Kinetics of the subtransition in dipalmitoylphosphatidylcholine.

The kinetics of the interconversions of the subgel and gel phases in dipalmitoylphosphatidylcholine have been studied by using differential dilatometry, differential scanning calorimetry (DSC), and neutral buoyancy centrifugation as a function of incubation temperature and deuteriation of the solvent. As seen by others, DSC scans show two peaks in the subgel transition region for incubation temperatures below 1 degree C. After incubation at 0.1 degree C, the DSC peak that occurs at the lower scanning temperature appears with an incubation half-time of 0.5 day and eventually converts into a peak at higher scanning temperature with an incubation half-time of 18 days. By varying the scanning rate, we show that these two peaks merge into one at slow scanning rates with a common equilibrium transition temperature of 13.8 degrees C, in agreement with equilibrium calorimetry and dilatometry (delta V = 0.017 +/- 0.001 mL/g). For incubation temperatures above 4.6 degrees C, only one peak appears in both scanning dilatometry and calorimetry. While the initial rate of subgel conversion is smaller at the higher incubation temperatures, after 300 h a higher percentage of the sample has converted to subgel than at the lower incubation temperatures. We suggest that higher incubation temperatures (near 5 degrees C) are preferable for forming the stable subgel phase, and we present a colliding domain picture that indicates why this may be so. Our results in D2O and the similarity of the kinetics of volume decrease with the kinetics of wide-angle diffraction lines also support the suggestion that the partial loss of interlamellar water plays a kinetic role in subgel formation.