The mechanism of regulation of membrane phase behaviour, structure and interactions by lipid headgroups and electrolyte solution

Abstract

The effect of the polar membrane on the structure and phase transitions of glycerophospholipids is analysed by means of a phenomenological model which allows explicitly for the contributions of the lipid hydration, bilayer electrostatics and intermolecular hydrogen bonds to the free energy; arising from an end effect these have thermodynamic consequences which are larger for lipids with shorter chains. The main determinant of lipid phase behaviour and structure is lipid hydration, which is regulated predominantly by short-range headgroup–headgroup and headgroup–solvent interactions and is chiefly sensitive to the detailed structure of, and number of protons in, the lipid headgroups. Therefore, lipids with a similar non-hydrated molecular conformation attain their minimum chain-melting transition temperature and maximum lateral expansion when they are maximally hydrated; then they also experience the greatest short-range intermembrane repulsion. Lipids with different anhydride conformations, however, may melt at even lower temperatures, e.g. when they have highly tilted chains. Chain tilt appears to be more sensitive to the degree of headgroup protonation than to the net lipid charge, the latter, however, playing an important role in the relatively long-range interbilayer forces and lipid colloidal stability.