ELECTROCHEMICAL SUPEROXIDATION OF FLAVINS: GENERATION OF ACTIVE PRECURSORS IN LUMINESCENT MODEL SYSTEMS

— Using 3‐methyllumifiavin and tetraacetylriboflavin as examples, we have shown that the so‐called “fully oxidized” flavins can be “superoxidized” at an anodic potential of 1.8 to 1.9 V giving flavin radical cation transients which are rapidly transformed in subsequent chemical reactions. An attack by H 2 O subsequent to the superoxidation of 3‐methyllumifiavin provides a route for the formation of 4a‐hydroxy‐3‐methyllumiflavin radical cation, as evident from the subsequent decomposition to the protonated form of the starting flavin. When 3‐methyllumiflavin is superoxidized in the presence of a base, a recycling process occurs, allowing superoxidized flavin to be trapped in a slower, competitive conversion. The relatively more stable trapped product is active in reacting with H 2 O 2 to emit chemiluminescence. Electrochemical oxidation of H 2 O 2 in acetonitrile at 1.30 V in the presence of an oxidized flavin results in a direct protonation of the flavin by H+ generated from the electrolysis of H 2 O 2 . Minor reactions presumably provide alternative formations of the 4a‐hydroperoxy‐ and 4a‐hydroxy‐flavin radical cation transients by the direct addition of HOO and HO‐ radicals, which also arise in the oxidation of H 2 O 2 , to protonated flavin. Under such conditions the superoxidized flavin radical cation is apparently also formed, either directly or by process(es) such as decomposition of the flavin 4a‐adduct radical cations. Subsequent reductions of either the superoxidized flavin or the flavin 4a‐adduct radical cations lead to an almost steady level of luminescence. The superoxidized flavin, the trapped product, and the N 5 ‐unprotected, 4a‐derivatized flavin radical cations are considered valuable models in further studies on the validity of the chemically initiated electron exchange luminescence mechanism in flavin‐mediated luminescence reactions, including bacterial bioluminescence.