Hydration Dependent Lipid Phase Transitions in a Biological Membrane

Abstract

Phospholipids spontaneously form ordered structures in water, usually bilayers with the hydrocarbon chains in a fluid state, depending on the temperature, length of hydrocarbon chains, and the nature of the headgroup41,2). When these same phospholipids are dehydrated they undergo phase transitions to the gel state (again depending on the temperature) or to other crystalline states such as the well-known hexagonal II (H_II) phase, in which the lipids are reorganized into tubes with the polar head groups oriented into the aqueous phase in the center of the tube. These phase transitions are well documented in pure phospholipid systems, but they have been poorly studied in complex mixtures of phospholipids or in intact biological membranes, even though there is some suggestion that such mixtures of phospholipids may exhibit different phase behavior from that of pure phospholipids (3). We have undertaken such a study, using isolated sarcoplasmic reticulum (SR) as a model membrane, and we report some of our progress in this regard in the present paper.