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Photoinduced electron-transfer (ET) occurs between a negatively charged metallopeptide, [Ru(bpy)2(phen-am)-Cys-(Glu)(5)-Gly](3-) = RuCE(5)G, and ferricytochrome c = Cyt c. In the presence of Cyt c, the triplet state lifetime of the ruthenium metallopeptide is shortened, and the emission decays via biexponential kinetics, which indicates the existence of two excited-state populations of ruthenium peptides. The faster decay component displays concentration-independent kinetics demonstrating the presence of a preformed peptide-protein complex that undergoes intra-complex electron-transfer. Values of K-b = (3.5 +/- 0.2) x 10(4) M-1 and k(ET)(obs) = (2.7 +/- 0.4) x 10(6) s(-1) were observed at ambient temperatures. The magnitude of k(ET)(obs) decreases with increasing solvent viscosity, and the behavior can be fit to the expression k(ET)(obs) proportional to eta(-alpha) to give alpha = 0.59 +/- 0.05. The electron-transfer process occurring in the preformed complex is therefore gated by a rate-limiting configurational change of the complex. The slower decay component displays concentration-dependent kinetics that saturate at high concentrations of Cyt c. Analysis according to rapid equilibrium formation of an encounter complex that undergoes unimolecular electron transfer yields K-b' = (2.5 +/- 0.7) x 10(4) M-1 and k(ET)(obs) = (7 +/- 3) x 10(5) s(-1). The different values of k(ET)(obs) and k(ET)(obs) suggest that the peptide lies farther from the heme when in the encounter complex. The value of k(ET)(obs) is viscosity dependent indicating that the reaction occurring within the encounter complex is also configurationally gated. A value of alpha = 0.98 +/- 0.14 is observed for k(ET)(obs), which suggests that the rate-limiting gating processes in the encounter complex is different from that in the preformed complex.

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