Orientations Of Chemical Shift Tensors Research Articles

Chlorine-35 Solid-State Nuclear Magnetic Resonance Spectroscopy as an Indirect Probe of the Oxidation Number of Tin in Tin Chlorides

Ultrawideline 35Cl solid-state nuclear magnetic resonance (SSNMR) spectra of a series of 12 tin chlorides were recorded. The magnitude of the 35Cl quadrupolar coupling constant (CQ) was shown to consistently indicate the chemical state (oxidation number) of the bound Sn center. The chemical state of the Sn center was independently verified by tin Mössbauer spectroscopy. CQ(35Cl) values of >30 MHz correspond to Sn(IV), while CQ(35Cl) readings of <30 MHz indicate that Sn(II) is present. Tin-119 SSNMR experiments would seem to be the most direct and effective route to interrogating tin in these systems, yet we show that ambiguous results can emerge from this method, which may lead to an incorrect interpretation of the Sn oxidation number. The accumulated 35Cl NMR data are used as a guide to assign the Sn oxidation number in the mixed-valent metal complex Ph3PPdImSnCl2. The synthesis and crystal structure of the related Ph3PPtImSnCl2 are reported, and 195Pt and 35Cl SSNMR experiments were also used to investigate its Pt-Sn bonding. Plane-wave DFT calculations of 35Cl, 119Sn, and 195Pt NMR parameters are used to model and interpret experimental data, supported by computed 119Sn and 195Pt chemical shift tensor orientations. Given the ubiquity of directly bound Cl centers in organometallic and inorganic systems, there is tremendous potential for widespread usage of 35Cl SSNMR parameters to provide a reliable indication of the chemical state in metal chlorides.

Average orientation of a fluoroaromatic molecule in lipid bilayers from DFT-informed NMR measurements of 1H-19F dipolar couplings.

Fluorine is often incorporated into the aromatic moieties of synthetic bioactive molecules such as pharmaceuticals and disease diagnostics in order to alter their physicochemical properties. Fluorine substitution may increase a molecule's lipophilicity, thereby enabling its diffusion across cell membranes to enhance bioavailability or to exert a direct physiological effect from within the lipid bilayer. Understanding the structure, dynamics and orientation of fluoroaromatic molecules in lipid bilayers can provide useful insight into the effect of fluorine on their mode of action, and their interactions with membrane-embedded targets or efflux proteins. Here we demonstrate that NMR measurements of 19F chemical shift anisotropy combined with 1H-19F dipolar coupling measurements together report on the average orientation of a lipophilic fluoroaromatic molecule, 4-(6-fluorobenzo[d]thiazol-2-yl)aniline (FBTA), rapidly rotating within a lipid bilayer. The 19F chemical shift tensor orientation in the molecular frame was calculated by density functional theory and corroborated by 1H-19F PISEMA NMR. It was then possible to analyse the line shapes of proton-coupled and proton-decoupled 19F spectra of FBTA in chain perdeuterated dimyristoylphosphatidylcholine (DMPC-d54) bilayers to restrict the average axis of molecular reorientation of FBTA in the bilayer to a limited range orientations. This approach, which exploits the high sensitivity and gyromagnetic ratios of 19F and 1H, will be useful for comparing the membrane properties of related bioactive fluoroaromatic compounds.

Accurate determination of chemical shift tensor orientations of single-crystals by solid-state magic angle spinning NMR

