The interaction of alpha-tocopherol with phosphatidylserine vesicles and calcium

The interaction of alpha-tocopherol with dimyristoylphosphatidylserine (DMPS) has been studied in the presence and in the absence of Ca2+ by using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR) and 45Ca2+-binding. In the absence of Ca2+, DSC showed that alpha-tocopherol decreases the temperature of the lamellar gel to lamellar liquid crystalline phase transition as well as it decreases delta H of this transition. Two different peaks were detected at 10 mol% of alpha-tocopherol and probably one of the peaks correspond to pure or nearly pure DMPS and the other to DMPS incorporating most of the alpha-tocopherol. The phase transition was totally abolished at 30 mol% of alpha-tocopherol. In the presence of Ca2+ this L(beta) to L(alpha) phase transition of DMPS was even more perturbed by alpha-tocopherol, so that it was totally abolished by only 7 mol% of alpha-tocopherol, at Ca2+ concentrations which were clearly non-saturating, like those giving DMPS/Ca2+ molar ratio of 4:1 and 10:1. Furthermore, the transition of the DMPS/Ca2+ complex observed at 91.6 degrees C was perturbed by the presence of alpha-tocopherol, indicating a change in the structure of the crystalline complex. The FT-IR analysis of the effect of alpha-tocopherol on DPMS phase transition confirmed the decrease in the phase transition temperature of the phospholipid, and also that alpha-tocopherol increases the number of gauche isomers in the gel state but has no effect in the liquid crystalline state. The binding of 45Ca2+ was also affected by the presence of alpha-tocopherol, so that the number of binding sites was decreased, and this may be interesting for situations in which phosphatidylserine and Ca2+ are simultaneously implicated in biological functions, such as membrane fusion and enzyme activation.