Nucleophilic addition at the photoexcited flavin cation: synthesis and properties of 6- and 9-hydroxy-flavocoenzyme chromophores.

Abstract

1 The method of “photohydroxylation” of azaheteroaromatics, known in the phenazine and alloxazine series, has been applied to flavin (= isoalloxazine) derivatives FloxH. The products habe been identified as 6- and 9-hydroxyflavins (Flox-6/9-OH). On this basis it was possible to assign a 6-hydroxyflavin structure to the new flavocoenzyme chromophores described by Mayhew et al. in the accompanying paper. 2 In agreement with the data obtained by Dekker et al. in a recent laser study on alloxazine, we find that the reaction proceeds via addition of water, alcohols or carboxylic acids by the lowest excited singlet state of flavin with synchronous intramolecular hydrogen shift from the site of RO-addition [= C(6) or C(9)] towards N(5) and subsequent oxidation of the 6/9-hydroxy-1,5-dihydroflavin formed. 3 The photoaddition reaction occurs in competition with photodehydrogenation of available CH-bonds by the lowest excited flavin triplet, in which case the CH-substrate may be provided by the solvent ROH or, when R = H, by methyl groups of a second flavin molecule (= flavin autophotolysis). This photodehydrogenation may be suppressed by O2-dependent triplet quenching and/or by applying a positive charge to the flavin (RFIOxH+, R = H or alkyl) in order to inhibit the reaction with itself. 4 Differentiation of 6- and 9-hydroxyflavin isomers was possible by double resonance evaluation of the proton coupling between the residual Ar-H and the adjacent methyl group, by evaluation of the electron paramagnetic resonance hyperfine coupling for the residual Ar-H in the radical cations 1,5-RHFl-6/9-OH, and by formation of cupric chelates from 6-hydroxy-flavin as compared to the lack of metal affinity in the 9-isomer. 1H-Nuclear magnetic resonance, electron paramagnetic resonance and visible spectra and their dependence on the state of ionisation of hydroxyflavins are discussed. Hydroxyflavin anions as well as their N(1)-alkylated meso-ionic derivatives are shown to exhibit characteristic long-wavelength absorption bands ranging from 50 to 800 nm with maxima around 600 nm and low molar absorption coefficients between 500 and 3000 M−1cm−1 (as compared to ∼ 104 for the first π, π* transition at ∼ 400nm), which might be readily mistaken for “flavin-substrate charge transfer” bands. 5, 9-Methoxy-flavins could be spectroscopically related to 9:10-bridged cyclic hemiacetals obtained upon ring closure of 9-hydroxy groups with 10β-carbonyl functions. Such cyclic hemiacetals can be trapped in the acid photolysis of 10-(hydroxyalkyl)-flavins, e.g. riboflavin or flavocoenzymes, in a sequence of 10-side-chain dehydrogenation and N(1)-cycloaminal cation formation of the 10β-carbonyl formed, which results in a quenching of the further photolysis in favor of 9-photoaddition.