PHOTOCHEMISTRY OF FLAVINS—I. CONVENTIONAL AND LASER FLASH PHOTOLYSIS STUDY OF ALLOXAZINE

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Abstract
Abstract— The photobleaching of alloxazine in buffered aqueous solution has been studied by means of flash photolysis using conventional and laser excitation sources. Several transient species have been characterized. The alloxazine triplet state (Λmax 420 nm and 550 nm, times; = 9 μs) was identified with the aid of low‐temperature comparison experiments in ethanol. Transient absorption with Λmax 440 nm, which appears after decay of the triplet state, and whose second‐order decay is pH‐dependent, is postulated to be due to the semiquinone radical (AH2*) and a radical derived from alloxazine by addition of water and loss of a hydrogen atom (HAOH*), which are in equilibrium with their conjugate cation radicals. The results of experiments in the presence of oxygen indicate that these species are not primarily formed from the triplet state. The enhanced second‐order decay of the flavin radicals in oxygen‐containing solutions is interpreted in terms of their reaction with the peroxy radicals. The proposed mechanisms account for the production of hydroxylated alloxazines.