Kinetics of the oxidation—reduction reactions of the photosystem II quinone acceptor complex, and the pathway for deactivation

When the photosystem II quinone acceptor complex has been singly reduced to the state Q A Q − B , there is a 22 s half‐time back‐reaction of Q − B with an oxidized photosystem II donor (S 2 ), directly measured here for the first time. From the back‐reaction kinetics with and without inhibitors, kinetic and equilibrium parameters have been estimated. We suggest that the state Q A Q − B of the complex is formed by a second‐order reaction of vacant reaction centers in the state Q − A with plastoquinone from the pool, and discuss the physico‐chemical parameters involved.