Methods for the analysis of motional effects on 2H NMR solid state line shapes are described. Several simple models of anisotropic motions in solids are presented from which line shapes and relaxation experiments are calculated. Using these methods order parameters of fast limit spectra can be explicitly evaluated. These methods are extended to include analysis of intermediate exchange spectra in terms of specific motional models. The effects of differential T2’s arising from exchange broadening on the line shape in powders are described. Methods for the calculation and analysis of T1 anisotropy in partially relaxed spectra of powders for both fast and intermediate exchange regimes are presented.
The theory required to extract detailed motional information from NMR relaxation times of nuclei in an amino acid side chain containing multiple internal rotation axes attached to a large macromolecule of at least cylindrical symmetry is developed. Emphasis is placed on the analysis of 13C-NMR of protonated carbons where dipolar relaxation is predominant. Extension to other relaxation processes is straightforward. The spectral density from which the relaxation times can be calculated is obtained for various models for the motion of the side chain. The existing theory which assumes that internal rotations are both independent and free is generalized to incorporate excluded volume effects in a heuristic way by restricting the amplitude of the internal rotations. It is found that small amplitude motions are ineffective in causing relaxation. Thus jump models involving a relatively small number of configurations are appropriate to describe the motion of the side chain. The advantage of jump models is twofold: (1) excluded volume effects can be handled simply by including only sterically allowed configurations, and (2) the unrealistic assumption of diffusional models that rotations about different bonds are independent can be removed by considering concerted motions in which the positions of only a small number of atoms are altered. An explicit expression for the spectral density, which is computationally practical when the number of configurations is less than 200, is obtained for a class of jump models in which the dynamics is described by a master equation. A detailed application of the general formalism is made to the case of a lysine side chain whose carbon–carbon backbone is constrained to lie on a tetrahedral lattice. Finally, numerical calculations are presented to illustrate some of the qualitative features of the different models, and the strategy that can be used to obtain motional information from experiment within the framework of these models is discussed.
Complete 17O 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°(CS) or perpendicular (QC) to the CdO 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.
Since the sign of V(P,F) in these metal complexes can be assumed to be negative52 from the twodimensional correlation experiments (cf. Figure 4b), it follows that V(F,Fe) is positive. Of particular value is the combination of two-dimensional experiments, e.g. ('H,57Fe) and (31,57Fe) for complex 1. In this way, the relative signs of a variety of scalar couplings in quasitrigonal and -tetragonal complexes can be obtained (cf. Chart IV). To the best of our knowledge, the signs (52) Staplin, D. C.;
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