4a,5-Dihydroflavin

Merenyi G, Lind J, Mager HIX, Tu S-C. Properties of 4a-hydroxy-4a,5-dihydroflavin radicals in relation to bacterial bioluminescence. J Phys Chem 1992 96 10528-33. 

C. Kemal and T. C. Bruice (1976), Simple synthesis of a 4a-hydroperoxy adduct of a 1,5-dihydroflavine Preliminary studies of a model for bacterial luciferase. Proc. Nat. Acad. Sci. USA 73, 995-999. 

Eckstein, J.W and Ghisla, S., On the mechanism of bacterial luciferase. 4a,5-Dihydroflavins as model compounds for reaction intermediates, in Flavins and Flavoproteins, Curti, B., Ronchi, S., and Zanetti, G., Eds., Walter deGruyter, Berlin, 1991, p. 269. 

Next to isomerism, the planarity of a dihydroflavin is the most important structural problem (95). For 1,8- and 4a,5-isomers this question is relatively easy to answer in that these chromophores are planar, apart from the one tetrahedral carbon center where the hydride equivalent is localized. In the 1,5-isomer, or flavohydroquinone, however, the two electron equivalents of reduction are delocalised to an extent which depends on the pyramidality of the N-centers 5 and 10. And 

Eckstein JW, Hastings JW, Ghisla S. Mechanism of bacterial bioluminescence 4a,5-dihydroflavin analogs as models for luciferase hydroperoxide intermediates and the effect of substitutions at the 8-position of flavin on luciferase kinetics. Biochemistry 1993 32 404-11. 

Thiols may be oxidised to disulphides by the intermediate formation of covalent 4a flavin-sulphur adducts subsequent nucleophilic attack by another thiol anion displaces dihydroflavin as a good leaving group (XXXXIV). 

Figure 4.2. The flavin coenzymes, flavine adenine dinucleotide/dihydroflavine adenine dinucleotide (FAD/FADH2), and flavin mononucleotide/dihydroflavin mononucleotide (FMN/FMN). The locations of N(5), N(10), and C(4a) are indicated. The groups Q are important only for binding and orientation in

As true reduced flavins , however, and apart from flavohydroquinone, one example of each subclass needs discussion, viz. 4a,5-dihydro-flavin( 5-blocked ) 181,182)and l,8-dihydroflavin( l/2a-blocked ) 12). Both isomers exhibit chromophores which are essentially different from flavohydroquinone, as outlined in Scheme 9. The three dihydro isomers exhibit, furthermore, totally different chemical reactivity 1,8-dihydro-flavin is aromatic with respect to electron distribution in the center (pyrazine) subnucleus, 1,5-dihydroflavin is, contrarily, antiaromatic (unless the pyrazine subnucleus is deplanarized, see below). It contains 

Of the various proposals for the mechanism of activation of molecular oxygen by flavoprotein oxygenases, we have chosen to believe that oxygen reacts with dihydroflavin to yield a 4a-hydroperoxyflavin (Equation 32). The inability to detect 4a-FlHOOH in the reaction of 

The peroxy-anion transfer reaction of Equation 45 has been established (51). This finding was intriguing because we had shown previously that FlEt- reacts with molecular oxygen to provide 4a-FlEtOO" (Equation 37). Combination of Equations 37 and 45 yields Equation 46, establishing that the dihydroflavin anion is a catalyst for the reaction of oxygen with the phenolate ion. 

Finally, the useful subscripts ox and red must be clearly defined as to their correlation either with true redox states or with flavin chromophores. For example, the addition of water to an alkyl-flavo-quinonium ion 5-RFltx 181) leads to a dihydroflavin chromophore, though the redox state did not change The true redox state of 5-RFl-4a-OH formed in this fashion is indicated by the oxygen atom, though it stems from hydroxyl ion. The picture becomes indeterminate in the case of 

The functional end of the flavin coenzymes FMN and FAD is the tricyclic isoalloxazine system, with the numbering system shown in structure I, the air-stable, yellow, oxidized form. The other two functionally important redox states are the one-electron-reduced semiquinone, II (pKa = 8.4 for dissociation at N(5)), and the two-electron-reduced, colorless dihydroflavin, III. In the dihydro form N(5), C(4a), C(la), andN(l) form a diaminoethylene system and it was anticipated that nitrogen at the 5 and 1 positions would be key to coenzymatic function. 

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