ELECTRON-TRANSFER THROUGH THE HYDROGEN-BONDED INTERFACE OF A BETA-TURN-FORMING DEPSIPEPTIDE

Hydrogen-bonding networks are believed to play an important role in el ectron-transfer pathways in a protein medium. A porphyrin-quinone dono r-acceptor compound with a depsipeptide bridge which forms a beta-turn has been synthesized to study hydrogen bond-mediated electron transfe r. The placement of the donor and acceptor has been chosen to favor el ectron transfer through the hydrogen bond interface of the beta-turn. Use of ester linkages also allows control of the hydrogen-bonding patt ern within the beta-turn-forming depsipeptide. Infrared spectroscopy i n the amide A (NH stretch) and amide I (carbonyl stretch) regions indi cates that the beta-turn conformation is about 85% populated in dichlo romethane and essentially completely disrupted in dimethyl sulfoxide a t 296 K. The electron-transfer rate constant, k(et), was evaluated usi ng the singlet excited-state lifetimes of the porphyrin in the presenc e and absence of an electron acceptor. The lifetimes were obtained usi ng time-correlated single-photon-counting fluorescence spectroscopy. V ery fast electron transfer (k(et) = (1.1 +/- 0.1) x 10(9) s(-1)) was o bserved in the presence of the beta-turn conformation. When the beta-t urn structure was disrupted using the solvent DMSO, electron transfer was no longer competitive with the intrinsic fluorescence emission. An alysis of the data in terms of Marcus theory and the pathway model for electronic coupling yielded a value for the hydrogen bond coupling de cay factor, epsilon(hb), of 0.8 +/- 0.4, which is of the same order of magnitude as the theoretically predicted value of 0.36.