Effects of pH and exocyclic substitution on flavosemiquinone reactivity with redox proteins and inorganic oxidants.
The effect of pH on the reaction of free flavosemiquinone analogs generated by laser-flash photolysis with oxidized Chromatium vinosum high-potential iron-sulfur protein, other iron-containing redox proteins, and nonbiological one-electron oxidants has been investigated. The results demonstrate that the second-order rate constant for the oxidation of lumiflavin flavosemiquinone increases dramatically with increasing pH for the redox proteins and some of the other oxidants. The pH-rate constant profiles for the redox proteins closely follow the ionization of the proton at the N-5 position of the neutral lumiflavin flavosemiquinone, suggesting a higher intrinsic reactivity for the anionic lumiflavin flavosemiquinone. This increased reactivity apparently results from changes in the redox potential and in the electron spin density distribution between the two protonic forms of the semiquinone. Similar pH dependencies are observed for a number of other flavin structural analogs, yielding estimates of the N-5 pK values for these analogs. The data are consistent with the involvement of both the N-5-dimethylbenzene ring portion and the C-4a position of the flavin macrocycle in flavosemiquinone oxidation by one-electron oxidants.
D055598:Chemical Phenomena; D002621:Chemistry; D002844:Chromatium; D005292:Ferricyanides; D005415:Flavins; D006863:Hydrogen-Ion Concentration; D007506:Iron-Sulfur Proteins; D007834:Lasers; D008667:Metalloproteins; D010084:Oxidation-Reduction / Q000187:drug effects; D010777:Photochemistry; D011809:Quinones / Q000378:metabolism*; D013329:Structure-Activity Relationship
D005292:Ferricyanides; D005415:Flavins; D007506:Iron-Sulfur Proteins; D008667:Metalloproteins; D011809:Quinones; C001636:flavosemiquinone; C007931:hexacyanoferrate III