Interdigitated hydrocarbon chains in C20 and C22 phosphatidylcholines induced by hydrostatic pressure

Saturated diacyl-phosphatidylcholines (PCs) can exist in an unusual gel phase in which the acyl chains from opposing monolayers interdigitate. This phase is induced by a variety of small amphiphilic molecules such as ethanol and by hydrostatic pressure. Pressure-temperature (P-T) phase diagrams previously established conditions for interdigitation of dipalmitoyl-PC(C16) and distearoyl-PC (C18). Measurements have now been made for P-T phase diagrams up to 1 kbar pressure for diarachidoyl-PC (C20) and dibehenoyl-PC (C22) using the small-angle neutron scattering facilities at NIST. The minimum pressure for interdigitation was found to be about 0.6 kbar for diarachidoyl-PC and slightly less for dibehenoyl-PC. Together with previous results for distearoyl-PC and dipalmitoyl-PC, these results establish that the minimum pressures for interdigitation, which occur at about 60 degrees C except for dibehenoyl-PC, decrease monotonically with acyl chain length. The data are fit reasonably well by an exponential. Also, the curved phase boundaries for interdigitation show that entropy changes associated with interdigitation change sign with increasing temperature at about 60 degrees C. (C) 1998 Elsevier Science B.V. All rights reserved.