The Reduction and Quenching of Photoexcited Flavins by Edta

Abstract

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

Effect of Fe-Catalyzed Photooxidation of EDTA on Root Growth in Plant Culture Media

“…EDTA is an effective electron donor for photoreducing flavins (4,5,6,11), and photoreduction of Fe3" to Fe2" by UV and blue radiation is enhanced by certain di-and tricarboxylic acids which can serve as electron donors (1). Moreover, the aerobic reaction products from flavincatalyzed oxidation of EDTA in the light are glyoxylic acid, carbon dioxide, an amine residue, and formaldehyde, which originates from the carbon adjacent to the carboxyl group (4,6).…”

Reactions of lumiflavin with amine radicals

“…The photochemical technique produces equal yields of -FIH and -AH through the sequence of reactions shown in Scheme 1. The values of many of the rate constants for the photophysical steps are known for representative flavins (2,(10)(11)(12)(13)(14), 'The pK of FlH2 is -6.7 and that of .F1H is -8.3 (7). Our equations have been written for pH just above 7. and the overall quantum yield for .FIH formation has been reported for several amines ( I 1 , 12).…”

The dependence of the photochemically induced reduction of lumiflavin on pH, light intensity, and concentration of sulphydryl compounds

“…Flavin molecules are prosthetic groups in a large number of essential oxidation-reduction enzymes (I) and there is a strong interest in the redox chemistry of their ground and excited states (2)(3)(4)(5). Scheme 1 depicts the sequences of reactions which have been postulated to explain products formed on photolysis with light in the wavelength range 400-500 nm.…”

“…Detailed photochemical and flash photolysis experiments (2)(3)(4)(5)(8)(9)(10) have been performed with a large variety of DH? species, so that values of k, have been determined for phenols, amines, amino acids, and certain sulphur-containing compounds, as well as the photophysical constants kr and k,,,.…”