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-diacylphosphatidylcholin es 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 o ther is the isobaric thermotropic phase transition. The temperature of the main transition from the ripple gel (P-beta') phase to the liquid crystal (L-alpha) 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 (L-beta') phase to the P-be ta' phase for each lipid was also elevated by pressure. The slope of p hase boundary, dTldP, for the pretransition was in the range of 0.12-0 .14 K MPa-1 Both temperatures of the main and pretransition under ambi ent pressure increased with an increase in acyl chain length. The chai n length dependences of the pretransition and main transition temperat ures describe smooth curves with no evidence of odd/even discontinuiti es. Pressure-induced interdigitated gel (LbetaI) phase was observed be yond 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 pre ssure for the interdigitation of lipid bilayer membranes decreased wit h an increase in acyl chain length in a manner of non-linear relation. The slopes of phase boundary between L-beta' and LbetaI phases transf ormed from the negative slope to the positive slope as the pressure in creases. (C) 1998 Elsevier Science B.V. All rights reserved.