Residual chemical shift anisotropy

Instead of measuring only the time-dependent dipolar interaction via NOE, it is also possible to determine dipolar couplings directly if the solute molecule is partially aligned in so-called alignment media. The most important resulting anisotropic parameters are RDCs, but residual quadrupolar couplings (RQCs), residual chemical shift anisotropy (RCSA) and pseudo-contact shifts (PCSs) can also be used for structure determination if applicable. 

It should be noted that the activation energies for motional processes in the same crosslinked polymer gel calculated from line widths in 13C NMR spectra are higher than those calculated from 1H NMR spectra under the magic angle conditions (residual line width). These findings indicate that 13C line widths (which are of the order of 10 Hz) are probably more affected by sample inhomogeneity and by relatively small residual chemical shift anisotropies 162). 

RDCs belong to the so-called anisotropic NMR parameters which cannot be observed in isotropically averaged samples as, for example, is the case in liquids. Besides RDCs, a number of other anisotropic parameters can be used for structure elucidation, like residual chemical shift anisotropy, residual quad-rupolar couplings for spin-1 nuclei, or pseudo-contact shifts in paramagnetic samples. Here, we will focus on RDCs where we give a brief introduction into the dipolar interaction, then into the averaging effects with the description by the alignment tensor and concepts to deal with the flexibility of molecules. For the other anisotropic NMR parameters, we refer the reader to ref 19 for an introduction and to refs. 6-8 for a detailed description. 

Liu YZ, Prestegard JH (2010) A device for the measurement of residual chemical shift anisotropy and residual dipolar coupling in soluble and membrane-associated proteins. J Biomol NMR 47(4) 249-258... 

Nuclear magnetic resonance has been used extensively in the last decade to study the structure and dynamics of model and biological membranes. However, the complexity of these systems, which should manifest itself in a corresponding richness of their NMR snectra, has in most cases not been observed because of the substantial breadth of the NMR lines. It is now understood that this breadth is due nrimarily to residual chemical shift anisotropy and dipole-dipole interactions. 

The problems referred to here were experienced earlier with NMR of membranes and to a large part were resolved by means of a quadrupole echo (Davis et ai, 1976). Considerable improvement in P-NMR spectra of membranes has been reported by use of the analogous Hahn echo based on residual chemical-shift anisotropy (Ranee and Byrd, 1983). A schematic of... 

---