Reactions of lumiflavin with amine radicals

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Abstract
The amino radicals (.cntdot.AH) formed by the 60Co radiolysis of N2O-saturated 0.05 M solutions of EDTA at pH 7 and 11.2, and of glycine at pH 11.2 brought about an efficient 2 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 .cntdot. CO2- radicals. The products were reoxidized 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 pH, respectively. The overall 2 electron reduction can be explained by the mechanism: [4] .cntdot.AH + Fl .fwdarw. .cntdot.FIH + A [5] .cntdot.FlH + .cntdot.FlH .fwdarw. Fl + FlH- H+ [6] .cntdot.FlH + .cntdot.AH .fwdarw. FlH- + A + H+. The rate constants of reaction [4] were found by pulse radiolysis to be 1.8 .+-. 0.3 .times. 109 and 1.5 .+-. 0.3 .times. 109 M-1 s-1 for the radicals of glycine and EDTA at pH 7 and 3.6 .+-. 0.3 .times. 10-8 M s-1 or glycine radicals at pH 11.2. The spectrum of .cntdot.FlH formed by glycine radicals at pH 7 is similar to that produced by .cntdot.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 2 electron reduction. However, the radicals from glycine anhydride and the primary amine ethylamine yielded significant amounts of non-oxidizable products. The reaction mechanisms are discussed and effects of pH are considered. [These reactions may be relevant to the mechanism of monoamine oxidases.].