Solvent anisotropy selection

In studies where cholesteric phase order appears to influence the course of bimolecular reactions, solute diffusion and collisional orientations appear to be affected by solvent anisotropy. An example is the stereoselective photodimerization of 1,3-dlmethylthymine(24). While all four possible cis-fused cyclobutane photodimers are produced in isotropic solutions and disordered glasses, the cis-syn dimer is formed almost exclusively in liquid-crystalline media. Furthermore, selectivity of reaction products in the mesophases is greatly decreased upon addition of an isotropic diluent which disturbs local solvent order (e.g., dioxane or DMSO). Since all of the... 

It is quite common to observe that the lines in a nitroxide spectrum show variations in width. One feature which contributes to this is the anisotropy of the nitroxide grouping, as a consequence of which the high field component of the nitrogen triplet may be perceptibly broadened. This effect is particularly noticeable when free tumbling of the nitroxide molecule is restricted either by the viscosity of the solvent or when the radical is incorporated into a very large (e.g. polymer) molecule. This selective line broadening is, of course, one of the principal sources of information in spin-labelling experiments (Berliner, 1976). 

Selected entries from Methods in Enzymology [vol, page(s)] Anisotropy effects, 261, 427-430 determination by dynamic laser light scattering (quasi-elastic light scattering), 261, 432-433 determination for nucleic acids by NMR [accuracy, 261, 432-433 algorithms, 261, 11-13, 425, 430 carbon-13 relaxation, 261, 11-12, 422-426, 431, 434-435 cross-relaxation rates, 261,419-422, 435 error sources, 261, 430-432 phosphorus-31 relaxation, 261, 426-427, 431 proton relaxation, 261,51,418-422 relaxation matrix calculations, 261,12] deuterium solvent viscosity effects, 261,433 effect... 

In the context of ESR studies on macromolecules, four solvent properties are of main interest. First, the viscosity of the solvent directly influences the rotational correlation time If. A rotational correlation time of 3.5 ns provides X-band (9.6-GHz) spectra that are most sensitive to details of the rotational motion (restrictions, anisotropy) and to small variations of x. Hence, it may be advantageous to select a solvent in which dynamics is as close as possible to this regime. The situation may... 

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