Electron transfer through a prenucleated bimetalated alanine-based peptide helix

We have synthesized a 22 residue alanine-based peptide with a tris(bipyridyl)ruthenium(II) amino acid near the middle of the peptide which can act as a photoinducible electron donor. Two histidines spaced i, i + 4 near the C-terminus of the peptide were then cross-linked with a tetraammineruthenium(III) moiety to prenucleate the helix and provide an electron acceptor site. Introduction of the cross-link enhances the average helix content from 67% to 84% at 0 °C, as judged by circular dichroism spectroscopy. The temperature dependence of the mean molar residue ellipticity at 222 nm, [Θ]₂₂₂, for the bimetalated peptide was fit to a modified Lifson-Roig helix-coil model to permit extraction of the population of helical conformation at each residue separating the electron donor and acceptor. On average, the residues between the donor and acceptor are 92% helical. Photoinduced electron transfer with a driving force of -1.0 eV and an estimated reorganization energy of 0.82 eV was measured by fluorescence quenching methods in H₂O and D₂O, yielding rate constants, k_ET, of 7 ± 3 × 10⁶ s^-1 and 5 ± 1 × 10⁶ s^-1 at 0 °C. Calculation of the electronic coupling matrix element, H_ab, with the Marcus equation yields a value of 0.19 ± 0.4 cm^-1. Analysis in terms of the pathway model for electronic coupling indicates that this magnitude of H_ab is consistent with the participation of hydrogen bonds in electronic coupling for an isolated α-helix.