Delta G(0) dependence of the electron transfer rate in the photosynthetic reaction center of plant photosystem I: Natural optimization of reaction between chlorophyll a (A(0)) 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 (Delta G(0)) of the reaction was varied between -1.1 and +0.2 eV by the reconstitution of 13 different quinone/quinonoid quinonoid compounds after the extraction of the intrinsic phylloquinone. Phylloquinone and its natural analog menaquinone, both of which show a Delta G(0) value of -0.34 eV, gave the highest rate constant (k) of (23 ps)(-1) Analysis of log k versus Delta G(0) plot according to the quantum mechanical electron transfer theory gave the total reorganization energy (lambda(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 Delta G(0) = -lambda(total) condition. The lambda(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).