A differential scanning calorimetric study of the thermotropic phase behavior of model membranes composed of phosphatidylcholines containing linear saturated fatty acyl chains

Abstract

The thermotropic phase behavior of a series of 1,2-diacylphosphatidylcholines containing linear saturated acyl chains of 10-22 carbons was studied by differential scanning calorimetry. When fully hydrated and thoroughly equilibrated by prolonged incubation at appropriate low temperatures, all of the compounds studied from an apparently stable subgel phase (the Lc phase). The formation of the stable Lc phase is a complex process which apparently proceeds via a number of metastable intermediates after being nucleated by incubation at appropriate low temperatures. The process of Lc phase formation is subject to considerable hysteresis, and our observations indicate that the kinetic limitations become more severe as the length of the acyl chain increases. The kinetics of Lc phase formation also depend upon whether the acyl chains contain an odd or an even number of carbon atoms. The Lc phase is unstable at higher temperatures and upon heating converts to the so-called liquid-crystalline state (the L.alpha. phase). The conversion from the stable Lc to the L.alpha. phase can be a direct, albeit a multistage process, as observed with very short chain phosphatidylcholines, or one or more stable gel states may exist between the Lc and L.alpha. states. For the longer chain compounds, conversions from one stable gel phase to another become separated on the temperature scale, so that discrete subtransition, pretransition, and gel/liquid crystalline phase transition events are observed. We have also examined some aspects of the hysteresis in the pretransition, and our observations indicate that the behavior can be approximated by a reversible process which is subject to some modest kinetic limitations, these limitations can become more severe for the longer chain phosphatidylcholines.