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 centres in isolated Photosystem II membranes were determined with very high resolution and analysed. Quenching due to photochemistry is maximal near 687 nm, minimal in the chlorophyll b emission interval and displays a distinctive 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 isolated chlorophyll-protein complexes, it is shown that these quenching structures can be exactly described by assuming that photochemistry lowers the fluorescence yield of the reaction centre complex (D1/D2/cytb (559)) plus CP47, 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 models are derived from present structural knowledge of the arrangement of the chlorophyll-protein complexes in Photosystem II and incorporate the reversible charge separation characteristic of the exciton/radical pair equilibration 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 imposes a diffusion limitation of about 30%, irrespective of the structural model assumed.