The Reduction of Flavins by Borohydride: 3,4‐Dihydroflavin

Abstract

The reduction of flavins by BH 4 − is slow, but strongly catalyzed by light. The main product of the reaction is 1,5‐dihydroflavin, i. e. normal leucoflavin, which is rapidly reoxidized upon admission of air. Under continuous aeration, however, a second reaction, irreversible by oxygen in the dark, becomes dominant yielding a tetrahydroflavin which, upon exhaustive reaction of BH 4 − , is autoxidized to give a new dihydroflavin of structure I. This compound is stable toward further oxidation in the crystalline state and in solution with organic solvents. Compound I has to be considered as a formal 3,4‐dihydroflavin. It is photooxidizable to give starting flavin, reacts very slowly with S 2 O 4 = compared to flavoquinone (II), but is easily reduced by EDTA in the presence of light. The compound was also synthesized by an unequivocal route and found identical with the product obtained by BH 4 − ‐reduction of lumiflavin. The light absorption spectrum of I shows absorption maxima at about 405, 320, 270, and 220 nm and is practically identical with that of the BH 4 − ‐reduction product of the flavoproteins d ‐ and l ‐amino acid oxidase. The BH 4 − ‐reduction products of the amino acid oxidases are catalytically active. From this it follows that position 4 of the flavocoenzyme is not involved in enzymatic dehydrogenation. The first photo‐excited triplet state of I has been investigated by optical and electron paramagnetic resonance spectroscopy. The structure of I was elucidated by means of infrared and nuclear magnetic resonance spectroscopy.