Positron lifetime studies on phase transitions of phospholipids

Positron-lifetime measurements have been made on the anhydrous crystalline form of DPPC (DL-α-dipalmitoylphosphatidylcholine) and on bilayer aggregates of 40% water–DPPC systems in the temperature range 23 to 63 °C. The lifetimes of o-Ps atoms for 40% water–DPPC systems vary slowly with respect to temperature starting before the pretransition temperature of the system. The increase of o-Ps lifetime is associated with the pretransition as well as with the main transition of the lipid aggregates. The determination of the phase-transition temperature in terms of the onset temperature and the differential lifetime methods is discussed and compared with the known results obtained by the differential-scanning-calorimetry and the positron Doppler-broadening methods.