An improved implementation of single-crystal magic-angle-spinning (MAS) NMR is presented which gives access to chemical shift tensors both in orientation (relative to the crystal axis system) and principal axis values. For mounting arbitrary crystals inside ordinary MAS rotors, a mounting tool is described which allows to relate the crystal orientation determined by diffraction techniques to the rotor coordinate system. The crystal is finally mounted into a MAS rotor equipped with a special insert which allows a defined reorientation of the single-crystal by 90°.The approach is based on the idea that the dispersive spectra, which are obtained when applying read-pulses at specific rotor-phases, not only yield the size of the eigenvalues but also encode the orientation of the different chemical shift (rank-2) tensors. For this purpose two 2D-data sets with orthogonal crystal orientation are fitted simultaneously. The presented analysis for chemical shift tensors is supported by an analytical formula which allows fast calculation of phase and amplitude of individual spinning side-bands and by a protocol which solves the problem of finding the correct reference phase of the spectrum. Different rotor-synchronized pulse-sequences are introduced for the same reason. Experiments are performed on L-alanine and O-phosphorylethanolamine and the observed errors are analyzed in detail. The experimental data are opposed to DFT-computed chemical shift tensors which have been obtained by the extended embedded ion method.

Solid-State 91Zr NMR Spectroscopy Studies of Zirconocene Olefin Polymerization Catalyst Precursors

(91)Zr (I = 5/2) solid-state NMR (SSNMR) spectra of the zirconocene compounds, Cp(2)ZrCl(2), Cp*(2)ZrCl(2) (1), Cp(2)ZrBr(2) (2), (Me(3)SiC(5)H(4))(2)ZrBr(2) (3), O(Me(2)SiC(5)H(4))(2)ZrBr(2) (4), (1,3-C(5)H(3))(SiMe(2)OSiMe(2))(2)(1,3-C(5)H(3))ZrBr(2) (5), Ind(2)ZrCl(2) (6), Cp(2)ZrMeCl (7), Cp(2)ZrMe(2) (8), and [Cp(2)ZrMe][MeB(C(6)F(5))(3)] (9) have been acquired. Static (91)Zr SSNMR spectra have been acquired for all complexes at magnetic fields of 9.4 and 21.1 T. Cp(2)ZrCl(2) and complexes 1 to 5 possess relatively narrow central transition powder patterns which allows for magic-angle spinning (MAS) (91)Zr solid-state NMR spectra to be acquired at a moderate field strength of 9.4 T. Complexes 6 to 9 possess ultrawideline central transition SSNMR spectra necessitating piece-wise acquisition techniques. From the static and MAS (91)Zr SSNMR spectra, it is possible to measure (91)Zr electric field gradient (EFG) and chemical shift (CS) tensor parameters, as well as the Euler angles which describe their relative orientation. Basis sets and methods for the accurate quantum chemical calculation of (91)Zr EFG and CS tensors have been identified. The origin of the observed EFG and CS tensor parameters are further investigated by visualization of the EFG and CS tensor orientations within the molecular frames. Correlations between the observed and calculated NMR tensor parameters and molecular symmetry and structure are made. All of these observations suggest that (91)Zr SSNMR spectroscopy can be utilized to probe the molecular structure of a variety of homogeneous and heterogeneous olefin polymerization catalysts.

A Solid-State 17O NMR Study of l-Tyrosine in Different Ionization States: Implications for Probing Tyrosine Side Chains in Proteins

We report experimental characterization of (17)O quadrupole coupling (QC) and chemical shift (CS) tensors for the phenolic oxygen in three l-tyrosine (l-Tyr) compounds: l-Tyr, l-Tyr.HCl, and Na(2)(l-Tyr). This is the first time that these fundamental (17)O NMR tensors are completely determined for phenolic oxygens in different ionization states. We find that, while the (17)O QC tensor changes very little upon phenol ionization, the (17)O CS tensor displays a remarkable sensitivity. In particular, the isotropic (17)O chemical shift increases by approximately 60 ppm upon phenol ionization, which is 6 times larger than the corresponding change in the isotropic (13)C chemical shift for the C(zeta) nucleus of the same phenol group. By examining the CS tensor orientation in the molecular frame of reference, we discover a "cross-over" effect between delta(11) and delta(22) components for both (17)O and (13)C CS tensors. We demonstrate that the knowledge of such "cross-over" effects is crucial for understanding the relationship between the observed CS tensor components and chemical bonding. Our results suggest that solid-state (17)O NMR can potentially be used to probe the ionization state of tyrosine side chains in proteins.

