Determination of the kinetic pKa of a flavin 4a-pseudobase from a study of the hydrolysis of 4a-hydroxy-5-ethyl-3-methyl-lumiflavin in H2O. The oxygen donation potential of 4a-hydroperoxyflavins

Abstract

Below pH ca. 8.5, 4a-hydroxy-5-ethyl-3-methyl-lumiflavin (2) is converted into the spiro(imidazo-4,2-quinoxaline)(3), but between pH 8.5–10.5, the product is 1,6,7-trimethyl-4-ethylquinoxaline-2,3-dione (4) accompanied by a small amount of (3); under strongly basic conditions the product (4) is accompanied by a trace of a substance tentatively identified as an imidazo[4,5-b]quinoxalinone (6); from the pH dependence of the hydrolysis of (2) the kinetic pK_app= 9.9 for the dissociation of the 4a-hydroxy proton.