On the Mechanism of Flavin-Catalyzed Dehydrogenation α,β to an Acyl Function. The Mechanism of 1,5-Dihydroflavin Reduction of Maleimides

Article abstract of DOI:10.1021/ja00331a047

The dihydroflavin reductions of maleimide (MI) and N-methylmaleimide (MMI) have been investigated using the water-soluble 1,5-dihydro-3-(3-sulfopropyl)lumiflavin (FlH2).The reaction of the dihydroflavin with MI and MMI to yield oxidized flavin (Fl_ox) and succinimides is biphasic.The first phase involves general-acid-assisted nucleophilic attack of dihydroflavin anion (FlH^-) upon the maleimide to yield 4α-substituted 4a,5-dihydroflavin covalent adducts.The Broensted α for the general-acid catalysis is -0.1 to -0.2.This observation would be expected for a Michael addition to FlH^- upon MMI to yield an unstable anionic 4a adduct which is trapped by proton transfer from general-acid species.The rate of decomposition of the intermediate to yield Fl_ox and succinimide is not measurably dependent upon the concentration of the general-base species.The reaction was found to be first order in ->.A mechanism involving preequilibrium ionization of the N(5)-H followed by elimination of succinimide anion (specific-base catalysis) and formation of Fl_ox would require that the pK_a for dissociation of the aniline-like N(5)-H is less than 23 in order that the rate of elimination from the anion does not exceed that for the vibration of the C-C bond that is broken.It is concluded that the HO^--catalyzed elimination represents a concerted process with a Broensted β approaching 1.0.From the results of this study and by involving the principal of microscopic reversibility the mechanism of N-methylsuccinimide anion oxidation by Fl_ox to yield maleimide and FlH^- can be constructed.The possible implications of this mechanism to the mechanism of such dehydrogenating flavoenzymes as succinic acid dehydrogenase, fumarate reductase, acyl-CoA dehydrogenase , and acyl-CoA dehydrogenase, and acyl-CoA oxidase are considered.