Studien in der Lumiflavin-Reihe VI. Alkylierungs- und Desalkylierungs-Reaktionen an (Iso)alloxazinen; 1,3,10-Trimethylflavosemichinone

Mechanism of bacterial bioluminescence: 4a,5-dihydroflavin analogs as models for luciferase hydroperoxide intermediates and the effect of substituents at the 8-position of flavin on luciferase kinetics

Bioluminescence catalyzed by bacterial luciferases was measured using FMN, iso-FMN (6-methyl-8-nor-FMN), and FMN analogs carrying the following substituents at position 8: -H, -Cl, -F, SMe, SOMe, -SO2Me, or -OMe. The first-order rate constants for the decay of light emission correlate with the one-electron oxidation potentials of the 4a,5-dihydro forms of the FMN analogs. To determine the values of these potentials, isoalloxazine (flavin) derivatives having the 4a,5-propano-4a,5-dihydro structure and -H, -CH3, -Cl, -OCH3, and -NH2 as substituents at position 8 have been synthesized as models for the 4a-peroxy-4a,5-dihydroflavin intermediates occurring during catalysis by the flavin-dependent monooxygenase luciferase. The tetrahydropyrrole ring between positions 4a and 5 of these isoalloxazine derivatives stabilizes the 4a,5-dihydroflavin by impeding formation of the thermodynamically more stable 1,5-dihydro form. One-electron oxidation potentials (Eobs) were measured by cyclic voltammetry and used to determine the empirical coefficients in the Swain equation. On the basis of this, the one-electron oxidation potentials of 4a,5-propano-4a,5-dihydro analogs with other substituents in position 8 were calculated (Ecalc). The bioluminescence reaction rate is fastest with FMN analogs of lowest oxidation potential; i.e., the slope of the correlation is negative. This indicates that in the rate-limiting step the 4a,5-dihydroflavin moiety donates negative charge. The results are compatible with an intramolecular, chemically initiated electron exchange luminescence mechanism for the bacterial luciferase reaction.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between flavins and alcohols

The effect of alcohols on the spectral properties of riboflavin derivatives in non-polar solvent was studied by various spectroscopic methods in order to support the view point that alcohol may directly interact with the isoalloxazine moiety of FAD and enhance the catalytic activity of D-amino acid oxidase (DAAO). The most likely association complex between alcohol and riboflavin is 1 : 1 stoichiometric complex through the 3-N imino and the 2-C carbonyl groups of the isoalloxazine ring and the hydroxyl group of alcohols. It appears that methanol has a larger association constant than any other alcohols, and the association constant decreases with the increase in carbon number and with the steric requirement of the alkyl group of alcohols.

(Photo)chemistry of 5-deazaflavin. A clue to the mechanism of flavin-dependent (de)hydrogenation

The catalytic action of 5-deazaflavin in the photochemical reduction of flavin and iron proteins [Massey, V. and Hemmerich, P. (1978) Biochemistry, 17, 9--17] is shown to be due to the highly reactive 5-deazaflavosemiquinone. This radical is generated in a complex sequence of reactions, which involves (a) covalent photoaddition of the substrate residue to the deazaflavin, (b) fast secondary photoreaction of this adduct with starting deazaflavin to yield a covalent radical dimer, accompanied by the liberation of the oxidized substrate, and (c) deazaflavin-sensitized cleavage of the radical dimer to the monomers. The structure and properties of this radical (redimerisation or dismutation) and the precursor intermediates as well as the mechanism of the photoreaction are described. Deazaflavins and their natural parent compounds are compared with respect to their different redox behavior and radical stability. The syntheses of 5-deuterated deazaflavins are described and their redox reactions are compared with those of normal deazaflavins.

Light-mediated reduction of flavoproteins with flavins as catalysts

It has been found that small amounts of free flavins greatly accelerate the photochemical reduction of flavoproteins both to the radical and fully reduced oxidation states. This catalytic effect has been shown to be due to the rapid photochemical reduction of the free flavin to its fully reduced state, followed by its reaction with the flavoprotein to yield flavoprotein radical and by its reaction with flavoprotein radical to yield fully reduced flavoprotein. Evidence is presented that the same route may occur with flavoproteins in the absence of added flavins. In this case the photoreduction is mediated by the small equilibrium concentration of free flavin coenzyme present in a flavorprotein solution. Hence, it is suggested that flavoprotein reduction with EDTA as photosubstrate does not involve an excited state of the holoprotein, nor contact of EDTA with the enzyme, but exchange of electrons between enzyme flavin and free reduced flavin.