Characterization of electron donation to cytochrome c-555 in Chromatium vinosum from ferrocyanide, tetramethylphenylenediamine and reduced dimethylquinone. Effects of redox potential, pH and salt concentration

1. The dependences of the reduction of ferricytochrome c-555 in the reaction center-cytochrome c complex on the redox potential and pH were investigated using N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), ferrocyanide, and reduced 2,5-dimethyl-p-quinone as electron donors. 2. In the reduction of cytochrome c-555 by TMPD, the unprotonated form was the exclusive electron donor to the cytochrome with a second-order rate constant of 1.0 X 10(5) M-1.s-1. 3. Ferrocyanide reduced cytochrome c-555 slowly with a rate constant of 7.8 X 10(3) M-1.s-1 at infinite salt concentration. The value of -5.2 X 10(-4) elementary charge/A2 was estimated as the surface charge density in the vicinity of cytochrome c-555 by analyzing the salt effect on the cytochrome reduction using the Gouy-Chapman theory. 4. The characteristics of the dependences of the reduction of cytochrome c-555 by reduced 2,5-dimethyl-p-quinone on the redox potential and pH were well explained by the redox potential and pH dependences of the formation of the semiquinone. In the neutral-to-alkaline pH range the anionic semiquinone was the main electron-donating species with a second-order rate constant of 6.0 X 10(7) m-1.s-1.