Figure 1.

The catalytic photo-cycle of quinone reduction in the RC. Electron transfer to Q_B occurs in two sequential reactions, k_AB⁽¹⁾ and k_AB⁽²⁾ (steps 3 and 5). The transitions studied in this work are in bold. In transition 1 the donor bacterio-chlorophyll (D⁺) is excited by light and transfers an electron to Q_A. D⁺ is reduced by cytochrome c₂. In step 2 a fraction (δ) of a proton is taken up in the D⁺Q_A⁻ state by the amino acid (or cluster of amino acids) A. At pH > 8, A is predominantly Glu-L212. In step 3 the electron on Q_A is transferred to Q_B (k_AB⁽¹⁾), concomitant with additional fractional proton uptake (ɛ). In step 4 there is a second light-excited electron transfer from D to Q_A that involves several steps as in step 1. In step 5 the second electron transfer from Q_A⁻ to Q_B (k_AB⁽²⁾) takes place. k_AB⁽²⁾ is coupled to the uptake of H⁺(1) to the Q_B headgroup. In step 6, the second proton, H⁺(2), is transferred internally (from Glu-L212) to Q_B forming quinol, which in step 7 dissociates from the RC. A second quinone binds, completing the cycle.