Tunneling in electron transport

Abstract

Light excitation of chlorophylls and bacteriochlorophylls creates strong reductants to initiate guided electron transfer through chains of redox centers, converting light energy into electrostatic and chemical redox energy and largely avoiding the threat of charge recombination unless useful. Most electron-transfer reactions of photosynthesis are single-electron transfers between well-separated redox centers via electron tunneling through the insulating intervening protein medium. Tunneling rates are dominated by an exponential dependence on the edge-to-edge distance between cofactors. There is an approximately Gaussian dependence of rate on driving force, with a peak rate at the reorganization energy, as defined by classical Marcus theory and modified to include quantum effects. Complex quantum theoretical rate dependencies are well approximated by a simple empirical expression with three parameters: distance, driving force, and reorganization energy. Natural selection exploits distance and driving force to speed desirable electron transfers or slow undesirable electron transfer. Redox centers engaged in productive electron transfer are placed less than 14 Å apart. Natural photosynthetic proteins are far from ideal: they have high yields but a superabundance of cofactors and relatively large energy losses.