Direct interpretation of dipolar couplings

Fig. 4 Experimental order parameters, S, of NH bond vectors in GB3 derived from iterative Direct Interpretation of Dipolar Couplings (DIDC) using all six sets of RDCs. The red line marks the order parameters derived from relaxation. Filled symbols represent residues for which the fully anisotropic model was required to get a satisfactory fit to the data, while, for open symbols, the isotropic internal motion model was able to fit the RDCs to within the experimental noise. (Reprinted with permission from [41])...

The NMR observable most commonly exploited in studies of solid acidity is the chemical shift. While some NMR observables (e.g., dipolar couplings) lend themselves to a more or less direct quantitative evaluation, the chemical shift must be interpreted. Changes in the 13C or 15N isotropic shifts of adsorbates are observed upon complexation with Brpnsted sites, and the same is true of the H shift of the Brpnsted site, but one is hard pressed to interpret such changes quantitatively in terms of a detailed structure of the adsorption complex or even the extent of proton transfer. 

A very important aspect of solid-state NMR studies of uniaxiaUy oriented peptides is the possibility to directly determine the conformation of the protein in the bilayer. The PISA wheels 2 for regular secondary structures, which may be considered a direct NMR mapping of the so-called helical wheels, help interpreting the experimental spectra. More recently, Opella and co-workers have suggested to exclusively use the effective dipolar couplings in the analysis of such spectra to alleviate uncertainties from small residue/structure-specific variations in the chemical shifts. 

It is the interpretation of the parameters Dq that yields information on molecular geometry. The direct dipolar coupling parameter of nuclei i and / of a molecule can be written as an average over all molecular motions, including the vibrations and tumbling of the molecule... 

The measurement and interpretation of RDCs between spatially proximate spin-pairs in biopolymers such as proteins, nucleic acids, and oligosaccharides in partially oriented environments has become a popular method for structure determination." All observed RDCs, D s contain an unknown contribution from the anisotropic part of coupling tensor, J in addition to the direct dipolar contribution, Bryce and Wasylishen" evaluated the influence of J on RDCs... 

The interpretation of nmr spectra in isotropic media is considerably simplified by the well known rule that the indirect spin—spin-coupling between so called equivalent nuclei (nuclei with identical chemical shift) does not lead to an observable line splitting. This rule is no longer true for the direct nuclear coupling. The proton resonance spectrum of acetylene, for instance, is split up in a doublet by the dipolar coupling between the two equivalent protons. The spectrum of partially aligned benzene exhibits about fifty lines. 

Perhaps the first studies of nitroxide pairs with HF EPR were carried out by Lebedev and co-workers. The authors noted that although the number of spectral components of the rigid-limit 140 GHz EPR spectra of a nitroxide biradical is greater than at conventional X-band, the spectrum is still easier to interpret because of an excellent angular resolution which allows the spectral components and the corresponding dipolar coupling to be distinguished. Basically, for well-resolved spectra, the components of the dipolar tensor can be estimated directly from the spectrum. 

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