Peptide 17O chemical shielding and electric field gradient tensors

Complete (17)O chemical shielding (CS) and quadrupole coupling (QC) tensors and their molecular orientations were determined for the central residues in two tripeptides Gly-Gly-Val (GGV) and Ala-Gly-Gly (AGG) by single-crystal NMR methods. Tensor orientations in the two peptides are very similar, however, principal components are different. The most shielded CS and smallest magnitude QC components are normal to the peptide plane, while the most deshielded CS and largest QC components are in the peptide plane either at an angle of 17 degrees (CS) or perpendicular (QC) to the C=O bond. Comparisons of principal components from experiment and DFT calculations indicate that the smaller shielding tensor span in GGV (549 ppm) compared to AGG (606 ppm) is likely due to two factors: a shorter "direct" H-bond distance to the peptide carbonyl oxygen and an "indirect" H bond of the peptide NH to a carboxylate rather than a carbonyl. We anticipate that (17)O NMR should be generally useful for probing H-bonding and local electrostatic interactions in proteins and polypeptides. Using the single-crystal data as an accurate reference, we show that a useful subset of the NMR parameters, QC and CS principal components and their relative orientation, can be obtained with reasonable accuracy from a very high-field (21.2 T), stationary sample powder spectrum.     

15N Chemical shielding in glycyl tripeptides: measurement by solid-state NMR and correlation with X-ray structure

15N chemical shielding parameters are reported for central glycyl residues in crystallographically characterized tripeptides with alpha-helix, beta-strand, polyglycine II (3(1)-helix), and extended structures. Accurate values of the shielding components (2-5 ppm) are determined from MAS and stationary spectra of peptides containing [2-(13)C,(15)N]Gly. Two dipolar couplings, (1)H-(15)N and (13)C(alpha)-(15)N, are used to examine (15)N shielding tensor orientations in the molecular frame and the results indicate that the delta(11), delta(33) plane of the shielding tensor is not coincident with the peptide plane. The observed isotropic shifts, which vary over a range of 13 ppm, depend on hydrogen bonding (direct and indirect) and local conformation. Tensor spans, delta(span) = delta(11) - delta(33), and their deviations from axial symmetry, delta(dev) = delta(22) - delta(33), vary over a larger range and are grouped according to 2 degrees structure. Augmented by previously reported (13)C(alpha) shielding parameters, a prediction scheme for the 2 degrees structure of glycyl residues in proteins based on shielding parameters is proposed.     

Experimental and computational characterization of the 17O quadrupole coupling and magnetic shielding tensors for p-nitrobenzaldehyde and formaldehyde

We have used solid-state 17O NMR experiments to determine the 17O quadrupole coupling (QC) tensor and chemical shift (CS) tensor for the carbonyl oxygen in p-nitro-[1-(17)O]benzaldehyde. Analyses of solid-state 17O NMR spectra obtained at 11.75 and 21.15 T under both magic-angle spinning (MAS) and stationary conditions yield the magnitude and relative orientation of these two tensors: CQ = 10.7 +/- 0.2 MHz, etaQ = 0.45 +/- 0.10, delta11 = 1050 +/- 10, delta22 = 620 +/- 10, delta33 = -35 +/- 10, alpha = 90 +/- 10, beta = 90 +/- 2, gamma = 90 +/- 10 degrees. The principal component of the 17O CS tensor with the most shielding, delta33, is perpendicular to the H-C=O plane, and the tensor component with the least shielding, delta11, lies along the C=O bond. For the 17O QC tensor, the largest (chi(zz)) and smallest (chi(xx)) components are both in the H-C=O plane being perpendicular and parallel to the C=O bond, respectively. This study represents the first time that these two fundamental 17O NMR tensors have been simultaneously determined for the carbonyl oxygen of an aldehyde functional group by solid-state 17O NMR. The reported experimental solid-state 17O NMR results provide the first set of reliable data to allow evaluation of the effect of electron correlation on individual CS tensor components. We found that the electron correlation effect exhibits significant influence on 17O chemical shielding in directions within the H-C=O plane. We have also carefully re-examined the existing experimental data on the 17O spin-rotation tensor for formaldehyde and proposed a new set of best "experimental" 17O chemical shielding tensor components: sigma11 = -1139 +/- 80, sigma22 = -533 +/- 80, sigma33 = 431 +/- 5, and sigma(iso) = -414 +/- 60 ppm. Using this new set of data, we have evaluated the accuracy of quantum chemical calculations of the 17O CS tensors for formaldehyde at the Hartree-Fock (HF), density-functional theory (DFT), M?ller-Plesset second-order perturbation (MP2), and coupled-cluster singles and doubles (CCSD) levels of theory. The conclusion is that, while results from HF and DFT tend to underestimate the electron correlation effect, the MP2 method overestimates its contribution. The CCSD results are in good agreement with the experimental data.     

