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Riboflavin was irradiated anaerobically in aqueous EDTA solutions over the pH range 2.5-10. In other dye systems (Bonneau and Pereyre, 1975), only the trivalent anion of EDTA was found to have significant reactivity for photoreduction. For riboflavin. the reactivity begins with monoanionic EDTA. and the reactivity is markedly increased as the charge increases. This suggests that the charge on the reductant is more important to the electron transfer process for riboflavin than the formation of a nonhydrogen bonded nitrogen site on EDTA. At high concentrations of EDTA in the pH range 4-8, quenching of the photoreduction occurs, which can be explained by an energy transfer between the excited singlet state of riboflavin and trianionic EDTA, possibly as an association complex. The rate constants for the photoreduction of riboflavin by the monovalent, divalent, and trivalent anions of EDTA are 1.0 x lo' s -l , 4 . 8 x lo' M-'s-l, and 2.0 x I O * M -' s -' , respectively. The rate constant for the singlet state quenching by trianionic EDTA is 3 x 10'M-l s -' , and the limiting quantum yield for intersystem crossing for riboflavin in aqueous solution is 0.50 + 0.05. INTRODUCHON

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Acknowledgment. We are indebted to Professor P. Courtot for a collaboration that triggered this work. We thank Dr.. T. Indelli for her assistance in the laser work and for helpful discussions. We are also grateful to Professors F. Fagioli and A. De Battisti for help with elemental analyses and cyclic voltammetry. The work was supported by the Commission of the European Communities (Contract ESD-026-I) and by the Consiglio Nazionale delle Ricerche (Progetto FinalizzatoChimica Fine e Secondaria, Temática Ahl).