Interaction between alpha-tocopherol and heteroacid phosphatidylcholines with different amounts of unsaturation

Differential scanning calorimetry was used to study the influence of a alpha-tocopherol on the thermotropic properties of model membranes composed by a series of heteroacid phosphatidylcholines with different amount of unsaturation in the sn-2 chain. The effect of alpha-tocopherol on 1,2-distearoylglycerophosphocholine (18:0,18:0), 1-stearoyl-2-oleoylgylcerophosphocholine (18:0,18:1), 1-stearoyl-2-linoleoylglycerophosphocholine (18:0,18:2), 1-stearoyl-2-linolenoylglycerophosphocholine (18:0,18:3), and 1-stearoyl-2-arachidonoylglycerophosphocholine (18:0,20:4) was determined. The addition of alpha-tocopherol perturbed the thermotropic gel to liquid-crystalline phase transition of these phospholipids. alpha-Tocopherol broadened the endotherm, lowered the transition temperature and decreased the associated enthalpy change. Partial phase diagrams showed the presence of fluid immiscibilities giving rise to lateral phase separation of domains containing different amounts of alpha-tocopherol. We suggest that, in these alpha-tocopherol-rich domains, the influence exerted by the vitamin on the phospholipids is strong enough to alter their thermotropic properties such that an additional endotherm appears in the thermogram, a characteristic not observed in homoacid phosphatidylcholines. alpha-Tocopherol caused a concentration-dependent removal of the detectable phase transition in all cases. The magnitude of the influence of alpha-tocopherol on phospholipid was dependent on the degree of unsaturation of the sn-2 acyl chain. These 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 phospholipid considered, which are probably related to the change of molecular shape of phosphatidylcholines containing a polyunsaturated acyl chain.