Dipolar couplings bond with

Fig. 8.2 Ori entations of an amide NH dipolar coupling bond-vector of the protein ubiquitin. Each cone of orientations is compatible with two different alignment directions adopted by the protein in two different alignment media. The central lines defining each cone correspond to the orientations obtained from the measured dipolar couplings. The outer lines include orientations that are possible if the dipolar coupling values are either increased or decreased by 1 Hz. The angle at which the two cones intersect is defined by ft. The solid dot at the cone intersection determines the orientation of the dipolar coupling vector. (Reproduced with permission from B. E. Ramirez and A. Bax, J. Am. Chem. Soc. 1998, 720, 9106-9107.)...

Figure 1 The principal sources of structural data are the NOEs, which give information on the spatial proximity d of protons coupling constants, which give information on dihedral angles < i and residual dipolar couplings, which give information on the relative orientation 0 of a bond vector with respect to the molecule (to the magnetic anisotropy tensor or an alignment tensor). Protons are shown as spheres. The dashed line indicates a coordinate system rigidly attached to the molecule.

The NOESY spectrum of buxatenone shows four cross-peaks, A-D. Cross-peak B represents the dipolar coupling between the most upfield C-19 cyclopropyl proton (8 0.68) with the most downfield olefinic proton (8 6.72). This could be possible only when the double bond is located either between C-1 and C-2 or between C-11 and C-12. The possibility of placing a double bond between C-11 and C-12 can be excluded on the basis of chemical shift considerations, since conjuga-... 

The most important feature of the information in dipolar couplings is that it is independent of distance. The data can be envisioned as reflecting the relative orientation of pairs of bond vectors, with the intervening distance having no effect (Meiler et al., 2000). Thus dipolar couplings can potentially provide a method for characterizing the structure of denatured proteins, provided that the denatured state ensemble of conformations retains several levels of nonrandomness. 

Fig. 8.1 Orientation of two dipolar coupling vectors in a protein segment. The vectors connect the amide Hn and 15N atoms. In this case the interaction vector coincides with the chemical bond. The axis system of the alignment tensor is designated as A, Aw Aa. The angles ( n, y>n, and 02, define the orientation of the two dipolar vectors with respect to the alignment tensor. (Reproduced with permission from N. Tjandra, Structure 1999, 7, R205-R211.)...

The two-bond HNC dipolar coupling is observable in a 15N-HSQC experiment in which the J coupling between the carbonyl atom C and the 15N amide is active. The doublet components in the 15 N dimension that represent the C N coupling are displaced with respect to one other in the H dimension as in an E.COSY [39] because of this two-bond coupling. 

Dipolar couplings of side chains can provide valuable structural information. Dipolar couplings for methylene and methyl sites have been applied to protein structure refinement [49]. In a side chain with a C-CH3 moiety, rapid rotation of the methyl group results in an averaged C-H dipolar vector aligned in the direction of the C-C bond. This rapid rotation scales the real value of the C-H dipolar by a factor of —1/3. Therefore, when including methyl dipolar couplings in protein structure refinement, the orientation of the C-CH3 bond is the one that is restricted, rather than the C-H bond. 

NMR data [95]. This new method requires two sets of dipolar couplings from two different protein orientations. Together with the backbone dipolar couplings that are typically used (i.e., amide NH, C N, CaC, CaHa and the two-bond HNC ), CaCp dipolar couplings are also needed. Provided that the orientation of one peptide plane is known independently, the dipolar coupling data give rise to two possible orientations for the subsequent peptide plane, where the conformations about the alpha carbon in these two orientations are mirror images. One of the conformations can be ruled out because of chirality. 

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