Thermotropic and barotropic phase transition on bilayer membranes of phospholipids with varying acyl chain-lengths

The bilayer phase-transitions of a series of 1,2-diacylphosphatidylcholines containing linear saturated acyl chain of even-and odd-number carbons (C = 12, 13, 14, 15, 16, 17 and 18) were observed by two kinds of 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 phase to the liquid crystal phase for each lipid was elevated linearly by pressure in the range of 150 MPa. The slope of the temperature-pressure diagram, dT/dp, was in the range 0.20-0.23 K MPa-1 depending on the acyl chain-length. The chain length dependence of the main transition temperature under ambient pressure described a smooth curve with no evidence of odd/even discontinuities. The phase transition enthalpy, Delta H, which was determined by the differential scanning calorimetry (DSC), increased with an increase in the acyl chain-length. The Delta H vs. chain-length curve was non-linear and convex upward. The volume change, Delta V, associated with the transition was calculated from the values of Delta H and dT/dp by means of the Clapeyron-Clausius equation. The values of Delta V increased with an increase in the acyl chain-length, which were best described by a smooth curve and not a linear function. The increment of the transition volume tends to be moderated as the length of the hydrocarbon chain is increased and amounts to 1.4 cm(3) mol(-1) per one methylene group. Non linear properties of thermodynamic quantities with respect to the acyl chain-length seem to be attributable to the end-group effects of the fatty acyl chains. (C) 1997 Elsevier Science Ireland Ltd.