Photo-addition reactions in the benzenoid subnucleus of flavoquinones.

It has been shown by TE NIJENHUIS and BERENDS2 that the benzenoid (or better: o-quinoid) subnucleus of alloxazines can be attacked photochemically. We found that with flavins the reaction products bear a hydroxyl group either in the 6 or in the 9 position of the benzenoid nucleus. The reactive species is the protonated flavin, the rate of the reaction has an optimum at pH 0 — 1. At still lower pH dynamic proton quenching decreases the rate. The fastest reaction is given by formylmethylflavin (1) which is fully protonated at pH 1 (pK = 3 .5 2 ) and by 1,10-substituted flavinium salts (Schemel, No. 1 and 2 ) .