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

Abstract

Multichannel detection with μs 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 (F v), 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 F vH are ascertained. Under the experimental conditions pertaining to the present investigation (solute concentration (7.5±2.5)×10 −5 M, ca. 10% conversion into triplets), quenching of F v † (a riboflavin triplet) by F v 0 (an unexcited riboflavin molecule) was found to be insignificant; most triplets appear to decay through triplet–triplet annihilation, the loss of two triplets being accompanied by the gain of one neutral semiquinone radical (F vH ). It is proposed that (i) triplets deactivate mainly through triplet–triplet annihilation, (ii) the annihilation event leads to the formation of an ion pair, F v †+F v †→F v ++F v −, (iii) the anion rapidly converts to F vH , and (iv) the cation ejects a proton and splits into two neutral products, a molecule whose ground-state absorption spectrum resembles that of F v 0, and an odd-electronic species containing the remainder of the aliphatic side chain.