Hydrogen abstraction by triplet flavins. I: time-resolved multi-channel absorption spectra of flash-irradiated riboflavin solutions in water

Multichannel detection with microsecond time resolution has been used for recording flash-induced changes in the absorbance of deaerated solutions of riboflavin in phosphate buffer at pH 6.8. By comparing the spectra of solutions containing, in addition to riboflavin, different substrates (ascorbic acid, EDTA, indole acetic acid, tryptophan), and by recording the absorption spectrum of the neutral tryptophyl radical (produced by flash photolysis of aqueous solutions of tryptophan), the absorption spectrum and the extinction coefficient of the riboflavin radical (semiquinone form) are ascertained. Most triplets are shown to decay, under the particular experimental conditions employed in this study, through triplet-triplet annihilation, the loss of two triplets being accompanied by the gain of one neutral semiquinone radical.