THERMOTROPIC TRANSITIONS IN BIOMEMBRANES

Abstract

One of the most prominent characteristics of protein-free lipid bilayers and bio­ membranes alike is their ability to undergo a reversible thermotropic transition from a fluid state at high temperature to a crystalline state at low temperature. Such transitions were, in fact, first characterized in phospholipids before they were found in biomembranes. They resemble some transitions found in related systems such as soaps in water and share some features in common with the melting of more conventional materials such as hydrocarbons. Since the structural framework of bio­ membranes is a lipid bilayer, it is not surprising that a drastic change in its state has important effects upon the life of the cell. Hence, although the initial impact of the demonstration of such transitions in membranes (1) was the insight into the structure of the membrane matrix, more recent research has emphasized their physiological implications and has explored their use as a means of understanding lipid-protein associations. In this review we, too, begin by discussing transitions in phospholipid model systems, proceed to the physics of transitions in biomembranes, and finally examine their physiological aspects. Although cooperative effects have occasionally been suggested to occur between other membrane components such as proteins, only thermotropic lipid transitions of the type described above have been clearly demonstrated to be a general property of membranes. Our discussion is limited to such transitions and to those physiological effects upon which they have a direct bearing.