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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.
Robinson et al. (Mon,) studied this question.