Mechanism of Light-induced Reduction of Biological Redox Centers by Amino Acids. A Flash Photolysis Study of Flavin Photoreduction by Ethylenediaminetetraacetate and Nitrilotriacetate

The mechanism of flavin photoreduction by the amino acids, EDTA, and nitrilotriacetate is shown to be due to light-induced charge separation, which is irreversible in the dark. The irreversibility originates from the decarboxylation of the amino acid radical. This fast process changes the redox properties of the radical and makes a further donation of an electron equivalent possible. In the case of EDTA the electron acceptor of the second electron is flavin, which was left unexcited by the flash or is formed by dismutation from the flavosemiquinone, generated in the primary one-electron transfer process. In contrast to this, a mechanism for the flavin photoreduction by nitrilotriacetate is proposed, in which the decarboxylated nitrilotriacetate radical adds to the flavosemiquinone to yield an alkylated flavohydroquinone. The latter decays to free reduced or oxidized flavin, depending on the position of addition at the flavin chromophore. The difference in reaction mechanism between the nitrite anion, EDTA, and nitrilotriacetate is discussed in terms of differences in molecular structure.