The interaction between 1,2-dipalmitoyl phosphatidylcholine and ubiquinone-10 in aqueous systems was studied by difference i.r. spectroscopy. Binary mixtures of the two lipids in proportions of 2, 5 and 15 mol% were investigated in the spectral regions reporting on the hydrocarbon chains of the phospholipid and the polar phosphate group. No spectral shifts or significant broadening of any absorbances due to the phospholipid were detected at temperatures of 20 or 54 degrees C. Changes in the frequency of the maximum of the CH2 antisymmetric C-H stretching vibration with temperature indicated that the gel-to-liquid-crystal-line phase-transition temperature of the phospholipid was lowered by about 2 degrees C in the presence of between 2 and 15 mol% ubiquinone-10. Absorbance by the benzoquinone substituent of ubiquinone-10 was detected by spectral subtraction of dispersions of phospholipid alone. Bands due to C = O stretching and ester group vibrations of ubiquinone-10 in co-dispersion with phospholipid were compared with the same spectral region when ubiquinone-10 was dissolved in solvents or as a crystalline solid. Spectral changes could be detected when ubiquinone-10 in phospholipid was compared with solution in dodecane and chloroform. These may indicate that the benzoquinone ring system is located within a hydrocarbon domain in dispersions with dipalmitoyl phosphatidylcholine. It was concluded from the study that when ubiquinone-10 is co-dispersed with dipalmitoyl phosphatidylcholine in water the two lipids phase-separate. There is no evidence that ubiquinone-10 intercalates between phospholipid molecules, which undergo a gel-liquid-crystalline phase transition in only a slightly modified form. The data suggest that the benzoquinone substituent resides in a hydrophobic domain and that aggregates spanning the bilayer are a possible arrangement of the ubiquinone in the structure.
