CHLOROPHYLL FLUORESCENCE CHARACTERIZATION OF THE ACTION OF PHOTOSYSTEM-II ELECTRON-TRANSPORT INHIBITORS

The action of substituted 9,10-anthraquinones on the room-temperature chlorophyll fluorescence emission from photosystem II in plant chloroplasts is consistent with the inhibition of electron transport. Previous fluorescence measurements suggest that anthraquinones competitively displace the electron acceptor Q(B) from its binding sites on the D1 protein. Competition studies and molecular orbital calculations led to a hypothesis whereby the substituents on the 9,10-anthraquinone ring dictate whether an anthraquinone hydrogen bonds to one or both of the domains of the D1 polypeptide suggested as sites of Q(B) binding. This study seeks to confirm our binding site predictions for substituted anthraquinones using Stern-Volmer investigations of the chlorophyll fluorescence emission of spinach chloroplasts treated to alter the Q(B) function. Anthraquinone-induced fluorescence quenching is measured for chloroplasts depleted of Q(B) and the associated plastoquinone pool and for chloroplasts treated with additional inhibitors which are ascribed to different binding regions of the D1 protein. Comparisons of Stern-Volmer parameters, particularly f(a) values, for anthraquinones in normal, Q(B)-depleted, and inhibitor-treated chloroplasts reveal variations in the accessibility of anthraquinones to the Q(B) site. The preferred anthraquinone binding sites supported by these findings are in excellent agreement with our previous predictions.