A 30-residue polypeptide [H21(30-mer)] with the sequence Ac-K(IEALEGK) 2 (IEALEHK)-(IEALEGK)G-NH 2 was synthesized. The circular dichroism (CD) spectrum of the peptide shows minima at 208 and 222 nm and θ 222 /θ 208) 1.06, which indicates the formation of a self-assembled coiled-coil when dissolved in aqueous solution. The concentration dependence of the CD data can be fit to an expression that describes a two-state monomer-dimer equilibrium for the apopeptide (K d) 1.5 (0.4 µM and θ max)-23 800 (130 deg cm 2 dmol-1), showing that it has a maximum helicity of 69%. A [MTSL-C21(30-mer)] dimer was also prepared in which MTSL is the thiol-specific nitroxide spin label 1-oxyl-2,2,5,5-tetramethyl-∆ 3-pyrroline-3-methyl-methanethiosulfonate attached to C21 of the 30-mer. Fourier deconvolution analysis of the dipolar line broadening of the electron paramagnetic resonance (EPR) spectrum yields a measure of the interchain C R sC R distance of 13.5 (0.9 Å at position 21 of the coiled-coil, which is nearly identical to those distances observed for the isostructural family of bZip proteins. Two metallohomodimers, [Ru(trpy)(bpy)-H21(30-mer)] 2 and [Ru(NH 3) 5-H21(30-mer)] 2 , in which the ruthenium complexes were coordinated with the H21 site of the 30-mer, were prepared. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), chemical cross-linking studies, and analytical ultracentrifugation show that the peptides exist as a dimeric coiled-coil with a molecular weight of ∼7.5 kDa. The electron transfer (ET) heterodimer, [Ru(trpy)(bpy)-H21(30-mer)]/ [Ru(NH 3) 5-H21(30-mer)], was prepared, and molecular modeling shows that the two metal complexes are separated by a metal-to-metal distance of ∼24 Å across the noncovalent peptide interface. Pulse radiolysis was used to measure an ET rate constant of k et) 380 (80 s-1 for the intracomplex electron transfer (∆G°)-1.11 eV) from the Ru II (NH 3) 5-H21 donor to the Ru III (trpy)(bpy)-H21 acceptor. The value for k et falls within the range reported for modified proteins over comparable distances and supersedes the one reported in an earlier communication.

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(obs)(ET)= (2.7 +/- 0.4) x 10(6) s(-1) were observed at ambient temperatures. The magnitude of k(obs)(ET) decreases with increasing solvent viscosity, and the behavior can be fit to the expression k(obs)(ET) 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(obs')(ET)= (7 +/- 3) x 10(5) s(-1). The different values of k(obs)(ET) and k(obs')(ET) suggest that the peptide lies farther from the heme when in the encounter complex. The value of k(obs')(ET) 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(obs')(ET), which suggests that the rate-limiting gating processes in the encounter complex is different from that in the preformed complex.