Effect of exothermicity on electron transfer rates in photosynthetic molecular models

Abstract

To appreciate fully the significance of the high-resolution crystal structure of the reaction-centre complex from the purple bacterium Rhodopseudomonas viridis^1,2, there is increasing impetus to construct models to ascertain how the specific variables such as distance^3–5, exothermicity^6–9 and solvent^10–11 control the rates of electron transfer through the integral membrane protein complex. Recently, we have determined the effect of a 4 Å change in distance (at fixed exothermicity, solvent and temperature) on the rates of photon-induced electron transfer for two porphyrin-quinone assemblies separated by rigid spacers^3,4. From picosecond fluorescence lifetime measurements on zinc meso-phenyloctamethylporphyrin coupled to a benzoquinone moiety via one and two bicyclo[2.2.2]octyl spacers (10 and 14 Å, edge-to-edge), we deteremined that the rates of electron transfer are 10¹⁰ s⁻¹ and ≤10⁷s⁻¹, respectively (the exothermicity, −ΔG = 1.0eV in acetonitrile at 25 °C). This result revealed that the porphyrin-quinone separated by a 10-Å spacer would be ideally suited, with regard to picosecond fluorescence techniques for a study of exothermicity effects at fixed distance and solvent. Here we report the fluorescence lifetimes of a homologous series of seven porphyrin-quinone molecules, each with differently substituted ben-zoquinones separated by an identical rigid phenylbicyclo[2.2.2]octane spacer (10 Å, edge-to-edge) which vary with respect to driving force for electron transfer from the first excited singlet state of the porphyrin. The key features of this series of molecules lie in the fact that the edge-to-edge distance is fixed and ΔG is tunable. Thus, the effect of exothermicity on electron transfer can be measured while avoiding major perturbations of the electronic structure.