Solid-State 17O NMR and Computational Studies of C-Nitrosoarene Compounds

We report the first solid-state (17)O NMR determination of the (17)O quadrupole coupling (QC) tensor and chemical shift (CS) tensor for four (17)O-labeled C-nitrosoarene compounds: p-[(17)O]nitroso-N,N-dimethylaniline ([(17)O]NODMA), SnCl(2)(CH(3))(2)([(17)O]NODMA)(2), ZnCl(2)([(17)O]NODMA)(2), and [(17)O]NODMA.HCl. The (17)O quadrupole coupling constants (C(Q)) observed in these C-nitrosoarene compounds are on the order of 10-15 MHz, among the largest values found to date for organic compounds. The (17)O CS tensor in these compounds exhibits remarkable sensitivity toward the nitroso bonding scheme with the chemical shift anisotropy (delta(11) - delta(33)) ranging from just 350 ppm in [(17)O]NODMA.HCl to over 2800 ppm in [(17)O]NODMA. This latter value is among the largest (17)O chemical shift anisotropies reported in the literature. These extremely anisotropic (17)O NMR interactions make C-nitrosoarene compounds excellent test cases that allow us to assess the detection limit of solid-state (17)O NMR. Our results suggest that, at 21.14 T, solid-state (17)O NMR should be applicable to all oxygen-containing organic functional groups. We also show that density functional theory (DFT) calculations can reproduce reasonably well the experimental (17)O QC and CS tensors for these challenging molecules. By combining quantum chemical calculations with experimental solid-state (17)O NMR results, we are able to determine the (17)O QC and CS tensor orientations in the molecular frame of reference for C-nitrosoarenes. We present a detailed analysis illustrating how magnetic field-induced mixing between individual molecular orbitals (MOs) contributes to the (17)O shielding tensor in C-nitrosoarene compounds. We also perform a Townes-Dailey analysis for the observed (17)O QC tensors and show that (17)O CS and QC tensors are intrinsically related through the pi bond order of the N horizontal lineO bond. Furthermore, we are able for the first time to examine the parallelism between individual (17)O and (15)N CS tensor components in C-nitrosoarenes.

A Refinement Protocol to Define the Structure, Topology and Depth of Insertion of Membrane Proteins using Hybrid Solution/Solid-state NMR Restraints

To fully describe the folding space and ultimately the biological function of membrane proteins (MPs), it is necessary to determine their interactions with lipid membranes. This intrinsic property of MPs referring to as structural topology is not possible to resolve using x-ray crystallography or solution NMR. Here, we present a hybrid solution and solid-state NMR approach for the simultaneous determination of the structure, topology, and depth of insertion of MPs. Distance and angle restraints obtained from solution NMR of MPs solubilized in detergent micelles are combined with backbone orientational restraints (15N chemical shift anisotropy and 15N-1H dipolar couplings) derived from solid-state NMR in orientated lipid bilayers (PISEMA) into a hybrid objective function. In addition, a supplementary energy term, Ez (insertion depth potential), is used to ensure the correct positioning of helical MPs domains with respect to a virtual membrane. The hybrid objective function is optimized using a two-stage simulated annealing protocol and is implemented into XPLOR-NIH software for general use. To validate the hybrid method, the effects of chemical shift tensor orientations, principal tensor values, and dipolar constant magnitudes on the structural ensemble are determined. The hybrid method is applied to monomeric and pentameric PLN (phospholamban), a integral MP that regulates sarco(endo)plasmic reticulum Ca-ATPase (SERCA) function in cardiac muscle. The hybrid conformational ensemble defines the structure, topology and depth of insertion of PLN in lipid bilayer simultaneously. This ensemble is compared with solution NMR structures in DPC micelle obtained using conventional solution NMR data (NOEs, J-couplings) and residual dipolar coupling as orientational restraints. This research was supported by NIH Grants GM64742 and K02HL080081 to G.V. and we also thank University of Minnesota, Supercomputing Institute (MSI) for providing the computing resource.

