Phase Transitions in Lipids

Abstract

The physico-chemical characterization of phospholipid interactions is one of the important problems in membrane biology. The research groups of Chapman, van Deenen and Luzzati have shown that systems of phospholipid molecules may exhibit a large variety of organization forms, or phases, depending upon the chemical composition of the molecules, the water content of the system and the temperature (Chapman, 1965; Chapman and Wallach, 1968; Van Deenen, 1965; 1966; Luzzati, 1968). The present paper is concerned with some aspects of the so-called crystalline-liquid crystalline phase transition of phospholipids in the fully hydrated state. Systematic calorimetric studies of this highly endothermic phase transition were carried out by Chapman and coworkers (Chapman et al., 1967; Ladbrooke and Chapman, 1969; Phillips et al., 1969). According to these studies the characteristic transition temperature T_t depends upon the length of the lipid hydrocarbon chains, the degree of unsaturation of the hydrocarbon chains, the chemical structure of the lipid polar head groups and the water content of the system. With increasing degree of hydration a limiting lower transition temperature, the so-called Krafft point is reached. In the fully hydrated state the lipid molecules form mainly lamellar structures with the lipid molecules grouped into bilayers which are separated by layers of water (cf. Chapman et al., 1967). The characteristic transition temperature of some lecithin-water systems in the fully hydrated state are T_t= 24°C for dimyristoyllecithin, T_t = 41°C for dipalmitoyllecithin and T_t = 55°C for distearoyllecithin. The values of T_t are very sensitive against impurities.