 T. The Si and O atomic coordinate parameters were optimized to give the best agreement between experimentally measured and ab initio calculated principal components of the 29Si chemical shift tensors, with the closest Si–O, O–O, and Si–Si distances restrained to correspond with the distributions of the distances found in a set of single-crystal X-ray diffraction (XRD) structures of high-silica zeolites. An improved structure for the silica-ZSM-12 zeolite, compared to a prior structure derived from powder XRD data, is obtained in which the agreement between the experimental and calculated 29Si chemical shift tensors is dramatically improved, the Si–O, O–O, and Si–Si distances correspond to the expected distributions, while the calculated powder XRD pattern remains in good agreement with the experimental powder XRD data. It is anticipated that this “NMR crystallography” structure refinement strategy will be an important tool for the accurate structure determination of materials that are difficult to fully characterize by traditional diffraction methods.

Monday, September 08, 2008

ASAP J. Am. Chem. Soc., ASAP Article, 10.1021/ja8047317

Temperature-Dependent Interconversion of Phosphoramidite−Cu Complexes Detected by Combined Diffusion Studies, 31P NMR, and Low-Temperature NMR Spectroscopy
Katrin Schober, Hongxia Zhang, and Ruth M. Gschwind

For copper-catalyzed enantioselective conjugate additions, knowledge about the precatalytic and catalytic complexes has not yet been sufficiently developed to understand the strong influence of different temperatures on these famous reactions. Therefore, NMR experiments with four Cu(I) salts and two phosphoramidite ligands have been performed to elucidate the temperature dependence and the low-temperature structures of these copper complexes. The existence of the precatalytic binuclear complex with a mixed trigonal/tetrahedral coordination on copper is for the first time proven with direct NMR spectroscopic methods. Below 200 K, intermolecular interactions between free ligands and [Cu2X2L3] complexes induce binuclear [Cu2X2L4] complexes similar to the crystal structures. By combining diffusion experiments and 31P integrals at different temperatures, it is for the first time possible to follow the formation of stoichiometrically different complexes, even under experimental conditions in which the 31P signals of the complexes are spectroscopically not resolved due to exchange processes. This allows a first correlation between the complex species observed and the synthetic conditions reported. Furthermore, different preferences to build homo- or heterochiral complexes are detected for binaphthol and biphenol phosphoramidite complexes.

Friday, September 05, 2008

J. Am. Chem. Soc., 130 (34), 11266–11267, 2008.

On the Origin of NMR Dipolar Waves in Transient Helical Elements of Partially Folded Proteins

Malene Ringkjøbing Jensen and Martin Blackledge

The presence of dipolar coupling waves within helical elements of proteins implies an effective tilt of the main axis of the helical element relative to the magnetic field. Here, we investigate the origin of dipolar waves observed in helical elements of partially folded proteins. We find that the dipolar waves result from an effective tilt of the helix relative to the alignment axis that is determined by the directionality of the unfolded chains projected from the helix caps. The amplitude and phase of the dipolar wave depend in a predictable way on helix length, providing direct insight into helix stability, nucleation, and fraying in partially folded proteins.