AC calorimetry of dimyristoylphosphatidylcholine multilayers: hysteresis and annealing near the gel to liquid-crystal transition.

The gel to liquid-crystal phase transition in dimyristoylphosphatidylcholine liposomes was studied with 0.4-Hz ac calorimetry. The ac heat capacity on heating scans exhibited a peak in the vicinity of 23.9 degrees C with a full width at half-maximum of 0.15-0.20 degrees C. The enthalpy change was 4.8 kcal/mol, in good agreement with conventional differential scanning calorimetry. On cooling scans, the peak shifted to lower temperature by 0.1-0.5 degrees C, the width increased to 0.25-0.40 degrees C, and the apparent enthalpy change was only 40% of that observed on heating. Both the heating and cooling heat capacities were stable for at least 20 min in quasi-isothermal conditions. Following a 1 h anneal at 10 degrees C, the heating scans were quite reproducible. The results have been interpreted in terms of the nucleation and subsequent annealing of small ordered domains in the bilayer on freezing the acyl chains. No peak associated with the pretransition was observed, as expected since the relaxation time for the degrees of freedom the produce the pretransition is much longer than the period of the 0.4-Hz temperature wave.