Interpretation of deuteron magnetic resonance spectroscopic studies of the hydration of macromolecules

Abstract

Methods are given for the treatment of deuteron magnetic resonance data from macromolecular systems binding ²H₂O, using results from lecithin dispersions to provide examples. Doublet splittings in such spectra measure the degree of anisotropy of water motion. By studying these splittings as a function of water content of the sample, the water content and characteristic splitting of each hydration “shell” may be found. Lecithin dispersions contain up to four kinds of water: bulk, trapped, loosely bound to the lipid, and tightly bound.

Characteristic splittings in such systems are considerably less than those of ice and crystalline hydrates. Narrowing mechanisms are discussed, including water diffusion, motion of water-binding groups, and tumbling of water molecules in potential energy wells defined by the binding groups. The relative importance of each is found from the correlation times of each motion, using relaxation time measurements. In some cases the water spectrum gives information about motion of the binding group (e.g. “loosely bound” water for lecithin), while in others the splitting is related to the binding energy (> 0.3 kcal mol^–1 for the water most tightly bound to lecithin). Relaxation data yield details of the orientation and anisotropic motions of this water.