A solid-state nitrogen-15 NMR and ab initio study of nitrobenzenes

Insight into the unexpectedly small range of isotropic nitrogen chemical shifts in nitrobenzene derivatives is gained through measurements of the chemical shift (CS) tensor by solid-state NMR experiments and ab initio molecular orbital (MO) and density functional theory (DFT) calculations. The principal components, delta(ii), of the (15)N CS tensors have been measured for nitrobenzene, 4-nitroaniline, 4-nitrotoluene, 4-nitroanisole, 4-nitroacetophenone, nitromesitylene, and 2,4,6-tri-tert-butylnitrobenzene. No obvious correlations of the delta(ii) values with traditional reactivity parameters were observed. The CS tensor components change significantly for the para-substituted nitrobenzenes, but these variations nearly cancel to yield isotropic shifts that fall in a range of only 3 ppm. Ab initio calculations of the delta(ii) values at the HF level are in poor agreement with the experimental values, whereas MP2 calculations and DFT calculations employing the B3LYP functional are in better agreement with experiment. The calculated (B3LYP/6-311G) delta(ii) values follow a trend in which delta(11) and delta(33) increase while delta(22) decreases with the accepted electron withdrawing ability of the para substituent. These changes tend to cancel yielding a variation in delta(iso) of only 4 ppm. These calculations indicate that the CS tensor has the same orientation as the carbon CS tensor in the isoelectronic benzoate anion: delta(11) bisects the O-N-O angle, delta(33) is perpendicular to the NO(2) plane, and delta(22) is in the NO(2) plane and perpendicular to delta(11).     

Solid-state nitrogen-15 NMR and quantum chemical study of N,N-dimethylaniline derivatives

In this study the components of the nitrogen chemical shift (CS) tensor are examined for a series of para substituted N,N-dimethylaniline derivatives. This is done through measurement of the 15N NMR spectra of powder samples and through quantum chemical calculations on the isolated molecules. Experiments and calculations show that the isotropic CS, delta(iso), decreases with increasing electron donating ability of the para substituent, in agreement with previous solution studies. More importantly, this study shows that this decrease in the isotropic (solution) CS is due to decreasing values of the CS tensor component delta(11) and component delta(33). The component delta(22) is essentially invariant to the electron donating/withdrawing ability of the para substituent. Through Ramsey's theory of nuclear magnetic shielding, it can be seen that the variation in delta(11) and delta(33), and hence delta(iso), is due to changes in the n-pi* and the sigma-pi* energy gaps in N,N-dimethylaniline. This, in turn, is a result of the change in the energy of the pi* molecular orbital with change in the pi-electron donating ability of the para substituent. The effects of nitrogen inversion on the components of the nitrogen CS tensor components are also discussed. This study also shows the feasibility of performing 15N cross-polarization experiments on nonspinning powder samples at natural isotopic abundance.     

