Oxidation of 2-Thioflavins by Peroxides : formation of Flavin 2-S-Oxides

Abstract

The reaction of 2-thioriboflavin (sulfur replacing the oxygen substituent at position 2α) with hydrogen peroxide at pH ~10 leads to a blue flavin (λmax = 565 nm)which was purified in stable, homogeneous form. Titrations of 2-thioflavins with m-chloroperoxybenzoic acid also yield the same blue flavins with consumption of 1 eq of peracid. Anaerobic reduction of the blue flavin by sodium dithionite requires 4e¯ eq, and leads to formation of 1,5-dihydro-2-thioflavin. Oxidation of the latter with O2 restores the original 2-thioflavin. pH titration of the blue flavin shows two pKa values of 2.4 and 6.6, with no apparent ionization in the pH range 8-11. These results suggest that the blue flavin is a flavin 2-S-oxide. The visible absorption spectra of l flavin 2-S-oxides show a pronounced dependence on solvent polarity. This property suggests that these flavin analogs may be useful hydrophilic/hydrophobic probes of flavoprotein active sites. Flavin 2-S-oxides can be oxidized further to the 2-sulfinate and 2-sulfonate analogs, some properties of which are described.