| Abstract
| - We have synthesized a 22 residue alanine-based peptide with a tris(bipyridyl)ruthenium(II) amino acid near themiddle of the peptide which can act as a photoinducible electron donor. Two histidines spaced i, i + 4 near theC-terminus of the peptide were then cross-linked with a tetraammineruthenium(III) moiety to prenucleate the helixand 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 molarresidue ellipticity at 222 nm, [ϑ]222, for the bimetalated peptide was fit to a modified Lifson−Roig helix−coil modelto permit extraction of the population of helical conformation at each residue separating the electron donor andacceptor. On average, the residues between the donor and acceptor are 92% helical. Photoinduced electron transferwith a driving force of −1.0 eV and an estimated reorganization energy of 0.82 eV was measured by fluorescencequenching methods in H2O and D2O, yielding rate constants, kET, of 7 ± 3 × 106 s-1 and 5 ± 1 × 106 s-1 at 0 °C.Calculation of the electronic coupling matrix element, Hab, with the Marcus equation yields a value of 0.19 ± 0.4cm-1. Analysis in terms of the pathway model for electronic coupling indicates that this magnitude of Hab is consistentwith the participation of hydrogen bonds in electronic coupling for an isolated α-helix.
- Synthesis of a bimetalated peptide helix is presented. Circular dichroism data analyzed by Lifson−Roig helix−coil theory indicate an average helical content of 92% for the residues between the two metals. Electron transfer from photoexcited tris(bipyridyl)ruthenium(II) to a tetraammineruthenium(III) moiety occurs in H2O and D2O with a rate constant of 5 to 7 × 106 s-1, giving Hab = 0.19 ± 0.04 cm-1. This Hab is consistent with hydrogen bonds mediating electronic coupling through helical structure.
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