Selective quenching of the fluorescence of core chlorophyll-protein complexes by photochemistry indicates that Photosystem II is partly diffusion limited

The spectral characteristics of fluorescence quenching by open reaction cen tres in isolated Photosystem II membranes were determined with very high re solution and analysed. Quenching due to photochemistry is maximal near 687 nm, minimal in the chlorophyll b emission interval and displays a distincti ve structure around 670 nm. The amplitude of this 'quenching hole' is about 0.03 for normalised spectra. On the basis of the absorption spectra of iso lated chlorophyll-protein complexes, it is shown that these quenching struc tures can be exactly described by assuming that photochemistry lowers the f luorescence yield of the reaction centre complex (D1/D2/cytb(559)) plus CP4 7, with quenching of the former complex being approximately double that of the latter complex. These data, which qualitatively indicate that there are kinetically limiting processes for primary photochemistry in the antenna, have been analysed by means of several different kinetic models. These mode ls are derived from present structural knowledge of the arrangement of the chlorophyll-protein complexes in Photosystem II and incorporate the reversi ble charge separation characteristic of the exciton/radical pair equilibrat ion model. In this way it is shown that Photosystem II cannot be considered to be purely trap limited and that exciton migration in the antenna impose s a diffusion limitation of about 30%, irrespective of the structural model assumed.