Correlation between molecular shape and hexagonal H II phase promoting ability of sterols

Abstract

Sterols are main constituents of biological membranes. Their exact function is unknown, but many studies indicated that sterols greatly affect the physical properties of the lipid part of membranes. In particular, the observations that cholesterol by its condensing effect increases the chain order in the liquid crystalline state and by its liquifying effect decreases the chain order in the gel state, has led to the concept that cholesterol-containing membranes are in an ‘intermediate state of fluidity’ [ 11. which upon isolation, when dispersed in aqueous buffers, organize themselves in inverted structures such as the hexagonal HII phase or intrabilayer inverted micelles. For instance, natural phosphatidylethanolamines (PEs) undergo temperature-dependent bilayer -+ hexagonal HII transitions such that at physiological temperatures the hexagonal HII phase,is preferred [ 10,111. Phosphatidylcholines stabilize bilayer structure of PEs whereas cholesterol promotes hexagonal HII phase formation in mixed phosphatidylcholine-PE systems [ 12,131.