Residue-specific 13C' CSA tensor principal components for ubiquitin: correlation between tensor components and hydrogen bonding

The residue-specific 13C' CSA tensor principal components, sigma(11), sigma(22), sigma(33), and the tensor orientation defined by the rotation angles beta and gamma have been determined by solution NMR for uniformly labeled ubiquitin partially aligned in four different media. Spurious chemical shift deviations due to solvent effects were corrected with an offset calculated by linear regression of the residual dipolar couplings and chemical shifts at increasing alignment strengths. Analysis of this effect revealed no obvious correlation to solvent exposure. Data obtained in solution from a protein offer a better sampling of 13C' CSA for different amino acid types in a complex heterogeneous environment, thereby allowing for the evaluation of structural variables that would be challenging to achieve by other methods. The 13C' CSA principal components cluster about the average values previously determined, and experimental correlations observed between sigma(11), sigma(22) tensorial components and C'O...H(N) hydrogen bonding are discussed. The inverse association of sigma(11) and sigma(22) exemplify the calculated and solid-state NMR observed effect on the tensor components by hydrogen bonding. We also show that 13C' CSA tensors are sensitive to hydrogen-bond length but not hydrogen-bond angle. This differentiation was previously unavailable. Similarly, hydrogen bonding to the conjugated NH of the same peptide plane has no detectable effect. Importantly, the observed weak correlations signify the presence of confounding influences such as nearest-neighbor effects, side-chain conformation, electrostatics, and other long-range factors to the 13C' CSA tensor. These analyses hold future potential for exploration provided that more accurate data from a larger number of proteins and alignments become available.     

Acetylenic carbon-13 chemical shift tensors for diphenylacetylene and (eta2-Diphenylacetylene)Pt(PPh3)2: a solid-state NMR and theoretical study

The structure of (eta2-diphenylacetylene)Pt(PPh3) (2), as well as those of its dichloromethane and benzene solvates, is determined via X-ray crystallography. An investigation of the chemical shift (CS) tensors of the 13C-labeled carbons in Ph13C13CPh and (eta2-Ph13C13CPh)Pt(PPh3)2.(C6H6) is carried out via analysis of 13C NMR spectra from stationary solid samples. The principal components of the CS tensors as well as their orientations with respect to the 13C,13C internuclear vector are determined. DFT calculations of these CS tensors are in close agreement with the experimental values. For diphenylacetylene (tolane), the orientations and principal-component magnitudes of the alkynyl carbon CS tensors are comparable to those for other alkynyl carbons, although the CS tensor is not axially symmetric in this case. Coordination to platinum causes a change in the CS tensor orientation and a net increase in the isotropic chemical shift, resulting from a significant increase in two principal components (delta11 and delta33) while the third (delta22) decreases only slightly. The measured carbon CS tensors in the platinum complex bear a striking similarity to those of the alkenyl carbons in trans-Ph(H)C=C(H)Ph, and a short theoretical discussion of these observations is presented.     

X-ray, 31P CP/MAS,and single-crystal NMR studies,and 31P DFT GIAO calculations of inclusion complexes of bis[6-O,6-O'-(1,2:3,4- diisopropylidene-alpha-D-galactopyranosyl)thiophosphoryl] disulfide: the importance of C-H...S=P contacts in the solid state

Bis[6-O,6-O'-(1,2:3,4-diisopropylidene-alpha-D-galactopyranosyl) thiophosphoryl] disulfide shows a strong tendency to form inclusion compounds. The crystal and molecular structure of eight different solvates was established by X-ray analysis. The results indicate three different types of disulfide arrangements in the crystal lattice. By means of 31P CP/MAS NMR experiments the principal values delta 11, delta 22, and delta 33 of the 31P chemical shift tensor were obtained for each form. The orientation of its principal axes with respect to a molecular frame was investigated by means of 31P CP and single-crystal NMR for the complex with propan-2-ol. The principal axis 1 of both chemically equivalent phosphorus atoms is nearly parallel to the P-S bond and the principal axis 3 is very close to the P=S bond. DFT GIAO calculations of the model compound (EtO)2(S)P1SSP2(S)-(OEt)2 allowed assignment of the experimental chemical shift curves to the magnetically nonequivalent atoms P1 and P2. The maximum difference between calculated angles [symbol: see text] i-P-X)calcd and experimental angles [symbol: see text] i-P-X)exptl is 8.3 degrees and the rms distance 3.8 degrees (i = principal axes 1, 2, 3; X = S, -S-, -O1-, -O2-). The influence of C-H...S weak hydrogen bonding on phosphorus shielding was tested theoretically (31P DFT GIAO) employing the dimethoxythiophosphoryl disulfide.CH4 complex as a model compound. The sensitivity of 31P delta ii parameters to intermolecular forces is demonstrated.     

