Barotropic phase transitions and pressure-induced interdigitation on bilayer membranes of phospholipids with varying acyl chain lengths.

The bilayer phase diagrams of a series of 1, 2-diacylphosphatidylcholines containing linear saturated acyl chain (C=13, 14, 15, 16, 17 and 18) were constructed by two kinds of high-pressure optical methods. One is the observation of isothermal barotropic phase transition and the other is the isobaric thermotropic phase transition. The temperature of the main transition from the ripple gel (Pbeta') phase to the liquid crystal (Lalpha) phase for each lipid was elevated by pressure. The slope of the temperature-pressure diagram, dT/dP, was in the range of 0.21-0. 23 K MPa-1 depending on the acyl chain length. The temperature of the pretransition from the lamellar gel (Lbeta') phase to the Pbeta' phase for each lipid was also elevated by pressure. The slope of phase boundary, dT/dP, for the pretransition was in the range of 0. 12-0.14 K MPa-1. Both temperatures of the main and pretransition under ambient pressure increased with an increase in acyl chain length. The chain length dependences of the pretransition and main transition temperatures describe smooth curves with no evidence of odd/even discontinuities. Pressure-induced interdigitated gel (LbetaI) phase was observed beyond 300 MPa for 14:0-PC, 175 MPa for 15:0-PC, 100 MPa for 16:0-PC, 80 MPa for 17:0-PC and 70 MPa for 18:0-PC, respectively. The minimum pressure for the interdigitation of lipid bilayer membranes decreased with an increase in acyl chain length in a manner of non-linear relation. The slopes of phase boundary between Lbeta' and LbetaI phases transformed from the negative slope to the positive slope as the pressure increases.