The temperature dependence of the hydroxyl deuterium quadrupole coupling parameter and the rotational correlation time of the OD internuclear vector in neat ethanol-d 1

The temperature dependence of the hydroxyl proton chemical shift and deuterium quadrupolar relaxation time of neat ethanol were measured over the temperature range 190–350 K. The proton isotropic chemical shift varies from 6.2 ppm at 190K to 4.7 ppm at 350 K. The deuterium NMR relaxation time in ethanol-d₁ varies from 6.2 ms to 309 ms over the same range. Ab initio calculations performed on various ethanol clusters ranging in size from monomer to hexamer show a linear correlation (R² = 0.99) between XD, the deuterium quadrupole coupling parameter, and δ_H, the isotropic proton chemical shift in ppm relative to TMS: XD(kHz) = 297.60 ? 15.28δ_H. The temperature dependence of XD ranges from 199.5kHz at 190K to 221.4 kHz at 350 K. Using the values for XD and the relaxation time data, the temperature dependence of the OD rotational correlation time was found to vary from 282 ps at 190 K to 4.5 ps near the boiling point (350 K). Using these correlation times and bulk viscosity data, the Gierer-Wirtz model predicts a supramolecular cluster volume of about 317 ų, the approximate volume of a cyclic pentamer cluter of ethanol molecules. The cluster volume was nearly constant from 340 K to about 290 K.     

Evaluation of various DFT protocols for computing 1H and 13C chemical shifts to distinguish stereoisomers: diastereomeric 2‐, 3‐, and 4‐methylcyclohexanols as a test set

¹H and ¹³C NMR chemical shifts were measured for a set of six isomers—the cis and trans 2‐, 3‐, and 4‐methylcyclohexanols. ¹H and ¹³C NMR chemical shifts were computed at the B3LYP, WP04, WC04, and PBE1 density functional levels for the same compounds, taking into account the Boltzmann distribution among conformational isomers (chair–chair forms and hydroxyl rotamers). The experimental versus computed chemical shift values for proton and carbon were compared and evaluated (using linear correlation (r²), total absolute error (|Δδ|_T), and mean unsigned error (MUE) criteria) with respect to the relative ability of each method to distinguish between cis and trans stereoisomers for each of the three constitutional isomers. For ¹³C shift data, results from the B3LYP and PBE1 density functionals were not sufficiently accurate to distinguish all three pairs of stereoisomers, while results using the WC04 functional did do so. For ¹H shift data, each of the WP04, B3LYP, and PBE1 methods was sufficiently accurate to make the proper stereochemical distinction for each of the three pairs. Applying a linear correction to the computed data improved both the absolute accuracy and the degree of discrimination for most of the methods. The nature of the cavity definition used for continuum solvation had little effect. Overall, use of proton chemical shift data was more discriminating than use of carbon data. Copyright © 2007 John Wiley & Sons, Ltd.     

The roles of homonuclear line narrowing and the1H amide chemical shift tensor in structure determination of proteins by solid-state NMR spectroscopy

The seminal contributions of Ulrich Haeberlen to homonuclear line narrowing and the determination of¹H chemical shift tensors are crucial for protein structure determination by solid-state nuclear magnetic resonance spectroscopy. The¹H chemical shift is particularly important in spectra obtained on oriented samples of membrane proteins as a mechanism for providing dispersion among resonances that are not resolved with the¹H-¹⁵N dipolar coupling and¹⁵N chemical shift frequencies. This is demonstrated with three-dimensional experiments on uniformly¹⁵N-labeled samples of Magainin antibiotic peptide and the protein Vpu from HIV-1 in oriented lipid bilayers. These experiments enable resonances in two-dimensional¹H-¹⁵N dipolar coupling/¹⁵N chemical shift planes separated by¹H chemical shift frequencies to be resolved and analyzed. These three-dimensional spectra are compared to one-dimensional spectra of full-length Vpu, the cytoplasmic domain of Vpu, and Magainin, as well as to two-dimensional spectra of fd coat protein and Colicin El polypeptide. The¹H amide chemical shift tensor provides valuable structural information, and this is demonstrated with its contributions to orientational restrictions to one of the in-plane helical residues of Magainin.     

