Pseudo-Critical Heat Capacity of Single Lipid Bilayers

By an ac calorimeter the heat capacity of aqueous suspensions of dipalmitoylphosphatidylcholine has been measured near the pretransition and main transition. The anomaly at the main transition exhibits a sharp divergence, i.e., the fluctuation at the transition is certainly dominant, although they undergo first-order transition. The pseudo-critical behavior is analyzed in terms of critical-exponent expressions: α and α' are 0.40±0.05 and 0.4±0.1, respectively. Using Pippard's relation, the dependence of the transition temperature on pressure is discussed, which is almost consistent with that of the direct experiments. The anomaly at the pretransition observed by a differential scanning calorimeter is extremely reduced in ac calorimetry. This is due to the frequency dispersion of the heat capacity, since the relaxation at this transition is very long.