Effect of pH on Oxidation-Reduction Potentials of 8α-N-Imidazole-Substituted Flavins

The pK_a values for the various ionic forms of 8α-N-imidazolylriboflavin were determined in its oxidized and hydroquinone forms and estimated for its semiquinone form. The pH dependence of the absorption and fluorescence spectral properties and potentiometric titration data show the pK_a values for the oxidized form to be 6.02 ± 0.03 for the 8α-imidazole nitrogen and 9.67 ± 0.05 for the N(3) position of the flavin ring. The pH dependence of the oxidation-reduction potential was determined by spectrocoulometric titrations, and the data points were compared with computer-simulated plots. Two pK_a values for the hydroquinone form of the flavin were determined and assigned. The pK_a for the imidazole ring is found to be 6.9 ± 0.1 and for the N(l) position of the flavin hydroquinone is found to be 5.5 ± 0.1. Analysis of the pH dependence of the one-electron couples E2 (flavoquinone/flavin semiquinone, Fl_0X/F1.) and E1 (flavin semiquinone/flavin hydroquinone, Fl./Fl_red) resulted in an estimated pK_a of 6.5 for the 8α-imidazole ring in the flavin semiquinone form. These data show the possible involvement of the ionization of the 8α-imidazole substituent in the redox chemistry of flavoenzymes containing either an 8α-N¹- or an 8a-N³-histidyl-linked covalent flavin coenzyme. Future work on oxidation-reduction potentials of this class of enzymes must take into consideration the influence of the 8α-histidyl substituent.