INTERDIGITATED HYDROCARBON CHAINS IN C20 AND C22 PHOSPHATIDYLCHOLINESINDUCED BY HYDROSTATIC-PRESSURE

Saturated diacyl-phosphatidylcholines (PCs) can exist in an unusual ge l phase in which the acyl chains from opposing monolayers interdigitat e. This phase is induced by a variety of small amphiphilic molecules s uch 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 diarachidoy l-PC (C20) and dibehenoyl-PC (C22) using the small-angle neutron scatt ering 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 di palmitoyl-PC, these results establish that the minimum pressures for i nterdigitation, which occur at about 60 degrees C except for dibehenoy l-PC, decrease monotonically with acyl chain length. The data are fit reasonably well by an exponential. Also, the curved phase boundaries f or interdigitation show that entropy changes associated with interdigi tation change sign with increasing temperature at about 60 degrees C. (C) 1998 Elsevier Science B.V. All rights reserved.