Kinetics and Mechanisms of the 1,5-Dihydroflavin Reduction of Carbonyl Compounds and the Flavin Oxidation of Alcohols. 4. Interconversion of Formaldehyde and Methanol

The mechanism of the kinetically biphasic reduction of formaldehyde by dihydroflavin (ref 9) has been elucidated. In the first and rapid phase, dihydroflavin and formaldehyde yield, via two competing reactions: (a) oxidized flavin plus methanol; and (b) the N(5)-hydroxymethyl derivative plus the corresponding imine. In the slower second phase of reaction the oxidized flavin produced in the first phase acts as a catalyst in the conversion of the N(5)-hydroxymethyl derivative to reduced flavin plus formaldehyde, and these reactants reenter the reactions of the first phase. Concentration and pH dependencies have been investigated and the appropriate rate constants determined. l,5-Dihydro-3-methyllumiflavin is proposed to form an N( 1 )-hydroxymethyl adduct as has been shown (ref 25) for uracils. Methanol has been shown to reduce oxidized flavin at alkaline pH providing as products 1,5-dihydroflavin and formaldehyde in a 1:1 ratio. Previous studies are reviewed (Introduction) and a general radical mechanism presented (Discussion).