Solid-state NMR and quantum chemical investigations of 13Calpha shielding tensor magnitudes and orientations in peptides: determining phi and psi torsion angles

We report the experimental determination of the (13)C(alpha) chemical shift tensors of Ala, Leu, Val, Phe, and Met in a number of polycrystalline peptides with known X-ray or de novo solid-state NMR structures. The 700 Hz dipolar coupling between (13)C(alpha) and its directly bonded (14)N permits extraction of both the magnitude and the orientation of the shielding tensor with respect to the C(alpha)-N bond vector. The chemical shift anisotropy (CSA) is recoupled under magic-angle spinning using the SUPER technique (Liu et al., J. Magn. Reson. 2002, 155, 15-28) to yield quasi-static chemical shift powder patterns. The tensor orientation is extracted from the (13)C-(14)N dipolar modulation of the powder line shapes. The magnitudes and orientations of the experimental (13)C(alpha) chemical shift tensors are found to be in good accord with those predicted from quantum chemical calculations. Using these principal values and orientations, supplemented with previously measured tensor orientations from (13)C-(15)N and (13)C-(1)H dipolar experiments, we are able to predict the (phi, psi, chi(1)) angles of Ala and Val within 5.8 degrees of the crystallographic values. This opens up a route to accurate determination of torsion angles in proteins based on shielding tensor magnitude and orientation information using labeled compounds, as well as the structure elucidation of noncrystalline organic compounds using natural abundance (13)C NMR techniques.     

13C and (15)N chemical shift tensors in adenosine,guanosine dihydrate, 2'-deoxythymidine,and cytidine

The (13)C and (15)N chemical shift tensor principal values for adenosine, guanosine dihydrate, 2'-deoxythymidine, and cytidine are measured on natural abundance samples. Additionally, the (13)C and (15)N chemical shielding tensor principal values in these four nucleosides are calculated utilizing various theoretical approaches. Embedded ion method (EIM) calculations improve significantly the precision with which the experimental principal values are reproduced over calculations on the corresponding isolated molecules with proton-optimized geometries. The (13)C and (15)N chemical shift tensor orientations are reliably assigned in the molecular frames of the nucleosides based upon chemical shielding tensor calculations employing the EIM. The differences between principal values obtained in EIM calculations and in calculations on isolated molecules with proton positions optimized inside a point charge array are used to estimate the contributions to chemical shielding arising from intermolecular interactions. Moreover, the (13)C and (15)N chemical shift tensor orientations and principal values correlate with the molecular structure and the crystallographic environment for the nucleosides and agree with data obtained previously for related compounds. The effects of variations in certain EIM parameters on the accuracy of the shielding tensor calculations are investigated.     

Solid-state NMR and calorimetry of structural waters in helical peptides

The peptide hydrates Gly-Gly-Val x 2H(2)O (GGV) and Gly-Ala-Leu x 3H(2)O (GAL) are known to adopt alpha-helical configurations containing waters of hydration in which each water is H-bonded to three or four peptide groups. Herein we report a thermodynamic and solid-state NMR ((2)H and (17)O) study of these peptides. From TGA and DSC, the average enthalpy per H-bond is 15 kJ/mol. The dynamics and average orientation of the hydrate are studied by powder and single-crystal (2)H NMR. Whereas waters that are shown by the X-ray structure to be coordinated by four hydrogen bonds do not yield observable (2)H NMR signals at room temperature, two of the three triply coordinated waters yield residual (2)H quadrupole coupling tensors characteristic of rapid 180 degrees flip motions and the orientation of the residual tensor is that expected from the X-ray structure-derived H-bonding pattern. At -65 degrees C, the flip motions of triply coordinated water in GGV slow into the (2)H NMR intermediate exchange regime whereas the tetrahedrally coordinated water approaches the slow-exchange limit and yields an observable NMR signal. Extensive isotope exchange between water vapor and crystalline GGV establishes the presence of additional hydrate dynamics and solid-state proton transfer along a chain of water-bridged protonated alpha-amino groups.     

