Rotational diffusion of a steroid molecule in phosphatidylcholine membranes: effects of alkyl chain length, unsaturation, and cholesterol as studied by a spin-label method.

Abstract

Rotational diffusion of cholestane spin-label (CSL), a sterol analogue, in various phosphatidylcholine (PC)-cholesterol membranes was systematically studied by computer simulation of steady-state ESR spectra as a function of chain length and unsaturation of alkyl chains, cholesterol mole fraction, and temperature for better understanding of phospholipid-cholesterol and cholesterol-cholesterol interactions. CSL motion in the membrane was treated as Brownian rotational diffusion of a rigid rod within the confines of a cone imposed by the membrane environment. The wobbling rotational diffusion constant of the long axis, its activation energy, and the cone angle of the confines are obtained for various membranes in the liquid-crystalline phase. The wobbling diffusion constant decreases in the order dilauroyl-PC greater than dimyristoyl-PC greater than dioleoyl-PC approximately dipalmitoyl-PC greater than distearoyl-PC greater than dioleoyl-PC/cholesterol = 3/1 greater than dioleoyl-PC/cholesterol = 1/1 membranes. Activation energy for the wobbling diffusion of the long axis of CSL is strongly dependent on alkyl chain length, unsaturation, and cholesterol mole fraction. It decreases with decrease in alkyl chain length and by introduction of unsaturation in the alkyl chains. In dioleoylphosphatidylcholine membranes, activation energy decreases by a factor of approximately 3 in the presence of 50 mol % cholesterol. Activation energy for wobbling diffusion of CSL in phosphatidylcholine membranes is smaller than the activation energy for translational diffusion of a phospholipid. The former is more dependent on alkyl chain length and unsaturation.(ABSTRACT TRUNCATED AT 250 WORDS)