Abstract
It was found significant difference in reactivity of undissociated ascorbic acid AH 2 and its monoanion AH − towards singlet oxygen and in electron transfer to excited photosense. Kinetic analysis indicated that interaction of ascorbate with singlet oxygen in aqueous media results in quenching of the latter with rate constant ( k q+ k r)=(6.8±1)×10 8 mol −1 l s −1, where chemical quenching rate constant k r is 1.4×10 8 mol −1 l s −1. The value of k r for AH 2 is a factor of about 20 lower. The similar trend was observed for ascorbic acid oxidation in electron transfer to excited photosense. The corresponding rate constants were established for ascorbate AH − and were found to be: for total quenching ( k q+ k r) is of order 10 7 mol −1 l s −1 and for radicals photogeneration k r=(2.5±0.5)×10 3 mol −1 l s −1. Analysis of the photosense–ascorbic acid system had revealed the low efficiency of type I pathway, initiated by electron transfer from ascorbate to excited photosense, in comparison to efficiency of the route, mediated by singlet oxygen.
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