Pressure-induced elevation of phase transition temperature in dipalmitoylphosphatidylcholine bilayers: An electron spin resonance measurement of the enthalpy of phase transition

The application of 136 atm of helium pressure to an aqueous dispersion of dipalmitoylphosphatidylcholine increased the temperature of the primary phase transition at 40.4 ± 0.2 °C by 3.0 °C. The lower temperature pretransition at 30.5 ± 0.5 °C, thought to be due to phosphate headgroup reorganization, was increased by 1.7 °C. Addition of 4% dipalmitoylphosphatidic acid to the dipalmitoylphosphatidylcholine affected the phase transition in the head group region more than in the hydrocarbon chain region. The pressure and temperature data obtained, taken together with the literature value for the bilayer volume expansion during solid-fluid phase transition, and inserted into the Clausius-Clapeyron equation yield a ΔH value of 8.8 kcal/mole for this phase transition. This value is within experimental error of the ΔH value obtained from differential scanning calorimetry and serves to support the validity of the data and the experimental technique. Phase transition was observed by electron spin resonance measurement of the exclusion of the small spin label Tempo (2,2,6,6-tetramethylpiperidine-N-oxyl) from the solid domains of the bilayer. This result offers a possible explanation for the direct antagonism by high pressure of the effects of the inhalation anesthetics.