Electron transfer through the hydrogen-bonded interface of a β-turn- forming depsipeptide

David A. Williamson, Bruce E. Bowler

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Abstract

Hydrogen-bonding networks are believed to play an important role in electron-transfer pathways in a protein medium. A porphyrin-quinone donor- acceptor compound with a depsipeptide bridge which forms a β-turn has been synthesized to study hydrogen bond-mediated electron transfer. The placement of the donor and acceptor has been chosen to favor electron transfer through the hydrogen bond interface of the β-turn. Use of ester linkages also allows control of the hydrogen-bonding pattern within the β-turn-forming depsipeptide. Infrared spectroscopy in the amide A (NH stretch) and amide I (carbonyl stretch) regions indicates that the β-turn conformation is about 85% populated in dichloromethane and essentially completely disrupted in dimethyl sulfoxide at 296 K. The electron-transfer rate constant, k(et), was evaluated using the singlet excited-state lifetimes of the porphyrin in the presence and absence of an electron acceptor. The lifetimes were obtained using time-correlated single-photon-counting fluorescence spectroscopy. Very fast electron transfer (k(et) = (1.1 ± 0.1) x 109 s-1) was observed in the presence of the β-turn conformation. When the β-turn structure was disrupted using the solvent DMSO, electron transfer was no longer competitive with the intrinsic fluorescence emission. Analysis of the data in terms of Marcus theory and the pathway model for electronic coupling yielded a value for the hydrogen bond coupling decay factor, ε(hb), of 0.8 ± 0.4, which is of the same order of magnitude as the theoretically predicted value of 0.36.

Original languageEnglish
Pages (from-to)10902-10911
Number of pages10
JournalJournal of the American Chemical Society
Volume120
Issue number42
DOIs
StatePublished - Oct 28 1998

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