ESR studies of chlorophyll one-electron photochemistry in solution: kinetics of reversible quinone reduction and general redox mechanisms

Abstract— ESR studies have been made of the kinetics of semiquinone radical formation and disappearance resulting from the reversible photosensitization of reduction or oxidation, by chlorophyll, pheophytin or hematoporphyrin, of several quinone-hydroquinone pairs in various solvents. The rate of radical decay was found to be second order with respect to the radical concentration in all systems. Radical formation rates were determined by the initial production rate minus the decay rate. The kinetic constants for single electron transfer between triplet porphyrins and quinones or hydroquinones were determined usingβ-carotene as a quencher in aqueous pyridine, and by measuring the initial rate of radical formation at various concentrations of quinones and hydroquinones in methanol and ethanol. These constants were found to be approximately the same in a given solvent for benzoquinone and hydroquinone with all porphyrins, though the rates differed in different solvents: pyridine-water ～ 10⁶I./mole sec, and methanol and ethanol ～ 5X 10⁴l./mole sec. Trimethylquinone and its hydroquinone also give similar rate constants for radical formation in pyridine-water, ～ 10⁶ l./mole sec. The second order radical decay constants for both benzoquinone and hydroquinone in pyridine-water were the same, ～ 10⁵ I./mole sec, with either chlorophyll, pheophytin or hematoporphyrin as sensitizer. The same activation energy, 6900 cal/mole, was found for chlorophyll-benzoquinone and hydroquinone in aqueous pyridine; 5500 cal/mole was obtained for these systems in ethanol. In methanol and ethanol solutions of chlorophyll, the same radical decay rate constants, ～10⁶ I./mole sec, were observed for both benzoquinone and hydroquinone. Also, the same decay constants, ～ 10⁶ I./mole sec, were found for trimethylquinone and its hydroquinine in pyridine-water. These latter two compounds gave extremely small steady-state ESR signals in ethanol compared with aqueous pyridine. We have also observed that the steady-state signal obtained with chlorophyll-menadione in ethanol-water was much enhanced by the presence of NADH. In contrast, NAD⁺ was found to decrease radical production, by increasing the decay rate, in the chlorophyll-hydroquinone system in aqueous pyridine. These results are discussed in terms of possible mechanisms for radical formation and disappearance. The most likely possibility is considered to be a one-electron oxidation or reduction of the porphyrin triplet, followed by radical disproportionation and redox reactions between the disproportionation products.