AC Calorimetric Studies of Main Transition in Dipalmitoylphosphatidylcholine (DPPC)

Abstract

The main phase transition in dipalmitoylphosphatidylcholine (DPPC) liposomes was studied with 0.03 Hz ac calorimetry. Water suspensions with 6.25 wt% DPPC and 15.0 wt% DPPC were measured with a slow temperature scanning rate of 0.12 K/h through the transition on heating and on cooling. These studies revealed a fairly narrow C_p peak at the main transition with a hysteresis of about 0.1 K. The integrated area of the excess C_p peak observed on heating corresponds to an enthalpy of 6 kJ/mol of DPPC in both samples. This value is much smaller than the total latent heat of 38.8 kJ/mol obtained from adiabatic calorimetry, which indicates that the rate of phase conversion is quite slow. Pretransitional fluctuation contributions to C_p were very small both above and below the coexistence region for the main transition. No C_p anomalies were seen at the pre- or sub-transition even at the low frequency of 0.03 Hz.