Basicity, Visible Spectra, and Electron Spin Resonance of Flavosemiquinone Anions

The dependence on pH of the disproportionation of the flavosemiquinone has been investigated by ESR. For 3-alkylated lumiflavin [R(3)=CH2COO−] in aqueous solution pK (for the dissociation of the neutral flavosemiquinone) is 8.4; pKred is 6.3. Hydrolytic ring cleavage in position 2, 8,8′-dimerization, and 10-dealkylation interfere with the formation of the flavosemiquinone anion. However, in non-aqueous dimethylformamide and with stoichiometric amounts of strong base (t-BuOK), 100% flavosemiquinone anion is formed without interfering side reactions. Its light absorption spectrum is practically identical to that of the “red flavoprotein radical”. From the ESR spectra the individual isotropic hyperfine coupling constants have been determined.