Phospholipid hydration studied by deuteron magnetic resonance spectroscopy

Abstract

Deuteron magnetic resonance spectra were obtained from 2H 2O in mixtures with egg lecithin, egg phosphatidylethanolamine, and ox brain sodium phosphatidylserine. The acid form of phosphatidylserine does not hydrate. Details of the different hydration “shells” were obtained by studying the spectral splittings as a function of 2H 2O concentration. Several different types of water are present, including bulk water (exchanging only slowly with water associated with the lipid), “trapped” water (not present with phosphatidylethanolamine), and up to three types of bound water. The spectral splittings characteristic of each water environment yielded information about the water binding energies and degrees of anisotropy of motion of the phospholipid polar groups; lecithin polar groups have least motional restriction and sodium phosphatidylserine most, while phosphatidylethanolamine binds water most tightly. Spectra of some lecithin and phosphatidylserine dispersions varied with time, due to a slow reorganization of randomly oriented multilamellar regions into longer, more ordered systems, with a length of about 1 μm. At −20°C the timescales of the change were of the order of a week and a month for lecithin and phosphatidylserine respectively. Complex changes in the spectra were observed as the temperature was raised; these are interpreted in terms of changes in the motions of the phospholipid molecules.