Electron transport in Photosystem II of higher plants is particularly susceptible to inhibition at the site where electrons are transferred between the plastoquinone electron acceptors QA and QB. Substituted benzoquinones, naphthoquinones, anthraquinones, coumarins, and pyrones, as structural analogs of QB, compete with QB for a common binding domain and inhibit electron transfer from QA. We have used measurements of chlorophyll fluorescence intensity as well as CNDO molecular orbital calculations to (1) characterize the inhibitor strength; (2) correlate the chemical structure of the inhibitors to the extent of inhibitory activity; and (3) predict the binding site of the inhibitors on the QB-binding protein. Stern-Volmer analyses of the chlorophyll fluroscence data provide two parameters for assessing inhibitor strength. These parameters suggest that the variable inhibitory activities observed for the QB analogs reflect both the hydrophobicity and the electron-releasing character of the substituents. CNDO calculations reveal that inhibitors with the same π charge distribution on the atoms adjacent to the carbonyl carbons compete for a common binding domain. These studies permit the prediction of both the substituent identity and the substitution pattern in quinones, coumarins, and pyrones that lead to maximal inhibition of photosynthetic activity.
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