Pressure effect on the rate of crystalline phase formation of L-alpha-dipalmitoylphosphatidylcholines in multilamellar dispersions.

Fully hydrated dipalmitoylphosphatidylcholine (DPPC) in multilamellar dispersions undergoes a subtransition between the crystalline Lc phase and the gel L beta phase. This so-called subtransition occurs only if the DPPC-H2O system has been incubated at temperatures near 0 degrees C for an extended period. We have examined the effect of pressure, up to 336 atm, on the rate of crystalline Lc phase formation of multilamellar DPPC dispersions at 0 degrees C. The hydrostatic pressure is generated by a centrifugal field; the formation of the lamellar Lc phase in the multicomponent DPPC dispersions is monitored calorimetrically at ambient pressure. Results indicate that the rate of formation of the hydrated crystalline Lc phase decreases with increasing pressure. Based on transition state theory, the retardation of the formation of the hydrated crystalline Lc phase by pressure is due to an increase in the volume of the lipid-water system when the activated state is formed. We interpret that the positive value of activation volume is attributed primarily to the dehydration of the lipid polar head group. Although the acyl chain ordering and the head group dehydration are both associated with the L beta----Lc phase transition, the observation of the pressure effects on the rate of crystalline Lc phase formation is used to show that the head group dehydration plays a predominant role in controlling the kinetics of the L beta----Lc phase transition.