On the reduction of chlorophyll-A1 in the presence of the plastoquinone antagonist dibromothymoquinone.

In a preceding paper [ 1 ] we presented experimental results, which led to the identification of four protolytic reaction sites in the functional membrane of photosynthesis: two sites of proton uptake at the outer side of the membrane, in agreement with prior studies [2] , and two sites of proton release into the inner phase. One of these sites had to be attributed to the reduction of the terminal electron acceptor at the outer side of the membrane. Evidence was presented for the coupling of the other sites to the oxidation of water at the inner side of the membrane, to the reduction of plastoquinone at the outer side and its oxidation at the inner side, respectively. This interpretation was corroborated by kinetic studies on proton binding from the outer aqueous phase [3]. In this paper we report on studies on the protolytic reactions at both sides of the functional membrane on excitation with short flashes in the presence of the plastoquinone antagonist dibromothymoquinone (DBMIB)*. DBMIB is an effective inhibitor of photosynthetic electron transport between photosystem II and photosystem I [4,5]. DBMIB inhibits the oxidation of plastohydroquinone