The Photoreduction of Flavins by Amino Acids and Edta. A Continuous and Flash Photolysis Study

Abstract

Abstract— Primary and secondary photochemical processes in oxygen‐free aqueous solution have been characterised for FMN alone and in the presence of EDTA and four amino acids using nanosecond and microsecond flash photolysis and continuous photolysis techniques. The relative contributions of oneelectron and two‐electron (group or hydride transfer) reactions to the deactivation of the triplet has been determined by comparing the radical concentration (560 nm) with the bleaching of the ground state (446 nm). It was concluded that one‐electron reactions (hydrogen atom or electron abstraction) are the major mode of reactivity of the flavin triplet state with all the suhstrates studied. The nature of the reactions of the flavin semiquinone radical have been studied quantitatively by microsecond flash photolysis. These secondary reactions consist of either a ‘back reaction’ between the flavin and substrate radicals (tryptophan or glycyl‐tyrosine) or the transfer of a second electron (or hydrogen atom) from the substrate radical to the flavin radical (EDTA, methionine and possibly cysteine) to form reduced flavin and oxidised substrate. From a comparison of the quantum yields of formation of reduced flavin using ‘flash’ and continuous irradiation, an additional pathway for the decay of the flavin radical is suggested to occur at low light intensities in the presence of glycyl‐tyrosine or histidine.

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

“…1 b. This feature, the isobestic point at 333 nm and the nature of the spectral changes are in accord with the formation of dihydroflavin, a conclusion which also conforms with the earlier study of Heelis et al (3).…”

“…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,,,.…”

Photo-induced reduction of flavin mononucleotide in aqueous solutions

“…Photo-activation of flavins in air-saturated solutions with reducing agents converts them during light exposure to the semi and fully reduced form, but then they get re-oxidized in the dark. Under anaerobic conditions stable photo-reduction to the fully reduced form is achieved [14,16,[20][21][22][23][24][25][26][27][28][29] even in the absence of exogenous reducing agents [22,24,29]. The involved mechanisms in the photo-reduction of flavins are discussed in [20][21][22][23][24][25][26][27][28][29][30][31][32].…”

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).…”