Correlations between 31P chemical shift anisotropy and molecular structure in polycrystalline O,O'-dialkyldithiophosphate zinc(II) and nickel(II) complexes: 31P CP/MAS NMR and ab initio quantum mechanical calculation studies

Different potassium salts and zinc(II) and nickel(II) O,O'-dialkyldithiophosphate complexes were studied by solid-state 31P CP/MAS and static NMR and ab initio quantum mechanical calculations. Spectra were obtained at different spinning frequencies, and the intensities of the spinning sidebands were used to estimate the chemical shift anisotropy parameters. Useful correlations between the shapes of the 31P chemical shift tensor and the type of ligand were found: terminal ligands have negative values of the skew kappa, while bridging and ionic ligands have positive values for this parameter. The experimental results were compared with known X-ray diffraction structures for some of these complexes as well as with ab initio quantum mechanical calculations, and a useful correlation between the delta22 component of the 31P chemical shift tensor and the S-P-S bond angle in the O,O'-dialkyldithiophoshate zinc(II) and nickel(II) complexes was found: delta22 increases more than 50 ppm with the increase of S-P-S bond angle from ca. 100 degrees to 120 degrees , while the other two principal values of the tensor, delta11 and delta33, are almost conserved. This eventually leads to the change in sign for kappa in the bridging type of ligand, which generally has a larger S-P-S bond angle than the terminally bound O,O'-dialkyldithiophosphate group forming chelating four-membered P(ss)Me heterocycles.     

Determination of calpha chemical shift tensor orientation in peptides by dipolar-modulated chemical shift recoupling NMR spectroscopy

We present a new method for determining the orientation of chemical shift tensors in polycrystalline solids with site resolution and demonstrate its application to the determination of the Calpha chemical shift tensor orientation in a model peptide with beta-sheet torsion angles. The tensor orientation is obtained under magic angle spinning by modulating a recoupled chemical shift anisotropy (CSA) pattern with various dipolar couplings. These dipolar-modulated chemical shift patterns constitute the indirect dimension of a 2D spectrum and are resolved according to the isotropic chemical shifts of different sites in the direct dimension. These dipolar-modulated CSA spectra are equivalent to the projection of a 2D static separated-local-field spectrum onto its chemical shift dimension, except that its dipolar dimension is multiplied with a modulation function. Both (13)C-(1)H and (13)C-(15)N dipolar couplings can modulate the CSA spectra of the Calpha site in an amino acid and yield the relative orientations of the chemical shift principal axes to the C-H and C-N bonds. We demonstrate the C-H and C-N modulated CSA experiments on methylmalonic acid and N-tBoc-glycine, respectively. The MAS results agree well with the results of the 2D separated-local-field spectra, thus confirming the validity of this MAS dipolar-modulation approach. Using this technique, we measured the Val Calpha tensor orientation in N-acetylvaline, which has beta-sheet torsion angles. The sigma(11) axis is oriented at 158 degrees (or 22 degrees) from the C-H bond, while the sigma(22) axis is tilted by 144 degrees (or 36 degrees) from the C-N bond. Both the orientations and the magnitude of this chemical shift tensor are in excellent agreement with quantum chemical calculations.     

Solid-state 17O NMR study of the electric-field-gradient and chemical shielding tensors in polycrystalline amino acids

We have presented a systematic experimental investigation of carboxyl oxygen electric-field-gradient (EFG) and chemical shielding (CS) tensors in crystalline amino acids. Three 17O-enriched amino acids were prepared: L-aspartic acid, L-threonine, and L-tyrosine. Analysis of two-dimensional 17O multiple-quantum magic-angle spinning (MQMAS), MAS, and stationary NMR spectra yields the 17O CS, EFG tensors and the relative orientations between the two tensors for the amino acids. The values of quadrupolar coupling constants (CQ) are found to be in the range of 6.70-7.60 MHz. The values of deltaiso lie in the range of 268-292 ppm, while those of the delta11 and delta22 components vary from 428 to 502 ppm, and from 303 to 338 ppm, respectively. There is a significant correlation between the magnitudes of delta22 components and C--O bond lengths. Since C--O bond length may be related to hydrogen-bonding environments, solid-state 17O NMR has significant potential to provide insights into important aspects of hydrogen bonds in biological systems.     

