Intramolecular and intracomplex electron transfer in ruthenium modified cytochrome c derivative

Laser flash photolysis techniques have been used to investigate intramolecular and intracomplex electron transfer reactions involving ruthenium(II) polypyridine modified cytochrome c derivatives (Ru-Cyt c). The intramolecular rate constants k$sb1$ and k$sb2$ have been determined for Ru$sb{rm c}$-8-Cyt $c (1.5times 10sp5$ s$sp{-1}$ and $3.6times 10sp5$ s$sp{-1},$ respectively) and are added to those previously reported for the same model system. The sacrificial electron donors DMAB and PAD react efficiently with Ru(II*) by a reductive quenching mechanism and have been used to measure the k$sb4$ rate constant for the following derivatives: Ru$sb{rm c}$-7-Cyt $c = 1.0times 10sp6$ s$sp{-1},$ Ru$sb{rm c}$-8-Cyt $c = 5.7times 10sp5$ s$sp{-1},$ Ru$sb{rm c}$-86-Cyt $c = 3.7times 10sp5$ s$sp{-1},$ Ru$sb{rm c}$-87-Cyt $c = 2.5times 10sp5$ s$sp{-1}.$ Intraprotein electron transfer reactions involving Ru(I) (E$sp0$ = $-$1.38 eV) have not been previously reported. The complete set of intramolecular electron transfer rate constant data for the Ru$sb{rm c}$-Cyt c model system was analyzed to assess the affects of distance and driving force on the k$sb1$, k$sb2$, and k$sb4$ rate constants. The results are discussed in terms of the Marcus Equation parameters.;The reactions of horse Ru-Cyt c derivatives with beef cytochrome $csb1$ (including hinge protein) were studied by laser flash photolysis. The rate constants were independent of protein concentration at low ionic strength (5mM sodium phosphate, pH 7.0), indicating intracomplex electron transfer. The rate constants were 4,800, 6,800, 20,000 and 22,000 s$sp{-1}$ for the Ru-Cyt c derivatives modified at lysines 13, 27, 25 and 72. The observed rate constants were shown to be independent of ionic strength up to 50 mM. Additional increases in ionic strength caused the rate constants to decrease proportionally, indicating dissociation of the protein complex. Second-order rate constants measured at 310 mM ionic strength were $1.0times 10sp6, 1.6times 10sp7, 1.2times 10sp8$ and $3.0times 10sp7$ M$sp{-1}$ s$sp{-1}$ for the derivatives modified at lysines 13, 27, 25 and 72. The ionic strength dependence of the rate constants in Ru-Cyt cis similar to native cytochrome c and is consistent with electron transfer reactions between oppositely charged proteins.