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

Multichannel detection with mu s time resolution has been used for recordin g flash-induced changes in the absorbance of deaerated solutions of ribofla vin in phosphate buffer at pH 6.8. By comparing the spectra of solutions co ntaining, in addition to riboflavin (F-v), different substrates (ascorbic a cid, EDTA, indole acetic acid, tryptophan), and by recording the absorption spectrum of the neutral tryptophyl radical (produced by flash photolysis o f aqueous solutions of tryptophan), the absorption spectrum and the extinct ion coefficient of FvH* are ascertained. Under the experimental conditions pertaining to the present investigation (solute concentration (7.5 +/- 2.5) x 10(-5) M, ca. 10% conversion into triplets), quenching of F-v(dagger) (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 on e neutral semiquinone radical (FvH*). It is proposed that (i) triplets deac tivate mainly through triplet-triplet annihilation, (ii) the annihilation e vent leads to the formation of an ion pair, F-v(dagger)+ F-v(dagger)-> F-v( +) + F-v(-), (iii) the anion rapidly converts to FvH*, and (iv) the cation ejects a proton and splits into two neutral products, a molecule whose grou nd-state absorption spectrum resembles that of F-v(0), and an odd-electroni c species containing the remainder of the aliphatic side chain. (C) 1999 El sevier Science B.V. All rights reserved.