Probing Electron Transfer Reactions in Model Photosynthetic Systems by Raman Spectroscopy

Model systems which display the essential characteristics of the photo-induced electron transfer process in the photosynthetic reaction center have been studied using electrochemical methods together with resonance Raman (RR) and surface-enhanced Raman scattering (SERS) spectroscopy. The objective is to develop an understanding of the experimental factors which are important for producing highly efficient charge separation between an electron donor molecule and an electron acceptor. In the photosynthetic reaction center complex the quantum yield for charge separation is near 1.0. The primary electron transfer between the bacteriochlorophyll (BChl) special pair (P) and bacteriopheophytin (BPh) is extremely fast (ca. 3 ps) and the energy depleting charge recombination reaction is prevented by a series of dark electron transfer steps from BPh to secondary electron acceptors. The dark process is thermodynamically driven, as each of the successive acceptor species have decreasing reduction potentials. Various approaches have been used to mimic the photosynthetic process including covalently bonded donor-acceptor species, triads, tetrads, etc. as reviewed recently [1]. Another approach has been to prepare organized systems in which the donor and acceptor species are separated spatially. Vesicles [2], bilayers [3] and Langmuir-Blodgett [4] monolayers have all been used for this purpose. The success of the model system studies has been evaluated in terms of the kinetics of the charge separation step and the quantum yield.