Phosphorus chemical shift tensors of phosphido ligands in ruthenium carbonyl compounds: (31)P NMR spectroscopy of single-crystal and powder samples and ab initio calculations

The phosphorus chemical shift (CS) tensors of several ruthenium carbonyl compounds containing a phosphido ligand, micro), bridging a Ru [bond] Ru bond were characterized by solid-state (31)P NMR spectroscopy. As well, an analogous osmium compound was examined. The structures of most of the clusters investigated have approximate local C(2v) symmetry about the phosphorus atom. Compared to the "isolated" PH(2)(-) anion, the phosphorus nucleus of a bridging phosphido ligand exhibits considerable deshielding. The phosphorus CS tensors of most of the compounds have spans ranging from 230 to 350 ppm and skews of approximately zero. Single-crystal NMR was used to investigate the orientation of the phosphorus CS tensors for two of the compounds, Ru(2)(CO)(6)(mu(2)-C [triple bond] C [bond] Ph)(mu(2)-PPh(2)) and Ru(3)(CO)(9)(mu(2)-H)(mu(2)-PPh(2)). The intermediate component of the phosphorus CS tensor, delta(22), lies along the local C(2) axis in both compounds. The least shielded component, delta(11), lies perpendicular to the Ru [bond] P [bond] Ru plane while the most shielded component, delta(33), lies perpendicular to the C [bond]P [bond] C plane. The orientation of the phosphorus CS tensor for a third compound, Ru(2)(CO)(6)(mu(2)-PPh(2))(2), was investigated by the dipolar-chemical shift NMR technique and was found to be analogous, suggesting it to be the same in all compounds. Ab initio calculations of phosphorus magnetic shielding tensors have been carried out and reproduce the orientations found experimentally. The orientation of the CS tensor has been rationalized using simple frontier MO theory. Splittings due to (99,101)Ru [bond] (31)P spin-spin coupling have been observed for several of the complexes. A rare example of (189)Os [bond] (31)P spin-spin splittings is observed in the (31)P MAS NMR spectrum of the osmium cluster, where (1)J((189)Os, (31)P) is 367 Hz. For this complex, the (189)Os nuclear quadrupolar coupling constant is on the order of several hundred megahertz.     

Carbon chemical shift tensor components in quinolines and quinoline N-oxides

Chemical shift calculations are carried out for the quinoline carbons in 1,8-bis(2-isopropyl-4-quinolyl)naphthalene, 2-isopropylquinoline, amodiaquine, chloroquine, and quinine and the N-oxide of each compound. Ab initio calculations of the isotropic shielding values are in agreement with experimental chemical shifts. The calculations indicate that changes to the principal components of the shielding tensor upon N-oxidation are similar for each compound. Carbons 2, 4, 8, and 10 are largely shielded in each case as the nitrogen is oxidized. For C2, C4, and C10, this shielding is due to a large change in sigma11 and/or sigma22, indicating a change in pi-electron density. For C8, the large shielding change is due mainly to a change in sigma33, indicating a change in sigma-electron density. Upon examination and comparison of the calculated 13C shielding tensor components in the antimalarial drugs versus those in unsubstituted quinolines, it is found that amodiaquine and chloroquine have increased pi-electron density in the ring containing the amino side chain and quinine has increased pi-electron density in the opposite ring, containing the methoxy substituent.     

