Abstract
The reaction center of Photosystem II is photoactive when its primary electron acceptor, Q a, or one or more of its ancillary electron acceptors are oxidized. In this work two ancillary electron acceptors are distinguished. One of these ancillary acceptors is Q 400. It was found to have no photoinduced absorption changes in the ultraviolet and so is not a quinone. An additional ancillary electron acceptor, Aq, was identified. Aq has the following characteristics: it is a quinone based on photoinduced absorption change in the ultraviolet, it has a midpoint potential of 318 mV that is independent of pH in the 6.5–8.0 range, the extent of its oxidation depends on pH and the concentration of ferricyanide, it has no C550 bandshift, and the stoichiometry of Aq:Q 400:Q a is 0.38:0.54:1.00 based on the area above the chlorophyll a fluorescence rise curve. When Aq is oxidized it participates in double advancement in transitions of oxygen-evolution S-states. A contribution to double advancement solely by Q 400 could not be found.
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