Chemical shift tensor data

TABLE 8 Chlorine chemical shift tensor data ... 

The experimental data closely resemble the simulation based on the all-trans PSB values. The discrepancy between the two solid-state NMR studies on rhodopsin arises in part from a difference, in signal-to-noise ratio and in part from possible problems associated with a fatty acid resonance which overlaps with the centerband in the previous study. The simulations illustrate the sensitivity of the sideband intensities to changes in the chemical shift tensor, as well as the quality of data necessary to accurately determine the shift tensor values. 

The above data have been analyzed theoretically within the same model as described for the chemical shift tensor (5iso In order to understand the chemical shift observed within the KDP family, we concentrate here on the polarizability modified double-well potential of the protons, which has been modelled by two back-to-back Morse potentials (see Chap. 1 in this volume). The separation between the protons and the centre of the left (/) or right (r) PO4 shell... 

Measurements of the static 13C line shape or sideband intensities of acetone on many solid acids at room temperature underestimate the chemical shift anisotropy due to motion, but the principal components of the chemical shift tensor can be accurately measured at reduced temperature. Table V reports these data for acetone on a wide variety of Brpnsted and Lewis acids (43, 45) note that the largest contribution to the isotropic shift is <5n The shift induced by A1C13 and other Lewis acids is rationalized by... 

The origin of equations 1 and 2 was explained in more detail in (6). The calibration of the bond polarization parameters for, 3C chemical shift tensors for C(sp3/sp2)-C, C(sp3/sp2)-H, C(sp3/sp2)-0, and C(sp3)-N o-bonds as well as C(sp2)-C and C(sp2)-0 71-bonds was described in detail in (5). Structural data and tensorial single crystal, 3C NMR data of 20 crystalline substances including sugars, polycyclic aromatic compounds, and amino acids were used in this calculation. 

The study of nucleic acid bases by NMR has been reported in a number of monographs (/), but very little data is available on the, 3C and, 5N NMR chemical shift tensors in these compounds. The low sensitivity of NMR spectroscopy and the long relaxation times exhibited by many of these compounds have posed the main impediments for these studies. The use of sample doping with free radical relaxation reagents, to reduce the relaxation times facilitating 2D multiple pulse experiment (2, 3), enables one to measure and analyze the principal values of the chemical shift tensors in natural abundance samples. In previous papers from this laboratory we have presented, 5N NMR chemical shift principal values for adenine, guanine, cytosine, thymine and uracil (4, 5). 

The experimental data on the principal components of the, 3C and, 5N chemical shift tensors can be complemented by quantum mechanical calculations to gain information on the spatial assignment of the principal components. The calculations also provide valuable information for exploring the relationships between the NMR chemical shifts and the molecular geometry and environment (6, 7). Recent work from this laboratory has demonstrated that the inclusion of intermolecular interactions is necessary to reproduce accurately the, 5N experimental chemical shift principal values (8). 

The principal values, 6n, 622, 633, of the chemical shift tensor were obtained for each form by CP/MAS NMR experiments. The orientation of the principal axes with respect to the molecular frame was investigated by P CP/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. The comparison of experimental P 6 parameters with theoretical data calculated by the DFT GIAO approach provided complementary information about the most sensitive NMR parameters, which best characterize the nature of the C-H -S contacts. 

The principal components of the chemical-shift tensor are usually determined by fitting the theoretically predicted data to the experimentally obtained values. A number of simulation and fitting programs are currently available, e.g., SIMPSON, DMFIT, etc. Due to the strong and complicated dipole-dipole interactions of H nuclei, only the... 

To illustrate the power of PISA wheels and dipolar waves to determine the structure of helical peptides and proteins in uniaxiaUy oriented lipid bilayers. Fig. 6a-c show SIMPSON/SIMMOL-simulated PISEMA spectra of an ideal 18-residue a-helix with a tilt angle of 10°-30° relative to Bq. In these simulations, we have tried to mimic experimental conditions by including a random distribution of the principal components of the chemical shift tensor and the dipolar coupling. The chemical shift distribution is 6 ppm for each principal element and has been established as follows we obtained — 85000 N isotropic chemical shifts reported to the BioMagResBank and selected only the — 31000 located in helical secondary stractures to have a data set independent on secondary chemical shifts. The standard deviation on the N chemical shifts for these resonances was — 6 ppm. With the lack of other statistically reliable experimental methods to establish such results for the individual principal elements of the N CSA tensor, we assumed the above variation of 6 ppm for all three principal elements. The variation of the H- N dipolar coupling was estimated by investigating the structures for a small number of a-helical membrane proteins for which the structures were established by liquid-state NMR spectroscopy. These showed standard deviations... 

In order to establish structural constraints on proteins and peptides from solid-state NMR, it is important to consider all aspects from appropriate labeling of the sample, selection of the experiments providing the desired information, and to have appropriate reference data available to allow extraction of structural data from the (anisotropic interaction) parameters determined by the experiment. As an example. Cross and co-workers investigated the conformation of the ion channel gramicidin A using selectively N-labeled peptides in uniaxiaUy oriented lipid bilayers c.f.. Section 4.2). To translate the measured " N chemical shifts in the oriented samples into stractural constraints, it is necessary to determine the magnitude and orientation of the " N chemical shift tensors... 

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