Effect of unsaturated acyl chains on the thermotropic and barotropic phase transitions of phospholipid bilayer membranes

In order to understand the effect of unsaturation on the thermotropic and barotropic phase behavior of phospholipid bilayer membranes, the phase transitions of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dielaidoyl-sn-glycero-3-phosphocholine (DEPC), 1-oleoyl-2-stearoyl-sn-glycero-3-phosphocholine (OSPC), 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), 1-stearoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (SAPC), 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (SDPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer membranes were observed by high-pressure optical method and differential scanning calorimetry. For all of the lipids studied, the temperatures of the so-called main transition between the liquid crystalline (L-alpha) and the lamellar gel (L-beta) phases were almost linearly elevated by pressure. The values of dT/dP for monounsaturated lipids (SOPC, OSPC and POPC) lie around 0.18 K/MPa, which are smaller than that for the saturated lipid DSPC, 0.23 K/MPa. The polyunsaturated lipids (SAPC and SDPC) have rather small values of dT/dP. The temperatures of L-beta/L-alpha transition for DSPC, SOPC, OSPC and DOPC were 55.6, 6.7, 8.7 and -40.3 degrees C, respectively. The substitution of cis unsaturated chain for saturated chains of DSPC brings about the depression of transition temperature by 47 degrees C for sn-1 chain and 49 degrees C for sn-2 chain. The volume change (Delta V) associated with the transitions were calculated from the transition enthalpy (Delta H) and the value of dT/dP by means of the Clapeyron-Clausius equation. The values of Delta V for SOPC, OSPC and POPC, which have an only cis double bond in sn-1 or sn-2 chain, were 18.9, 17.4 and 15.5 cm(3)/mol, respectively. These values are smaller than that for the saturated DSPC, 31.6 cm(3)/mol, and larger than that for the polyunsaturated SAPC, 10.1 cm(3)/mol. It seems that the Delta V for the L-beta/L-alpha transition is obviously dependent on the number of cis double bonds in the acyl chains. With respect to the bilayer membranes of DEPC, SOPC and OSPC, the transition between the lamellar crystalline (L-c) and the L-beta phases was observed; the values of dT/dP were 0.108, 0.093 and 0.105 K/MPa, respectively, which are almost the same. The values of Delta V for the L-c/L-alpha transition of trans unsaturated DEPC and cis unsaturated DOPC were 28.4 and 36.7 cm(3)/mol, and the values of Delta H were 59.2 and 65.3 kJ/mol, respectively. This difference may be directly attributed to the different geometrical configurations of trans and cis double bonds. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.