The concentration dependence of the proton chemical shift and the deuterium quadrupole coupling parameter for binary solutions of ethanol

Concentration dependent experimental measurements of the ethanol hydroxyl proton chemical shift σ_H for binary solutions were carried out. The solvents used were carbon tetrachloride (CCl₄), benzene, chloroform, acetonitrile, acetone and dimethylsulphoxide (DMSO). The chemical shift values range from 0.69 ppm (relative to TMS) for dilute ethanol (extrapolated to infinite dilution) in CCl₄ to 5.34 ppm for neat liquid ethanol. Ab initio calculations of the ethanol-solvent hydrogen bond energies show a correlation with the values for the chemical shift. The hydrogen bond energies for ethanol-solvent dimers range from 0.63 kcal mol^?1 for ethanol-CCl₄ to 9.34 kcal mol^?1 for ethanol-DMSO. Theoretical calculations show a linear correlation between the deuterium quadrupole coupling parameter XD ^ar d the isotropic proton chemical shift σ_H: X_D(kHz) = 291.48 ? 14.96 σ_H, where σ_H is the proton chemical shift in ppm relative to TMS (R ² = 0.99). Using the concentration dependent chemical shift data and this equation, X_D ia observed to range from 280 kHz for very dilute concentrations in CCl₄, where the primary species is ethanol monomer, to 210 kHz for the neat liquid that is comprised primarily of cyclic pentamers.     

The influence of structural relaxation on the fluorescent properties of N-naphthyl-substituted pyridinium cations

N-naphthyl-substituted pyridinium cations fluoresce in liquids at 293 and 77 K and have an unusually large Stokes shift ((9−15)×10³ cm^-1). The Stokes shift is a result of the torsional relaxation of different aromatic groups. Characteristic fluorescence spectra demostrate dependence of the Stokes shift on the solvent viscosity.     

Toward direct determination of conformations of protein building units from multidimensional NMR experiments III

Chemical shielding anisotropy tensors have been determined, within the GIAO-RHF formalism using a smaller [6-31+G(d)] and two medium-size basis sets [6-311++G(d,p) and TZ2P], for all elements of the conformational library (altogether 27 structures) of the hydrophobic model peptide For-L-Phe-NH₂. The individual chemical shifts and their conformational averages have been compared to their experimental counterparts taken from the BioMagnetic Resonance Bank (BMRB). At the highest level of theory applied, for all nuclei but the amide proton, deviations between statistically averaged theoretical and experimental chemical shifts are as low as a few percent. One-dimensional (1D) chemical shift - structure plots do not allow unambiguous identification of backbone conformations. On the other hand, on chemical shift - chemical shift plots of selected nuclei, e.g., ¹H ^N with ¹⁵N or ¹⁵N with ¹³C ^α , regions corresponding to major conformational motifs have been found, providing basis for the identification of peptide conformers solely from NMR shift data. The 2D ¹H ^α -¹³C ^α as well as the 3D ¹H ^α -¹³C ^α -¹³C ^β chemical shift - chemical shift plots appear to be of special importance for direct determination of conformations of protein building units from multidimensional NMR experiments. 48 pairs of ¹H ^α -¹³C ^α data for phenylalanine residues have been extracted from 18 selected proteins and compared to relevant ab initio results, supporting the calculated results. Thus, the appealing idea of establishing backbone folding information of peptides and proteins from chemical shift information alone, obtained from selected multiple-pulse NMR experiments (e.g., 2D-HSQC, 2D-HMQC, and 3D-HNCA), has received further support.     

199Hg spin-lattice relaxation and chemical shift anisotropy in diphenylmercury

¹⁹⁹Hg spin-lattice relaxation times (T₁) have been measured for diphenylmercury at magnetic fields of 2.35 and 7.05 T. T₁ was nine times shorter at the higher field (0.033 sec at 310 K) than at the lower field (0.30 sec), showing relaxation by chemical shift anisotropy (CSA) to be the dominant mechanism even at the lower field. Variable-temperature ¹³C T₁ measurements at 6.35 T made it possible to determine a value for the correlation time for motion perpendicular to the axis of the molecule (50 psec at 300 K), which with the ¹⁹⁹Hg data allowed a value of 6800 ± 680 ppm to be calculated for the mercury chemical shift anisotropy. The activation energy for rotational motion was 13.3 kJ · mole⁻¹. Previous data on dimethylmercury have been reassessed and the importance of the CSA mechanism for ¹⁹⁹Hg at high fields is pointed out.     

