Reactions of lumiflavin with amine radicals

Abstract

The amino radicals (•AH) formed by the 60Co radiolysis of N2O-saturated 0,05 M solutions of ethylene diamine tetraacetate (EDTA) at pH 7 and 11.2 and of glycine at pH 11.2 brought about an efficient two-electron reduction of lumiflavin (Fl). The spectra of the products were identical to those formed by photolysis of the same solutions and by reduction of the lumiflavin with •CO2− radicals. The products were reoxidised to flavin by oxygen. The quantum yield for flavin disappearance was 0.52 ± 0.07 and 0.17 ± 0.01 in the presence of EDTA at pH 7 and 11.2 and 0.065 ± 0.008 and 0.17 ± 0.01 for glycine at the same pHs, respectively.The overall two-electron reduction can be explained by the mechanism:[Formula: see text]The rate constants of reaction [4] were found by pulse radiolysis to be 1.8 ± 0.3 × 109 and 1.5 ± 0.3 × 109 M−1s−1 for the radicals of glycine and EDTA at pH 7 and 3.6 ± 0.3 × 108 M s−1 for glycine radicals at pH 11.2. The spectrum of •FlH formed by glycine radicals at pH 7 is similar to that produced by•CO2−, but there was some perturbation, which is apparently due to interaction with the amine.The radicals formed from the secondary amines piperazine and diethylamine at pH 11.8 also effected reversible two-electron reduction. However, the radicals from glycine anhydride and the primary amine ethylamine yielded significant amounts of non-oxidisable products. The reaction mechanisms are discussed and effects of pH are considered.