J. Am. Chem. Soc. 117, 2600-2605 (1995)
Abstract:
High-pressure differential pulse voltammetry and cyclic voltammetry were employed to determine the reaction volume associated with electron transfer in cytochrome c and a series of ruthenium-modified cytochromes c. The reduction of Cyt cIII, either in the native or ruthenium-modified form, is characterized by a reaction volume of -14.0 ± 0.5 cm3 mol-1 when measured versus a Ag/AgCl, KCl(sat'd) reference electrode. A detailed study of the reference electrode system resulted in a value of -9.0 ± 0.6 cm3 mol-1 for the contribution to (V-bar) for the net reaction Cyt cIII + Ag(s) + Cl-(aq) --> Cyt cII + AgCl(s) from the reference electrode components Ag(s), Cl-(aq), and AgCl(s). It follows that the absolute molar volume of Cyt cIII exceeds that of Cyt cII by only 5.0 ± 0.8 cm3 mol-1 (µ = 0.1 M, pH = 7), i.e. much less than the value of 24 cm3 mol-1 reported in the recent literature. Reaction volumes for a series of intramolecular electron-transfer reactions of the type trans-(NH3)4RuIII(L)-Cyt cII --> trans-(NH3) 4RuII(L)- Cyt cIII were found to be 31.7 ± 1.2 (L = NH3), 21.1 ± 1.0 (L = isonicotinamide), 23.3 ± 0.6 (L = pyridine), and 18.6 ± 0.4 cm3 mol-1 (L = 3,5-lutidine). This volume increase is mainly assigned to a decrease in electrostriction during the reduction of the ruthenium center and can be correlated with the number of coordinated ammine ligands. It is concluded that cytochrome c undergoes only a small volume change during electron-transfer reactions.