Time-resolved electron paramagnetic resonance studies of transient peptide biradicals generated by intramolecular electron transfer

Transient peptide biradicals have been generated by photoinduced electron transfer in basic water and water/methanol solutions and studied by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. Peptide biradicals generated in this way can serve as model compounds for the study of electron transfer in biologically relevant systems since both the electronic coupling matrix element (V) involved in electron transfer and the exchange interaction (J) between electron spins in a biradical are proportional to orbital overlap. In all peptide systems deprotonated tyrosine served as the electron donor and various alkyl or aryl bromides as the acceptor. Electron transfer from the excited triplet state of the tyrosyl anion to the acceptor, followed by loss of the halide anion, leads to formation of an electronically neutral biradical. Simulations of the TREPR spectra suggest that the exchange interaction is very large, much larger than expected for alkyl biradicals of similar length. The large J values indicate that the peptides may have a conformational preference for the donor and acceptor moieties to be in close proximity to one another or that through-bond coupling (TB) through the amide linkages is very strong. The photochemistry and photophysics of the unlinked donors and acceptors have been studied by TREPR and other techniques. Steady-state absorption spectroscopy offers evidence for an interaction between tyrosine and the most common acceptor, 4-bromobenzoic acid. Photoinduced electron transfer in alkyl spaced donor/acceptor systems has been studied by TREPR for comparison to the peptide systems. The exchange interactions observed for these systems are typical of those observed from other alkyl biradicals of similar length.