ΔG0 Dependence of the Electron Transfer Rate in the Photosynthetic Reaction Center of Plant Photosystem I: Natural Optimization of Reaction between Chlorophyll a (A0) and Quinone

The rate of the electron transfer reaction from the reduced primary electron acceptor chlorophyll a (A0-) to the secondary acceptor quinone (Q) was measured by picosecond−nanosecond laser spectroscopy at 280 K in the photosynthetic reaction center (RC) complex of plant photosystem I (PS I). The free energy change (ΔG0) of the reaction was varied between −1.1 and +0.2 eV by the reconstitution of 13 different quinone/quinonoid compounds after the extraction of the intrinsic phylloquinone. Phylloquinone and its natural analog menaquinone, both of which show a ΔG0 value of −0.34 eV, gave the highest rate constant (k) of (23 ps)-1. Analysis of log k versus ΔG0 plot according to the quantum mechanical electron transfer theory gave the total reorganization energy (λtotal) of 0.30 eV and the electronic coupling (V) of 14 cm-1. The natural system is shown to be highly optimized to give a ΔG0= −λtotal condition. The λtotal value is smaller and the V value is larger than those estimated in the corresponding reaction...