31P double-quantum solid-state NMR study of phosphoroorganic compounds with (O)P–O–P-(O), (S)P–O–P(S) and (S)P–S–P(O) unit

In this work we have tested applicability of the commonly used double quantum recoupling sequence POST-C7 to study of 31P– 31P geometrical constraints for phosphoroorganic model compounds with different chemical shift anisotropy (CSA) and distinct molecular dynamics in the crystal lattice. Our results clearly show that even with large CSA, POST-C7 gives good efficiency of 31P double-quantum excitations. Moreover, large amplitude molecular motion only slightly disturb 31P build-up curve. χ 2 error analysis is used for verification of values and orientations of chemical shift tensors (CST) parameters employed for simulation of POST-C7 buildup curves.

Acetylenic Carbon-13 Chemical Shift Tensors for Diphenylacetylene and (η2-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.

The 13C Chemical Shift Tensor Principal Values and Orientations in Dialkyl Carbonates and Trithiocarbonates

The 13C chemical shift tensor principal values for the trigonal carbonate and thiocarbonate carbon atoms in the dialkyl carbonates, dimethyl carbonate, ethylene carbonate, and diphenyl carbonate, and in the trithiocarbonates, ethylene trithiocarbonate and dimethyl trithiocarbonate, respectively, were measured in various solid-state one-dimensional and two-dimensional nuclear magnetic resonance experiments. Furthermore, the chemical shift tensor principal values and orientations were calculated for the corresponding isolated molecules with quantum mechanically fully optimized geometries. Proton-optimized X-ray geometries of ethylene carbonate, ethylene trithiocarbonate, and diphenyl carbonate were used in embedded ion method (EIM) calculations and in calculations on the isolated molecules to obtain the theoretical principal values and to assign the chemical shift tensor orientations in these three compounds. Considerable improvement in the correlation between the experimental and calculated principal values is obtained when the electrostatic crystal potentials are included in EIM calculations. The chemical shift tensor orientations and principal values obtained for the dialkyl compounds in this study complement the previous data on a series of ionic potassium carbonates and thiocarbonates.

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.

Carbonates, Thiocarbonates, and the Corresponding Monoalkyl Derivatives: III. The 13C Chemical Shift Tensors in Potassium Carbonate, Bicarbonate and Related Monomethyl Derivatives

The principal values of the 13C chemical shift tensors in potassium carbonate (K 2CO 3), trithiocarbonate (K 2CS 3), bicarbonate (KHCO 3), methylcarbonate (KO 2COCH 3), S-methyl-monothiocarbonate (KO 2CSCH 3), O-methyl-monothiocarbonate (KOSCOCH 3), S-methyl-dithiocarbonate (KOSCSCH 3), and O-methyl-dithiocarbonate (KS 2COCH 3), were measured in solid-state nuclear magnetic resonance experiments. Chemical shift tensor calculations on the corresponding isolated anions were used to assign the chemical shift tensor orientations in the molecular frames of all anions. The correlation between experimental and calculated principal values improves significantly when the calculations are performed on isolated anions with proton-optimized X-ray geometries rather than on isolated anions with fully optimized geometries. Further considerable improvement in the correlation is achieved by utilizing the embedded ion method, which was recently developed to include electrostatic crystal potentials in chemical shift tensor calculations on ionic compounds. Similarities and differences in the chemical shift tensor orientations and principal values of the trigonal sp 2 carbon atoms in the carbonate and thiocarbonate anions are compared with those known for condensed polyaromatic hydrocarbons.

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.

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.

