Location of ubiquinone homologues in liposome membranes studied by fluorescence anisotropy of diphenyl-hexatriene and trimethylammonium-diphenyl-hexatriene.

The measurements of diphenyl-hexatriene (DPH) and trimethylammonium-diphenyl-hexatriene (TMA-DPH) fluorescence anisotropy in dipalmitoylphosphatidylcholine (DPPC) and egg yolk lecithin (EYL) liposomes containing different concentrations of various ubiquinone (UQ) homologues have been performed. UQ-4 induced the highest DPH anisotropy increase in DPPC liposomes, whereas for higher UQ homologues the anisotropy was lowered with the increase of UQ side-chain length. These differences were less pronounced in EYL liposomes. It was concluded that at a higher content in the membranes (3-4 mol%), the short-chain ubiquinones are arranged parallel to lipid fatty acid chains, whereas long-chain homologues are progressively removed from the lipid acyl chains into the midplane region of the membrane. At the lower (1-2 mol%) concentrations, long-chain quinones seem to be evenly distributed within the membrane, especially in EYL membranes. UQ-10 in EYL liposomes perturbed TMA-DPH to a similar extend as the short-chain ubiquinones indicating that UQ-10 penetrates the interface regions of the membrane where its redox reactions occur. The localization and physical state of UQ-10 in native membranes is discussed.