The molecular reorganization of lipid bilayers by osmium tetroxide: A spin-label study of orientation and restricted y-axis anisotropic motion in model membrane systems

Abstract

Phospholipid dispersions spontaneously form oriented lamellar multilayers when dried onto glass slides. These oriented multilayers form useful model systems for studying the molecular dynamics of lipid bilayers. In order to examine the effects of osmium tetroxide on the orientation and motion of hydrocarbon chains in lipid bilayers, lecithin multilayers containing the spin label 3-doxyl-5α-cholestane (the 4′,4′-dimethyloxazolidine- N-oxyl derivative of 5α-cholestan-3-one) were prepared and examined by electron spin resonance spectroscopy. In egg lecithin multilayers at room temperature and 81% relative humidity the osmium tetroxide causes nearly complete loss of orientation and severe reduction of molecular motion. In contrast, the high degree of order in l-α-dipalmitoyl lecithin multilayers is not affected by exposure to osmium tetroxide vapors. Experiments are also reported on macroscopically disordered lecithin preparations, and the data support the conclusions drawn from the ordered lecithin multilayers that rotational mobility of the probe is severely reduced by fixation of the lipid chains. A simple mathematical model has been developed to account for the amplitude of the high-frequency (τ < 10 −8 sec) restricted y-axis anisotropic motion occurring in the bilayer plane. Since the y-axis is roughly parallel to the molecular axis of the rigid steroid spin label, this model enables quantitative comparisons of various degrees of restricted motion about the molecular axis.