Two- and Three-Dimensional H/C PISEMA Experiments and Their Application to Backbone and Side Chain Sites of Amino Acids and Peptides

Two-dimensional ¹H/¹³C polarization inversion spin exchange at the magic angle experiments were applied to single crystal samples of amino acids to demonstrate their potential utility on oriented samples of peptides and proteins. High resolution is achieved and structural information obtained on backbone and side chain sites from these spectra. A triple-resonance experiment that correlates the ¹H–¹³C_α dipolar coupling frequency with the chemical shift frequencies of the α-carbon, as well as the directly bonded amide ¹⁵N site, is also demonstrated. In this experiment the large ¹H–¹³C_α heteronuclear dipolar interaction provides an independent frequency dimension that significantly improves the resolution among overlapping ¹³C resonances of oriented polypeptides, while simultaneously providing measurements of the ¹³C_α chemical shift, ¹H–¹³C dipolar coupling, and ¹⁵N chemical shift frequencies and angular restraints for backbone structure determination.     

Separating Structure and Dynamics in CSA/DD Cross-Correlated Relaxation: A Case Study on Trehalose and Ubiquitin

We present two new sensitivity enhanced gradient NMR experiments for measuring interference effects between chemical shift anisotropy (CSA) and dipolar coupling interactions in a scalar coupled two-spin system in both the laboratory and rotating frames. We apply these methods for quantitative measurement of longitudinal and transverse cross-correlation rates involving interference of ¹³C CSA and ¹³C–¹H dipolar coupling in a disaccharide, α,α- -trehalose, at natural abundance of ¹³C as well as interference of amide ¹⁵N CSA and ¹⁵N–¹H dipolar coupling in uniformly ¹⁵N-labeled ubiquitin. We demonstrate that the standard heteronuclear T₁, T₂, and steady-state NOE autocorrelation experiments augmented by cross-correlation measurements provide sufficient experimental data to quantitatively separate the structural and dynamic contributions to these relaxation rates when the simplifying assumptions of isotropic overall tumbling and an axially symmetric chemical shift tensor are valid.     

Measurement of Cross Correlation between Dipolar Coupling and Chemical Shift Anisotropy in the Spin Relaxation ofC,N-Labeled Proteins

We present a simple method for extracting interference effects between chemical shift anisotropy (CSA) and dipolar coupling from spin relaxation measurements in macromolecules, and we apply this method to extracting cross-correlation rates involving interference of amide¹⁵N CSA and¹⁵N–¹H dipolar coupling and interference of carbonyl¹³C′ CSA and¹⁵N–¹³C′ dipolar coupling, in a small protein. A theoretical basis for the interpretation of these rates is presented. While it proves difficult to quantitatively separate the structural and dynamic contributions to these cross-correlation rates in the presence of anisotropic overall tumbling and a nonaxially symmetric chemical shift tensor, some useful qualitative correlations of data with protein structure can be seen when simplifying assumptions are made.     

15N chemical shift referencing in solid state NMR

Solid-state NMR spectroscopy has much advanced during the last decade and provides a multitude of data that can be used for high-resolution structure determination of biomolecules, polymers, inorganic compounds or macromolecules. In some cases the chemical shift referencing has become a limiting factor to the precision of the structure calculations and we have therefore evaluated a number of methods used in proton-decoupled ¹⁵N solid-state NMR spectroscopy. For ¹³C solid-state NMR spectroscopy adamantane is generally accepted as an external standard, but to calibrate the ¹⁵N chemical shift scale several standards are in use. As a consequence the published chemical shift values exhibit considerable differences (up to 22 ppm). In this paper we report the ¹⁵N chemical shift of several commonly used references compounds in order to allow for comparison and recalibration of published data and future work. We show that ¹⁵NH₄Cl in its powdered form (at 39.3 ppm with respect to liquid NH₃) is a suitable external reference as it produces narrow lines when compared to other reference compounds and at the same time allows for the set-up of cross-polarization NMR experiments. The compound is suitable to calibrate magic angle spinning and static NMR experiments. Finally the temperature variation of ¹⁵NH₄Cl chemical shift is reported.     

