Photochemical and Photophysical Studies of Amines with Excited Flavins. Relevance to the Mechanism of Action of the Flavin-Dependent Monoamine Oxidase

The photochemical reactions between 3-methyllumiflavin (3-MLF) and β,γ-acetylenic, β,γ,δ-allenic, and saturated amines, in aqueous and nonhydroxylic solvents, have been investigated. The fluorescence of 3-MLF in nonhydroxylic solvents is efficiently quenched by all amines studied, with values of k_s (the rate constant for quenching of singlet 3-MLF) near the calculated rate of diffusion. Quenching rates in aqueous solution are pH and concentration dependent and indicate that the free amine is required for the observation of fluorescence quenching. Whereas amine quenching of singlet 3-MLF is nonproductive, quenching of triplet 3-MLF leads to adducts that can be isolated from the reaction of the allenic and β,γ-acetylenic amines but that are unstable in the case of saturated amines. The isolated products from β,γ,δ-allenic amines 2a and 2b are flavocyanines. The reactions of β,γ-acetylenic amines with triplet 3-MLF give more complicated product mixtures that include flavocyanines and C_4a, N₅ adducts in similar amounts. These studies are consistent with a pathway involving one-electron transfer from amine to triplet 3-MLF, followed by successive proton and one-electron transfers leading to reduced flavin and iminium ion intermediates.