Orientational Information from TEDOR Spectral Sidebands

In a dipolar-coupled spin-1/2 network of the type 15N1–13C–15N2, an assessment of the sensitivity of the N→C and C→N TEDOR sideband intensities to the Euler angles defining the orientation of the two heteronuclear dipolar vectors in the 13C and 15N chemical shift (CS) tensor principal axes system has been carried out via numerical calculations. The results clearly indicate the potential of TEDOR MAS NMR spectroscopy for the characterization of the CS tensor orientation in the molecular frame. The efficacy of the method has been experimentally illustrated by TEDOR studies on a polycrystalline sample of [1,3—15N2, 2—13C]uracil, which is one of the four bases in RNA.

2D relayed anisotropy correlation NMR: characterization of the 13C' chemical shift tensor orientation in the peptide plane of the dipeptide AibAib.

An approach to the determination of the orientation of the carbonyl chemical shift (CS) tensor in a 13C-15N-1H dipolar coupled spin network is proposed. The method involves the measurement of the Euler angles of the 13C'-15N and 15N-1H dipolar vectors in the 13C' CS tensor principal axes system, respectively, via a 13C-15N REDOR experiment and by a 2D relayed anisotropy correlation of the 13C' CSA (omega2) and 15N-1H dipolar interaction (omega1). Via numerical simulations the sensitivity of the omega1 cross sections of the 2D spectrum to the Euler angles of the 15N-1H bond vector in the 13C' CSA frame is shown. Employing the procedure outlined in this work, we have determined the orientation of the 13C' CS tensor in the peptide plane of the dipeptide AibAib-NH2 (Aib = alpha-aminoisobutyric acid). The Euler angles are found to be (chiCN, psiCN) = (34 degrees +/- 2 degrees, 88 degrees +/- 2 degrees) and (chiNH, psiNH) = (90 degrees +/- 10 degrees, 80 degrees +/- 10 degrees). From the measured Euler angles it is seen that the sigma33 and sigma22 components of the 13C' CS tensor approximately lie in the peptide plane.

Redor in IS1S2 systems.

An approach to the determination of the 2-(13)C' chemical shift (CS) tensor orientation in pyrimidine bases via heteronuclear MAS NMR spectroscopy is presented. Considering a dipolar coupled spin 1/2 network of the type S1-I-S2 consisting of directly bonded heteronuclear spins, we have carried out numerical simulations to assess the sensitivity of I-S REDOR spinning sidebands to the Euler angles defining the orientation of the I-S1 and I-S2 dipolar vectors in the I spin CS tensor principal axes system. Our investigations clearly demonstrate the potential of I-S REDOR studies in IS1S2 systems for obtaining with high reliability and accuracy the I spin chemical shift tensor orientation in the molecular frame spanned by the two internuclear vectors I-S1 and I-S2. The significant contribution to the observed REDOR sideband intensities from anti-phase operator terms which are present at the start of the data acquisition is illustrated. The procedure for the recording and analysis of the I-S REDOR spectra in IS1S2 systems is presented and the measurement of the 2-(13)C' CS tensor orientation in a polycrystalline sample of [1,3-(15)N2, 2-(13)C] uracil, which is one of the four bases in RNA, is experimentally demonstrated.

Phosphorus-31 Solid-State NMR Studies of Homonuclear Spin Pairs in Molybdenum Phosphine Complexes: Single-Crystal, Dipolar-Chemical Shift, Rotational-Resonance, and 2D Spin−Echo NMR Experiments