表面活性剂SDS/TX-100混合体系的NMR研究

用NMR测量了不同比例的SDS/TX-100混合溶液中质子化学位移,结合表面活性剂溶液的两态交换模型,分析了质子化学位移随浓度的变化趋势, 求出了不同比例混合溶液中两种表面活性剂各自的临界胶束浓度及混合胶束的临界胶束浓度. 依据理想混合溶液理论,预测了混合胶束的临界胶束浓度,计算了溶液中SDS与TX-100之间的相互作用参数和SDS在混合胶束中的摩尔分数. 根据所得参数讨论了混合胶束的形成过程. 利用文中和文献中混合体系的实验数据验证了协同作用理论改进前后的适用性,表明改进后的协同作用理论完善一些.     

Fast volumetric spatial-spectral MR imaging of hyperpolarized C-labeled compounds using multiple echo 3D bSSFP

Purpose

The goal of this work was to develop a fast 3D chemical shift imaging technique for the noninvasive measurement of hyperpolarized ¹³C-labeled substrates and metabolic products at low concentration.

Materials and Methods

Multiple echo 3D balanced steady state magnetic resonance imaging (ME-3DbSSFP) was performed in vitro on a syringe containing hyperpolarized [1,3,3-2H3; 1-¹³C]2-hydroxyethylpropionate (HEP) adjacent to a ¹³C-enriched acetate phantom, and in vivo on a rat before and after intravenous injection of hyperpolarized HEP at 1.5 T. Chemical shift images of the hyperpolarized HEP were derived from the multiple echo data by Fourier transformation along the echoes on a voxel by voxel basis for each slice of the 3D data set.

Results

ME-3DbSSFP imaging was able to provide chemical shift images of hyperpolarized HEP in vitro, and in a rat with isotropic 7-mm spatial resolution, 93 Hz spectral resolution and 16-s temporal resolution for a period greater than 45 s.

Conclusion

Multiple echo 3D bSSFP imaging can provide chemical shift images of hyperpolarized ¹³C-labeled compounds in vivo with relatively high spatial resolution and moderate spectral resolution. The increased signal-to-noise ratio of this 3D technique will enable the detection of hyperpolarized ¹³C-labeled metabolites at lower concentrations as compared to a 2D technique.     

Fluorine NMR Chemical Shifts in Certain Ionic Fluorides

NMR chemical shifts are known for various nuclei in numerous solid compounds. Especially detailed data were obtained for NMR of ¹⁹F. The present work cites the measurement results for fluorine NMR chemical shifts in several d¹⁰-element fluorides previously not investigated. At the same time, the fluorine chemical shift values in alkali, alkali-earth and some other metal fluorides were measured anew. (This was necessitated by the discrepancies in the shift magnitudes, cited by various authors, as well as by the inaccuracy of the mutual chemical shifts of some standards commonly used.     

Energies and chemical shifts of the sulphur 1s level and the KL2L3(1D2) Auger line in H2S,SO2 and SF6

The sulphur 1s binding energies and KL₂L₃(¹D₂) Auger energies have been measured in gaseous H₂S, SO₂ and SF₆. The experimental data, including the chemical shifts, are compared with various theoretical ab initio results. Theoretical and experimental values agree within 1-2 eV for the chemical shift and the binding energy of the 1s level, provided in the latter case relaxation, relativistic and correlation corrections are applied. Likewise, Shirley's method²⁰, which uses empirical energies, predicts the Auger energies satisfactorily. The measured S 1s binding energies are 2478.5(1) eV, 2483.7(1) eV and 2490.1(1) eV, and KL₂L₃(¹D₂) Auger energies are 2098.7(1) eV, 2095.5(2) eV, 2092.6(1) eV for H₂S, SO₂ and SF₆, respectively. The chemical shift for the 1s electron is found to be greater than for the 2s or 2p electron and in better accord with the prediction of the potential model. Data suggest the molecular relaxation energy to be small compared with the atomic relaxation energy.     

Fluorine-19 Solid-State NMR Magic-Angle-Turning Experiments Using Multiple-Pulse Homonuclear Decoupling

