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Abstract— The coenzyme ubiquinone, an isoprenoid benzoquinone present in the electron-transport chain of mitochondria, has been studied using nanosecond laser flash photolysis and pulse radiolysis. The hitherto undetected triplet excited state of the coenzyme has been identified and some of the physico-chemical properties determined. These measurements may assist the understanding in molecular terms of the degradative action of light upon biological materials, photophosphorylation and the possible initiation of biological electron transport via quinone light absorption.

Laser photolysis of ubiquinone in cyclohexane and pulse radiolysis of ubiquinone in benzene results in the formation of a transient absorption with maximum around 440 nm and a half-life of 650 nsec in cyclohexane and 450 nsec in benzene. Energy transfer sensitisation of the β-carotene triplet absorption by ubiquinone in cyclohexane at a rate consistent with the life-time of the 440 nm transient absorption, yields strong evidence that this transient species is triplet ubiquinone. The triplet reacts with oxygen with a rate constant of 2 × 10--9 mole-1 sec-1.

Photolysis studies of ubiquinone in ethanol and isopropanol and addition of ethanol to ubiquinone in cyclohexane show that little ubisemiquinone is formed by reaction of the triplet with alcohols. Electron spin resonance studies support this conclusion, and also show that some ubisemiquinone is however formed on photolysis of solutions of ubiquinone in methylcyclohexane.

Energy transfer experiments in the presence of various triplet energy donors and acceptors suggest that the triplet energy of ubiquinone lies between 176 and 123 W mole-1, and that the triplet extinction coefficient at 440 nrn is 19,000 mole-1 cm-1 in cyclohexane and 13,000 mole-1 cm-1 in benzene (at 430 nm).

The singlet to triplet crossover efficiency for ubiquinone in cyclohexane was estimated to be 0.04. The low triplet energy level, crossover efficiency and abnormal type of reaction with alcohols are reflections of the profound influence of the isoprenoid chain upon excited states of this quinone.