Abstract
A number of electron and nuclear magnetic resonance studies of model membranes, and biological membranes, involve time-dependent magnetic interactions diffusion relative to one another. The two-dimensional character of this motion can have a special, large effect on magnetic resonance line shapes, and relaxation rates, because of the long-time tail of the correlation function for magnetic interactions modulated by this motion. Equations are given for the specific case in which nuclear relaxation rates are enhanced due to dipolar interactions with membrane-bound spin labels. An experimental study of spin-label-enhanced 13-C nuclear relaxation in unsonicated dispersions of phosphatidylcholine is accounted for with this theory, together with the previously reported lipid diffusion constant of D congruent to 2 times 10- minus 8 cm-2/sec. Our analysis of previously reported 1-H and 13-C nuclear relaxation rates in small phospholipid vesicles produced by sonication suggests that the rate of lateral diffusion in these small vesicles may be significantly larger than 10- minus 8 cm-2/sec.