Properties of flavins where the 8-methyl group is replaced by mercapto- residues

Sulfur functions in position 8 of the flavin nucleus give rise to new modified flavin derivatives, which should prove useful as probes of the flavin binding domains of flavoproteins. Here, we report on some properties of 8-nor-8-alkylmercaptoflavins and 8-nor-8-mercaptoflavin which are readily formed by nucleophilic displacement by alkylmercaptides or sulfide, with 8-nor-8-chloroflavins as starting material. The new flavins are characterized by extensive shifts in spectral properties, with very high extinction coefficients. 8-nor-8-mercaptoriboflavin is easily and reversibly converted to its (-S-S-) dimer. Oxidation of the sulfur group by peracids forms first sulfoxides and then sulfones, in which the characteristic usual flavin spectrum is regained. A comparison of 8-SR-8-nor-flavins with 8-OR-8-nor-flavins (Ghisla, S., and Mayhew, S.G. (1976) Eur. J. Biochem 63, 373-390) indicates that in both classes of compounds, optical properties, ionization constants, and oxidation-reduction potentials follow similar patterns.