Several solid-state NMR techniques have been employed to characterize phosphorus chemical shift tensors and 31P−31P spin−spin coupling interactions for bisphosphine molybdenum complexes where only one phosphorus of the bisphosphine ligand is coordinated to the metal. The bisphosphine ligand of each complex was either tetraphenyldiphosphine, tpdp, or bis(diphenylphosphino)methane, dppm. For one compound, (OC)5Mo(η1-dppm), single crystals were examined by 31P NMR and X-ray diffraction. For the metal-bound phosphorus, the most shielded direction of the chemical shift tensor is near the Mo−P bond axis. For the non-coordinated phosphorus, the most shielded direction is oriented in the direction of the formal electron lone pair on phosphorus. For the other compounds, stationary powder samples were examined using dipolar-chemical shift and 2D spin−echo techniques. Powder samples were also examined under slow magic-angle spinning, variable-angle spinning, and rotational-resonance conditions. Analysis of these 31P NMR spectra suggested chemical shift tensor orientations analogous to those measured for the single crystal; however, in the case of powders only the relative orientations of the two chemical shift tensors with respect to the P−P axis can be determined. Our investigations indicate that spectra from powder samples should be analyzed at two or more applied magnetic fields. The 2D spin−echo experiment proved to be invaluable for obtaining the effective dipolar P,P coupling constant, R, and the relative signs of R and the indirect spin−spin coupling constant, J. Values of nJ(31P,31P) measured in the solid state often differ significantly from the conformationally averaged values obtained in solution. The tpdp derivatives appear to have a reduced R, whereas for the dppm systems, R agrees very well with the value calculated from the known P,P separations.

Determination of 51V quadrupole and chemical shift tensor orientations in V 2O 5 by analysis of magic-angle spinning nuclear magnetic resonance spectra

Magic-angle spinning (MAS) 51V nuclear magnetic resonance (NMR) spectra of V 2O 5 have been recorded at various fields to evidence the relative effects of the quadrupole interaction and electronic shielding at the nucleus. A refinement of the spectra using theoretical simulations has been performed assuming a simultaneous existence of these two anisotropic interactions. The relative orientation of the principal axes for both tensors has been obtained. The results agree with previous single-crystal studies. Reliability of the results is discussed. A fundamental indetermination of the actual relative tensor orientations remains, owing to the powder nature of the sample.

Tryptophans in membrane proteins: indole ring orientations and functional implications in the gramicidin channel.

Orientational constraints generated from solid-state NMR of uniformly aligned gramicidin A in hydrated lipid bilayers have been used to determine the indole ring orientations for the four tryptophans of the gramicidin A monomer with respect to the bilayer normal and the channel axis. 15N epsilon 1 labeled tryptophan has been incorporated into gramicidin at positions 9, 11, 13, and 15. The chemical shift tensor orientation has been oriented with respect to the N-H bond via doubly labeled sample in which the 15N epsilon 1-1H has been exchanged for 2H. By observation of the dipolar coupled 15N chemical shift powder pattern of the amino acid, sigma cc has been shown to be perpendicular to the plane of the ring and that sigma aa makes an angle of 25 degrees with respect to the N-H bond. The indole ring orientations were obtained from a consideration of both the chemical shift and the 15N-1H dipolar interaction. These four rings have very similar orientations with respect to the bilayer normal as given by the range of angles between the bilayer and ring normals (64-67 degrees). Furthermore, the N-H bond orientations with respect to the bilayer normal varies by only 10 degrees among the four sites. This orientational analysis has been based on an assumption that large amplitude librational motions in the hydrated bilayer samples are not averaging the nuclear spin interactions. This assumption was verified by analyzing the 2H quadrupole spectra of d5-Trp11-labeled gramicidin A in oriented preparations. The orientations predicted for the five C-H bonds in the indole ring from the 15N data agreed (root-mean-square deviation of 3.7 degrees) with the observed orientations from quadrupole splittings of the C-D bonds in the ring. From the orientation of the indole rings with respect to the bilayer normal and a polypeptide backbone conformation, the four typtophans of the gramicidin monomer are oriented with respect to the backbone of the channel conformation. The similarity among the indole orientations with respect to the bilayer normal is therefore consistent with the electrophysiological results that the individual replacement of the indole rings with phenyl rings results in a incremental decrease in the conductance of the channels formed. The indole orientations with respect to the backbone as defined by the side-chain torsion angles is not uniquely determined but yields a discrete set of possible values.(ABSTRACT TRUNCATED AT 400 WORDS)