Anisotropic NMR parameters

Several local labels need to be measured, usually one-by-one in individual samples in the case of 19F-NMR. The combined set of anisotropic NMR parameters then allows one to re-construct the geometry of the entire peptide and to determine its alignment in the membrane, as illustrated in Fig. 1 [35-37, 47, 48]. The only prerequisite is that the 19F-labelled moiety has to be rigidly attached to the peptide backbone, and that the peptide assumes a well-defined secondary structure. Provided that a sufficient number of local orientational constraints can be measured... 

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. 

Table 3.2 Isotropic and anisotropic NMR parameters for oxovanadium(V) alkoxides.

The NMR data for NMR coesite were derived from the anisotropic dimension of a DAS data set (Figure 6.9, Grandinetti et al. 1995). The NMR parameters for all the other samples were taken from ID static and MAS data. The static measurements of the Si02... 

In order to establish structural constraints on proteins and peptides from solid-state NMR, it is important to consider all aspects from appropriate labeling of the sample, selection of the experiments providing the desired information, and to have appropriate reference data available to allow extraction of structural data from the (anisotropic interaction) parameters determined by the experiment. As an example. Cross and co-workers investigated the conformation of the ion channel gramicidin A using selectively N-labeled peptides in uniaxiaUy oriented lipid bilayers c.f.. Section 4.2). To translate the measured " N chemical shifts in the oriented samples into stractural constraints, it is necessary to determine the magnitude and orientation of the " N chemical shift tensors... 

A key difference between crystalline samples and hydrated or lipid-embedded proteins can be the different degrees of molecular mobility. This aspect will impact on the utility of those solid-state NMR experiments that exploit anisotropic interactions in the magnetic field. Such aspects can, for example, play an important role if proteins reconstituted into lipid bilayers are studied at high temperatures (i.e., in the liquid crystalline phase). Here, the degree of molecular mobility may depend on the polypeptide topology in the membrane and a theoretical understanding how solid-state NMR parameters are affected by molecular motion is crucial. The basic interactions, present in a solid-state NMR experiment on spin-V2 nuclei, are introduced in the next section. 

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