NMR Relaxation Studies of Solute-Solvent Interactions

Abstract

This review focuses on the application of nuclear magnetic resonance relaxation measurements to the study of solvent-solute interactions. It primarily deals with solutions of low molecular weight solutes and with aqueous protein solutions in an attempt to compare the approaches to these rather different systems. Magnetic resonance has been so widely used to characterize solution chemistry of all sorts that it is difficult to distinguish clearly between those experiments that deal directly with solvent-solute interactions and those that do not. This review therefore omits major areas of activity and makes no attempt to be comprehensive. Discussion of solvent or solute relaxation measurements in systems containing liquid crystals, lipids, polynucleotides, polysaccharides, inorganic solids such as clay, and most of the data on whole tissues is largely omitted for lack of space, not interest. Fundamental aspects of NMR have previously been presented (1-7). Reviews of magnetic resonance appear regularly in several series (8-12). Discussions of NMR relaxation and related topics appear in related review series (13-15). Grandly stated, the goal is to understand solutions. Unfortunately one is limited by the observations that are possible and the preconceived notions that are brought to the problem. For example, the existence of a solvent in the first coordination sphere of the transition metal ions is clearly documented. The first coordination sphere water molecules remain bonded to chromium(III) ions in water for times exceeding a day at room temperature, while other metal ions exchange solvent more rapidly (16). In either case the first coordination sphere is included in formu­ lating the chemistry of these ions; we write Cr(H20)�+, rather than Cr3+. The situation is less clear for other electrolyte ions such as the halide or alkali metal ions. One may expect to discover the coordination number for an ion by using a new experimental approach; however, if the solvent lifetime in the first coordination sphere region of the ion at a temperature of interest is not significantly longer than the time required for a solvent molecule to translate one jump length in the pure