Applications of Calorimetry to Lipid Model Membranes

Abstract

Thermal techniques have found wide application in the study of properties of biological and lipid model membranes. Particularly differential scanning calorimetry (DSC) has become a standard technique for studying the thermally induced transition of biological membranes and bilayers of model lipids from an ordered, crystallinelike state at low temperature (gel phase) to a more disordered liquid-crystalline state at higher temperature (Chapman et al, 1967; Chapman, 1968; Ladbrooke and Chapman, 1969; Lee, 1977a, b; Barisas and Gill, 1978; Mabrey and Sturtevant, 1978; McElhaney, 1982; Bach, 1984; Donovan, 1984). This gel to liquid-crystalline phase transition is a highly cooperative transition. The endothermic effect observed on heating the system through the phase transition is caused by changes in internal energy of the system as the number of gauche conformations in the fatty acyl chains increases, and by concomitant changes in van der Waals interactions between the chains and polar interactions at the lipid bilayer-water interface. The “melting” of the fatty acyl chains is accompanied by an increase in total volume of the bilayer by ca. 2%-4% (Sheetz and Chan, 1972; Blazyk et al, 1975; Melchior and Morowitz, 1972), the lateral expansion in the plane of the bilayer being much larger (ca.20%- 25%). This increase in molecular area at the bilayer-water interface is thus compensated to a large extent by a thinning of the membrane in the liquid-crystalline state.