INTERACTION BETWEEN ALPHA-TOCOPHEROL AND HETEROACID PHOSPHATIDYLCHOLINES WITH DIFFERENT AMOUNTS OF UNSATURATION

Differential scanning calorimetry was used to study the influence of a lpha-tocopherol on the thermotropic properties of model membranes comp osed by a series of heteroacid phosphatidylcholines with different amo unt of unsaturation in the sn-2 chain. The effect of alpha-tocopherol on 1,2-distearoylglycerophosphocholine (18:0,18:0), 1-stearoyl-2-oleoy lglycerophosphocholine (18:0,18:1), 1-stearoyl-2 linoleoylglycerophosp hocholine (18:0,18:2), 1-stearoyl-2-linolenoylglycerophosphocholine (1 8:0,18:3) and 1-stearoyl-2-arachidonoylglycerophosphocholine (18:0,20: 4) was determined. The addition of alpha-tocopherol perturbed the ther motropic gel to liquid-crystalline phase transition of these phospholi pids. alpha-Tocopherol broadened the endotherm, lowered the transition temperature and decreased the associated enthalpy change. Partial pha se diagrams showed the presence of fluid immiscibilities giving rise t o lateral phase separation of domains containing different amounts of alpha-tocopherol. We suggest that, in these alpha-tocopherol-rich doma ins, the influence exerted by the vitamin on the phospholipids is stro ng enough to alter their thermotropic properties such that an addition al endotherm appears in the thermogram, a characteristic not observed in homoacid phosphatidylcholines. alpha-Tocopherol caused a concentrat ion-dependent removal of the detectable phase transition in all cases. The magnitude of the influence of alpha-tocopherol on phospholipid wa s dependent on the degree of unsaturation of the sn-2 acyl chain. Thes e results are explained on the basis of the effect of alpha-tocopherol which will reduce the differences between gel and liquid crystalline states, the magnitude of these differences depending on the type of ph ospholipid considered, which are probably related to the change of mol ecular shape of phosphatidylcholines containing a polyunsaturated acyl chain.