For compounds giving “crowded” 1-dimensional magic-angle-spinning spectra, information about the local atomic environment in the form of the chemical shift anisotropy (CSA) is sacrificed for high resolution of the less informative isotropic chemical shift. Magic-angle-turning (MAT) NMR pulse sequences preserve the CSA information by correlating it to the isotropic chemical shift in a 2-dimensional experiment. For low natural abundance nuclei such as ¹³C and ¹⁵N and under ¹H heteronuclear dipolar decoupling conditions, the dominant NMR interaction is the chemical shift. For abundant nuclei such as ¹H, ¹⁹F, and ³¹P, the homonuclear dipolar interaction becomes a significant contribution to the observed linewidth in both F₁ and F₂ dimensions. We incorporate MREV8 homonuclear multiple-pulse decoupling sequences into the MAT experiment to give a multiple-pulse MAT (MP-MAT) experiment in which the homonuclear dipolar interaction is suppressed while maintaining the chemical shift information. Extensive use of computer simulation using GAMMA has guided the pulse sequence development. In particular, we show how the MREV8 pulses can be incorporated into a quadrature-detected sequence such as MAT. The MP-MAT technique is demonstrated for a model two-site system containing a mixture of silver trifluoroacetate and calcium difluoride. The resolution in the isotropic evolution dimension is improved by faster sample spinning, shorter MREV8 cycle times in the evolution dimension, and modifications of the MAT component of the pulse sequence.     

Incorporating H chemical shift determination into C-direct detected spectroscopy of intrinsically disordered proteins in solution

Exclusively heteronuclear ¹³C-detected NMR spectroscopy of proteins in solution has seen resurgence in the past several years. For disordered or unfolded proteins, which tend to have poor ¹H-amide chemical shift dispersion, these experiments offer enhanced resolution and the possibility of complete heteronuclear resonance assignment at the cost of leaving the ¹H resonances unassigned. Here we report two novel ¹³C-detected NMR experiments which incorporate a ¹H chemical shift evolution period followed by ¹³C-TOCSY mixing for aliphatic ¹H resonance assignment without reliance on ¹H detection.     

1H and 13C NMR Chemical Shifts and N-Substituent Effects of Some Unsymmetrically N,N-Disubstituted Acetamides

The ¹H and ¹³C NMR chemical shift assignments of a series of (E) - and (Z)-N,N-Dialkylacetamides [CH₃C(O)NR¹R², with R¹/R²=Me/Et (1), Me/n-Bu (2), Et/n-Bu (3), Et/t-Bu (4), Me/Hydrcxyethyl (5)., Et/Hydroxyethyl (6), Et/Acetylhydroxyethyl (7)] are reported. The ¹H chemical shifts for the N-substituents of the amides 1–7 recorded in benzene-d₆ and in chloroform-d₁ are in agreement with the Hatton and Richards (ASIS) and Paulsen-Todt models, respectively. The ¹³C chemical shifts for the N-substituents of compounds 1–3 were compared with data of the corresponding symmetrical amides, and the results can be explained by the reciprocal steric compression effect of one N-substituent on the other. The validity of this explanation is confirmed by ¹³C spin-lattice relaxation time (T₁) measurements.     

A Definitive Example of Aluminum-27 Chemical Shielding Anisotropy

Solid-state²⁷Al NMR spectra have been obtained for a crystalline 1:1 complex of AlCl₃and OPCl₃. Aluminum chloride phosphoryl chloride, AlCl₃· OPCl₃(1), is unusual in that the Al–O–P bond angle is close to 180°. From analysis of the²⁷Al MAS NMR spectra, it was determined that the²⁷Al nuclear quadrupole coupling constant is 6.0(1) MHz, the asymmetry in the electric field gradient (efg) tensor is 0.15(2), and the isotropic chemical shift, δ_iso(²⁷Al), is 88(1) ppm. Solid-state²⁷Al NMR of a stationary sample reveals a line shape affected by a combination of anisotropic chemical shielding and second-order quadrupolar interactions. Analysis of this spectrum yields a chemical shift anisotropy of 60(1) ppm and orientations of the chemical shift and electric field gradient tensors in the molecular frame. Experimental results are compared with those calculated usingab initioHartree–Fock and density functional theory.     

Natural Abundance Nitrogen - 15 Nuclear Magnetic Resonance Studies of Some Twin - Site Donors

The natural abundance ¹⁵N-NMR chemical shifts of 1, 10-phenanthroline, pyridazine and 7-azaindole have been measured as a function of the nature of the solvent. Hydrogen bonding and protonation result in upfield shift of both the pyridine type nitrogens in 1, 10-phenanthroline and pyridazine where as in 7-azaindole, the effect is larger at the pyridine ring nitrogen. The chemical shifts due to protonation far exceeded those from hydrogen bonding. In addition to identifying the site of donation, the ¹⁵N-NMR data establishes the mechanism of charge transfer and hydrogen bonded complex formation.