ΔG0Dependence of the Electron Transfer Rate in the Photosynthetic Reaction Center of Plant Photosystem I: Natural Optimization of Reaction between Chlorophylla(A0) and Quinone

The rate of the electron transfer reaction from the reduced primary electron acceptor chlorophyll a (A 0 - ) 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 (Δ G 0 ) 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 Δ G 0 value of −0.34 eV, gave the highest rate constant ( k ) of (23 ps) -1 . Analysis of log k versus Δ G 0 plot according to the quantum mechanical electron transfer theory gave the total reorganization energy (λ total ) 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 Δ G 0 = −λ total condition. The λ total value is smaller and the V value is larger than those estimated in the corresponding reaction between the reduced primary acceptor bacteriopheophytin (H - ) and the secondary acceptor ubiquinone (Q A ) in the purple bacterial RC complex. It is concluded that the A 0 - Q → A 0 Q - reaction in the PS I RC occurs in protein environments, which give a low dielectric property, with a shorter electron transfer distance compared to the reaction between H and Q A .