Photochemistry of Flavins—i. Conventional and Laser Flash Photolysis Study of Alloxazine

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 (Amax 420 nm and 550 nm, T = 9 ps) was identified with the aid of low-temperature comparison experiments in ethanol. Transient absorption with A, .. 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 (AH,') 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.

Efficiency of singlet oxygen generation by alloxazines and isoalloxazines

“…The previously reported lifetimes of the triplet states are 12.0 ls for Lch12 and 9.4 ls for alloxazine in aqueous solution at pH 2.2,12 and 13 ls for alloxazine in ethanol. 28 The lifetimes reported by others correspond very well with those in Table 2. Triplet state quantum yields are also reported in Table 2 for the compounds studied.…”

Nucleophilic Addition at the Photoexcited Flavin Cation: Synthesis and Properties of 6- and 9-Hydroxy-Flavocoenzyme Chromophores

“…dH do not yield alkoxyflavins upon illumination, neither in the presence, nor in the absence of RFloxH+ as potential sensitizer. Hence we are left with the choice of assuming nucleophilic photoaddition in an excited state RF1*oxH+ ; preliminary iodide quenching experiments point to the singlet state as the reactive species in agreement with the laser study by Dekker et al [2].…”

“…The photoaddition reaction (Scheme 1) described in the present paper works only under acid condi- The neutral flavoquinone species Flo,H can, however, enter the same type of photoaddition a t pH 2 and 6, as demonstrated by Dekker et al [2] in a recent laser study. These authors were dealing with the parent compound alloxazine which is devoid of all the -CH, groups of the flavin species with which we are dealing and they find the excited singlet lAOxH* to be the photoreactant.…”

Photophysics of dimethyl-alloxazines and 1-methyllumichrome on celluloseDedicated to Professor Frank Wilkinson on the occasion of his retirement.Electronic Supplementary Information available. See http://www.rsc.org/suppdata/cp/b1/b106953e/

“…For example, the possible products include: cation radicals, a semiquinone radical, a hydroxyalloxazine radical, anion radicals, dihydroalloxazine, hydroxyalloxazine, and the fully reduced form. 8,[27][28][29][30][31][32][33][34][35][36][37] For lumichrome in air-equilibrated water, Dekker et al 29 have also suggested the formation of a superoxide and singlet oxygen. The photochemistry on cellulose appears to be more specific, however, it is too early to conclude about the nature of the long-lived transient product or products.…”