How membrane chain melting properties are regulated by the polar surface of the lipid bilayer

The principle of regulation of various membrane properties by the hydrocarbon membrane interior is now well understood. The mechanism by which the interfacial membrane region including aqueous solution affects the state of the lipid bilayer matrix, however, is as yet unclear, despite its great biological and physiological significance. Data and analysis presented in this paper show that apart from the lipid chain type, length, and degree of unsaturation the main factors determining the characteristics of lipid membranes are surface polarity and interfacial hydration. These incorporate the effects of head group dipole and multipole moments as well as the head group ability for hydrogen bonding and can account for most of the changes in the physicochemical membrane state caused by the lipid head group structure, bulk pH value, salt content, solute adsorption, etc. The effects of membrane potential are much less, only 10-30% of the former. Variations in hydration thus not only govern the short- and medium-range intermolecular and intermembrane interactions but also provide a fast and energetically inexpensive regulatory mechanism for lipid membranes to adapt their characteristics, at least locally or transiently, to new requirements.