Conformational anisotropy

Complexes of near octahedral stereochemistry have / etr 1.9 BM and correspond to equation (73). The behaviour of CuS04 5H20 can be taken as typical iefr(300 K) = 1.88 BM, obeying the Curie-Weiss law with 10 <1 °C and a small TIP contribution. In lower stereochemistries, such as approximate square-planar, the average magnetic susceptibihty behaviour is similar, but there can be quite substantial anisotropy, conforming to the requirements of equation (75). 

The sensitivity of the NMR anisotropy to the conformation of the glycol residue, which was a complication in polyethylene terephthalate, can be turned to advantage in 4GT, as shown by some recent research 36). The anisotropy of the second moment was determined for three cases, starting with a highly oriented tape ... 

Despite its weakness, the anisotropy of the g tensor of iron-sulfur centers can be used to determine the orientation of these centers or that of the accommodating polypeptide in relation to a more complex system such as a membrane-bound complex. For this purpose, the EPR study has to be carried out on either partially or fully oriented systems (oriented membranes or monocrystals, respectively). Lastly, the sensitivity of the EPR spectra of iron-sulfur centers to structural changes can be utilized to monitor the conformational changes induced in the protein by different factors, such as the pH and the ionic strength of the solvent or the binding of substrates and inhibitors. We return to the latter point in Section IV. 

French workers have studied the 1H- and 13C-NMR parameters of disubstituted selenophenes.37 38 The proton chemical shifts are discussed in terms of magnetic anisotropy and electric field effects of the substituents in order to study the conformational equilibrium of the carbonyl group. The relationship between the H- and 13C-chemical shifts and 7t-electron distribution calculated by the PPP method are examined. Shifts and coupling constants are discussed in additivity terms. 

B22 G (0, l/-y/6) in the Wybourne s notation [19, 32]. If literature-reported or experimentally determined parameters do not conform to this convention, rotation of the reference system should be applied, resulting in a standardized form of CF parameters [33]. This is of fundamental importance if different sets of parameters are to be compared to derive magnetostructural correlations and the direction of the quantization axis, and thus of the principal anisotropy axis, appropriately defined. 

When a strong static electric field is applied across a medium, its dielectric and optical properties become anisotropic. When a low frequency analyzing electric field is used to probe the anisotropy, it is called the nonlinear dielectric effect (NLDE) or dielectric saturation (17). It is the low frequency analogue of the Kerr effect. The interactions which cause the NLDE are similar to those of EFLS. For a single flexible polar molecule, the external field will influence the molecule in two ways firstly, it will interact with the total dipole moment and orient it, secondly, it will perturb the equilibrium conformation of the molecule to favor the conformations with the larger dipole moment. Thus, the orientation by the field will cause a decrease while the polarization of the molecule will cause an... 

As another example, the three-dimensional structure of Cytochrome c has been determined on the basis of structural information from pseudocontact paramagnetic chemical shifts, Curie-Dipolar cross-correlation, secondary structure constraints, dipolar couplings and 15N relaxation data [103]. This protein has a paramagnetic center, and therefore the above-mentioned conformational restraints can be derived from this feature. Dipolar couplings do not average to zero because of the susceptibility tensor anisotropy of the protein. The structure determination of this protein without NOE data gives an RMSD (root... 

It should be mentioned that rotational anisotropy of the molecule will result in an increase in the R2 values for NH vectors having particular orientation with respect to the diffusion tensor frame [46]. This increase could be misinterpreted as conformational exchange contributions, and, vice versa, small values of Rex, usually of the order or 1 s 1 or less, could be mistaken for the manifestation of the rotational anisotropy. Therefore, identification of residues subjected to conformational exchange is critical for accurate analysis of relaxation data. Additional approaches are necessary to distinguish between the two effects. As suggested earlier [27] (see also Ref. [26]), a comparison between R2 and the cross-correlation rate r]xy could serve this purpose, as tjxy contains practically the same combination of spec-... 

The major reasons for using intrinsic fluorescence and phosphorescence to study conformation are that these spectroscopies are extremely sensitive, they provide many specific parameters to correlate with physical structure, and they cover a wide time range, from picoseconds to seconds, which allows the study of a variety of different processes. The time scale of tyrosine fluorescence extends from picoseconds to a few nanoseconds, which is a good time window to obtain information about rotational diffusion, intermolecular association reactions, and conformational relaxation in the presence and absence of cofactors and substrates. Moreover, the time dependence of the fluorescence intensity and anisotropy decay can be used to test predictions from molecular dynamics.(167) In using tyrosine to study the dynamics of protein structure, it is particularly important that we begin to understand the basis for the anisotropy decay of tyrosine in terms of the potential motions of the phenol ring.(221) For example, the frequency of flips about the C -C bond of tyrosine appears to cover a time range from milliseconds to nanoseconds.(222)... 

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