The polyisoprenoid chain length influences the interaction of ubiquinones with phospholipid bilayers

The interaction between ubiquinone homologues with polyisoprenoid chain lengths varying from 3 to 10 units and dipalmitoylphosphatidylcholine bilayers has been examined by differential scanning calorimetry and wide angle X-ray diffraction analysis. Decreasing the polyisoprenoid chain lengths of ubiquinone in mixed dispersions with phospholipid in mol ratios of about 10 mol% caused a decrease in the gel-liquid crystalline phase transition temperature of the phospholipid and a broadening of the transition. Enthalpy measurements showed that most of the phospholipid (greater than 92%) was involved in the transition endotherm and the formation of a gel phase was also confirmed by the presence of a sharp X-ray reflection of 0.42 nm. These results are consistent with a model in which all of the ubiquinone homologous ultimately undergo a phase separation from phospholipid molecules entering a gel phase on cooling below the phase transition temperature. Reducing the length of the polyisoprenoid chain alters the amphipathic balance of the ubiquinone molecules and is reflected in the tendency of shorter chain ubiquinones to intercalate between the phospholipid molecules upon reheating through the main phase transition.