In order to understand the effect of trans and cis unsaturation on the thermotropic and barotiopic phase transition of lipid bilayer membranes, the phase transitions of 1,2-distearoyl-sn-glycero-3-phosphoethanolamine (DSPE), 1,2-dielaidoyl-sn-glycero-3-phosphoethanolamine (DEPE), 1,2-dioleoyl-sn-grycero-3-phosphoethanolamine (DOPE) and the corresponding phosphocholines (i. e., DSPC, DEPC and DOPC, respectively) were observed by high-pressure optical method and differential scanning calo-rimetry. Forall of the lipid bilayers studied, the temperatures of transition from the gel phase to the liquid crystalline phase were almost linearly elevated by pressure. The substitution of trans or cis unsatuiated acyl chain for the saturated acyl chain brought about the depression of phase-transition temperature. The values of △H and △V for the transition were remarkably reduced by the introduction of trans or cis double bond into the acyl chains. This situation is attributable to the geometrical configurations of cis and trans double bonds, which brings about the random packing of acyl chains in the gel state.
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