Solid-state (63)Cu and (65)Cu NMR spectroscopy of inorganic and organometallic copper(I) complexes

Solid-state 63Cu and 65Cu NMR experiments have been conducted on a series of inorganic and organometallic copper(I) complexes possessing a variety of spherically asymmetric two-, three-, and four-coordinate Cu coordination environments. Variations in structure and symmetry, and corresponding changes in the electric field gradient (EFG) tensors, yield 63/65Cu quadrupolar coupling constants (CQ) ranging from 22.0 to 71.0 MHz for spherically asymmetric Cu sites. These large quadrupolar interactions result in spectra featuring quadrupolar-dominated central transition patterns with breadths ranging from 760 kHz to 6.7 MHz. Accordingly, Hahn-echo and/or QCPMG pulse sequences were applied in a frequency-stepped manner to rapidly acquire high S/N powder patterns. Significant copper chemical shielding anisotropies (CSAs) are also observed in some cases, ranging from 1000 to 1500 ppm. 31P CP/MAS NMR spectra for complexes featuring 63/65Cu-31P spin pairs exhibit residual dipolar coupling and are simulated to determine both the sign of CQ and the EFG tensor orientations relative to the Cu-P bond axes. X-ray crystallographic data and theoretical (Hartree-Fock and density functional theory) calculations of 63/65Cu EFG and CS tensors are utilized to examine the relationships between NMR interaction tensor parameters, the magnitudes and orientations of the principal components, and molecular structure and symmetry.     

Magnetic shielding tensor components in a highly strained phosphorus molecule: 1,2,3-triphenylphosphirene

Using the magic-angle spinning (MAS) technique the principal values of the ³¹P NMR magnetic shielding tensor were measured in 1,2,3-triphenylphosphirene: σ₁₁ = −58; σ₂₂ = 23 and σ₃₃ = 636 ppm. The anisotropy is quite large for a three-coordinate phosphorus compound. The high-field component σ₃₃ corresponds to the direction perpendicular to the ring, and is responsible for the high-field resonance in the isotropie phase.     

Proton magnetic shielding tensor in liquid water

The nuclear magnetic shielding tensor is a sensitive probe of the local electronic environment, providing information about molecular structure and intermolecular interactions. The magnetic shielding tensor of the water proton has been determined in hexagonal ice, but in liquid water, where the tensor is isotropically averaged by rapid molecular tumbling, only the trace of the tensor has been measured. We report here the first determination of the proton shielding anisotropy in liquid water, which, when combined with chemical shift data, yields the principal shielding components parallel (sigma(parallel)) and perpendicular (sigma(perpendicular)) to the O-H bond. We obtained the shielding anisotropy sigma(parallel)-sigma(perpendicular) by measuring the proton spin relaxation rate as a function of magnetic induction field in a water sample where dipole-dipole couplings are suppressed by H/D isotope dilution. The temperature dependence of the shielding components, determined from 0 to 80 degrees C, reflects vibrational averaging over a distribution of instantaneous hydrogen-bond geometries in the liquid and thus contains unique information about the temperature-dependent structure of liquid water. The temperature dependence of the shielding anisotropy is found to be 4 times stronger than that of the isotropic shielding. We analyze the liquid water shielding components in the light of previous NMR and theoretical results for vapor and ice. We show that a simple two-state model of water structure fails to give a consistent interpretation of the shielding data and we argue that a more detailed analysis is needed that quantitatively relates the shielding components to hydrogen bond geometry.     

Experimental and theoretical investigation of the 13C and 15N chemical shift tensors in melanostatin-exploring the chemical shift tensor as a structural probe

The determination of backbone conformations in powdered peptides using 13C and 15N shift tensor information is explored. The 13C and 15N principal shift values in natural abundance 13C and 15N melanostatin (L-Pro-L-Leu-Gly amide) are measured using the FIREMAT technique. Furthermore, the orientation of the C-N bond in the 13C shift principal axis system for the backbone carbons is obtained from the presence of the 13C-14N dipolar coupling. The Ramachandran angles for the title compound are obtained from solid-state NMR data by comparing the experimentally determined shift tensor information to systematic theoretical shielding calculations on N-formyl-L-amino acid-amide models. The effects of geometry optimization and neglect of intermolecular interactions on the theoretical shielding values in the model compounds are investigated. The sets of NMR derived Ramachandran angles are assembled in a set of test structures that are compared to the available single-crystal X-ray structure. Shift tensor calculations on the test structures and the X-ray structure are used to further assess the importance of intermolecular interactions when the shift tensor is used as a structural probe in powdered peptides.