Dose-dependent nonlinear response of the main phase-transition temperature of phospholipid membranes to alcohols.

The effect of 1-alkanols upon the main phase-transition temperature of phospholipid vesicle membranes between gel and liquid-crystalline phases was not a simple monotonic function of alkanol concentration. For instance, 1-decanol decreased the transition temperature at low concentrations, but increased it at high concentrations, displaying a minimal temperature. This concentration-induced biphasic effect cannot be explained by the van't Hoff model on the effect of impurities upon the freezing point. To explain this nonlinear response, a theory is presented which treats the effect of 1-alkanols (or any additives) on the transition temperature of phospholipid membranes in a three-component mixture. By fitting the experimental data to the theory, the enthalpy of the phase transition delta H* and the interaction energy, epsilonAB between the additive and phospholipid molecules may be estimated. The theory predicts that when epsilonAB greater than 2 (where epsilonAB = epsilon AB/RT0, T0 being the transition temperature of phospholipid), both minimum and maximum transition temperatures should exist. When epsilonAB = 2, only one inflection point exists. When epsilonAB less than 2, neither maximum nor minimum exists. The alkanol concentration at which the transition temperature is minimum (Xmin) depends on the epsilonAB value: the larger the epsilonAB values, the smaller the Xmin. When epsilonAB is large enough, Xmin values become so small that the plot delta T vs. X shows positive delta T in almost all alkanol concentrations. The interaction energy between 1-alkanols and phospholipid molecules increased with the increase in the carbon chain-length of 1-alkanols. In the case of the dipalmitoylphosphatidylcholine vesicle membrane, the carbon chain-length of 1-alkanols that caused predominantly positive delta T was about 12.