Fluorescence Probe Studies of the Distribution of Ubiquinone Homologues in Bilayers of Dipalmitoylglycerophosphocholine

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Abstract
Fluorescent probes have been used to examine the effect of ubiquinones with varying polyisoprenoid chain lengths varying from 3–10 units on the thermotropic properties of dipalmitoylglycerophosphocholine bilayers. The quenching of fluorescence from n‐(9‐anthroyloxy) fatty acids (n= 9 or 12 for stearates; n= 16 for palmitate) added to the dispersion by the benzoquinone substituent of ubiquinone was also used as a method of establishing the mode of interaction of the different homologues with the bilayer structure. Fluorescent intensity and polarisation values showed that ubiquinone homologues with polyisoprenoid substituents shorter than six units tended to perturb the thermotropic properties of hydrocarbon domain whereas longer‐chain homologues were without effect when present in proportions of up to about 5 mol %. The short‐chain homologues also caused extensive fluorescence quenching of the probes suggesting that they were accessible to the interior hydrocarbon domain of the structure and tended to be more concentrated in this region at temperatures above compared to those below the gel‐liquid crystalline phase transition of the phospholipid. Ubiquinones with polyisoprenoid chains longer than 6–7 units did not cause fluorescence quenching of the n‐(9‐anthroyloxy) fatty acid probes either in gel or liquid crystalline bilayers. Addition of the detergent, Triton X‐100, to phospholipid dispersions induced fluorescence quenching by long‐chain homologues. Ubiquinone‐10 was also found to quench fluorescence of diphenylhexatriene, a probe accessible to all of the hydrocarbon domain of the dispersion. The results suggest that the long‐chain homologues of ubiquinone are located within bilayer dispersions of phospholipid in a region not sensed by the fatty acid probes or that the bilayer structure prevents the mutual orientations of the fluorophore and the benzoquinone substituent required for formation of a quenching complex.