Interaction of vitamin E with saturated phospholipid bilayers

The effect of α-tocopherol and α-tocopheryl acetate on the physical properties of saturated phospholipid bilayers was studied by differential scanning calorimetry and fluorescence polarization. The addition of α-tocopherol and α-tocopheryl acetate to dimyristoylphosphatidylcholine modifies the thermotropic gel to liquid crystalline transition by decreasing the enthalpy and lowering the transition temperature (T_C). The transition is abolished at 25 mole % of the vitamin E derivative. The same effects are seen with phospholipids with different polar headgroups. These results are consistent with vitamin E being aligned principally with the prevailing direction of the phospholipid acyl chains. The modification of the phase behavior of DMPC can be explained by the methyl substituents of the phytanoyl chain of vitamin E preventing co-crystallization with the all trans gel-phase acyl chains. This perturbation lowers the amount of crystallizable phospholipid and thus lowers the enthalpy. This phenomenon induces the preferential partitioning of the derivative into the fluid phase which results in lowering the transition temperature. Polarization of fluorescence measurements indicates that α-tocopherol perturbs lipid packing even in the liquid-crystalline state. Vitamin E derivatives are slightly more effective than cholesterol in modifying the physical properties of saturated phospholipids. The effects demonstrated here occur at a much higher mole fraction of Vitamin E than is normally seen in biological membranes and thus suggests that only small structural modification would be seen under physiologic conditions.