Alignment medium

For aprotic polar organic solvents only few widely applicable alignment media are known. The lyotropic mesophase of poly(y-benzyl-L-glutamate) (PBLG) with DMF [37] has the disadvantage of a relatively strong minimum alignment. Bicellar 

In all cases, alignment media will interact with the solute. The choice of alignment medium therefore is mainly determined by the solute compatibility with respect to charge, specific hydrophobic interactions or even chemical reactions if for example, radicals are stiU present in radically synthesized polymers. 

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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. 

Stretched gelatin gels as chiral alignment media for the discrimination of enantiomers by NM R spectroscopy. Angew. Chem. Int. Ed. 2005, 44, 3145-3147. 

Freudenberger,. C Spiteller, P., Bauer, R Kessler, H Luy, B. Stretched poly(dimethylsiloxane) gels as NMR alignment media for apolar and weakly polar organic solvents an ideal tool for measuring RDCs at low molecular concentrations. /. Am. Chem. Soc. 2004, 326, 14690-14691. 

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.)...

RDC data can allow the five averages (i.e., the elements of r1- in Eq. (23)) describing each dipolar interaction tensor to be determined, and subsequently interpreted in terms of mean internuclear vector orientations and an associated description of dynamics. To date, there are a couple of different approaches which have been introduced for the analysis and interpretation of RDCs acquired in multiple alignment media. 

The first step in the model free approach57,58 is to determine the corresponding order tensors for each separate RDC dataset, based on a set of structural coordinates. The irreducible tensorial descriptions of each of these order tensors are arranged into the so-called F matrix. This matrix is of dimension 5 x M in which M is the total number of distinct RDC datasets acquired using different alignment media. The following formulation appears in the work of Peti et al.,5H... 

In a recent work by Tolman,50 a refinement approach similar to the model free approach was described which extends the realm of applicability to cases in which RDC datasets acquired in five different alignment media are not available. The theoretical development proceeds from recognition that, in general, the RDC data in its entirety can be represented by a single... 

Tolman has shown that prior knowledge of a structure is not necessary when RDCs can be measured in at least five different alignment media.50 This... 

Keywords NMR spectroscopy RDCs residual dipolar couplings alignment media structure chirality configuration conformation... 

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