Observation of Lateral Diffusion in Biomembranes by Excitation Transfer 31P NMR: Estimation of Vesicle Size Distributions

Abstract

An excitation transfer 31P NMR experiment is described which directly measures translationally induced rotation of lipid molecules in large membrane vesicles. The experiment involves selective excitation of a small subset of the lipid molecules whose axially symmetric chemical shift tensors are oriented at approximately 90° to the magnetic field. Line shapes due only to the selected molecules are then recorded as a function of the mixing time. The spectra directly reflect the time dependent orientational distribution of lipid molecules and may be analyzed in terms of the distribution of effective rotational diffusion constants, related analytically to the distribution of vesicle radii. Employing this 31P NMR method, size distributions have been evaluated for multilamellar vesicles of dimyristoylphosphatidylcholine (DMPC) and DMPC/1.12% (mol/mol) bovine rhodopsin. In both cases the radius distributions are extremely broad, spanning more than 2 orders of magnitude in the case of the pure DMPC vesicles.