Dynamic heat capacity at the gel to liquid-crystalline phase transition in large unilamellar vesicles of dimyristoylphosphatidylcholine in the ultralow frequency region

摘要

Dynamic heat capacity has been measured near the gel to liquid-crystalline phase transition of large unilamellar vesicle (LUV) samples of dimyristoylphosphatidylcholine with a vesicle size of 100 nm using a heat capacity spectroscopic technique. A large broad heat capacity peak was observed at 24.35 °C. Almost no frequency dependence of the excess heat capacity was seen in the frequency region of 2–50 mHz. The transition enthalpy obtained by integrating the excess heat capacity was 22 kJ/mol, which roughly agrees with the static transition enthalpy of 18.9–26.4 kJ/mol for LUV samples obtained by differential scanning calorimetry. These facts indicate that there is no relaxation process below the frequency region of the present measurement. The heat capacity peak showed a significant rounding over a 200 mK range, which has been ascribed to the intrinsic nature of the transition. As a possible origin of this rounding, the external-field effect and the finite-size effect have been discussed. In particular, the result of the data fitting with a parametric representation of the equation of state suggests a possibility that the gel phase of LUV is a hexatic phase.