Figure 5.

Proposed model for conformational gating of electron transfer from Q_A^−⋅ to Q_B. Two conformational states are shown, distal (D; oval), which is inactive, and proximal (P; rectangle), which is active in electron transfer. The main sequence of steps in the reaction are (1) excitation of RCs in the distal state, (2) conversion to the proximal state, and (3) electron transfer. A fraction of RCs in the proximal state before light excitation may give rise to a faster observed rate through reactions 1′ and 3.