Study of the relationship between bilayer water permeability and bilayer physical state

The effects of high pressure on the phase state and molecular structure of pure lipid bilayers are discussed. The relations of $\Delta$ H, $\Delta$ S and $\Delta$ V in phase transitions are straightforward and are discernible in heterogeneous bilayers in natural membranes. The effects of pressure on the dynamic properties of bilayer constituents are less clearly understood, but order parameters obtained at pressure by different techniques show agreement. The extent and significance of hydration is poorly understood. Four physiological functions are discussed: passive permeability, active transport, membrane excitability and synaptic transmission. It is shown that a full interpretation of the kinetic effects of pressure on these processes requires much more detailed molecular information than is available at present.

The effects of pressure on the molecular structure and physiological functions of cell membranes

Differential scanning calorimetry (DSC) currently finds wide application in studies of the thermotropic behavior of lipids in biological membranes and related systems (Chapman, 1975a,b; Scheidler and Steim, 1975) Almost without exception these studies have employed calorimetric equipment which is limited to very small sample sizes and relatively high scan rates, so that only low sensitivity is realized with the aqueous solutions and suspensions which are of interest in biology The recent development of scanning calorimeters especially adapted for applications involving relatively dilute solutions and suspensions (Gill and Beck, 1965; Tsong et al, 1970; Jackson and Brandts, 1970; Ross and Goldberg, 1974; Grubert and Ackermann, 1974; Privalov et al, 1975; and Suurkuusk et al, 1976) opens up the possibility of more searching and quantitative studies of these systems The present chapter outlines the fundamental principles of DSC, and discusses several recent applications of high-sensitivity DSC to membranes and related systems, with particular reference to the most highly developed scanning calorimeter currently available, the one designed by Privalov (Privalov et al, 1975)

High-Sensitivity Differential Scanning Calorimetry in the Study of Biomembranes and Related Model Systems

The interactions of cholesterol and dipalmitoyl-phosphatidylcholine in bilayers were investigated by differential scanning dilatometry and related techniques. Dipalmitoyl-phosphatidylcholine bilayers ranging from 0 to 50 mol % cholesterol were studied over a temperature range of 0-50 degrees C. These investigations allowed construction of a three-dimensional surface with dimensions of mole fraction of cholesterol, temperature, and apparent partial specific volume. Much of the phenomenology reported for dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylcholine-cholesterol bilayers appears and can be interrelated on this surface. In addition to the thermotropic events associated with the system, two cholesterol-induced events at 17.5-20 and 29 mol % cholesterol are particularly in evidence.

Dilatometry of dipalmitoyllecithin-cholesterol bilayers

Publisher Summary Normal cell function requires fluid membrane bilayers. In addition to selecting mixtures that form stable bilayers for their membrane lipids, prokaryotes maintain their membranes in a fluid state under varying environmental conditions. This chapter discusses the reason for the cells requirement of fluid membranes; and presents some of the strategies they use to maintain their membranes in a fluid state. The roles different lipid classes play in membrane function is considered from the perspective of their physical behavior. The properties of membrane lipids in water are also described. Because of the hydrophobic effect, lipid molecules aggregate in water to minimize the contact of nonpolar regions with water while allowing the contact of head groups with water. The membrane bilayers of most organisms are entirely or mostly fluid at physiological temperatures, but at lower temperatures undergo a major reversible change of state. Membrane bilayers are composed of many different lipids and melt over a wide range of temperatures. Membranes from a variety of prokaryotes crystallized at lower than physiological temperature. The lipid molecules act to keep bilayers in a stable state despite changes in environmental conditions, but are not themselves bilayer formers. The functions of the membrane bilayer as acceptors and sinks are interrelated; and the two functions provide a sensitive, unified means to control the fatty acid composition and hence the physical state of biological membranes, whether fatty acids are supplied exogenously in the growth medium or are endogenously biosynthesized by the organism.

Mary T. Walsh

Papers

Thermal techniques in biomembrane and lipoprotein research