Primary charge separation in Photosystem II

In this Minireview, we discuss a number of issues on the primary photosynth etic reactions of the green plant Photosystem II. We discuss the origin of the 683 and 679 nm absorption bands of the PS II RC complex and suggest tha t these forms may reflect the single-site spectrum with dominant contributi ons from the zero-phonon line and a pronounced similar to 80 cm(-1) phonon side band, respectively. The couplings between the six central RC chlorins are probably very similar and, therefore, a 'multimer' model arises in whic h there is no 'special pair' and in which for each realization of the disor der the excitation may be dynamically localized on basically any combinatio n of neighbouring chlorins. The key features of our model for the primary r eactions in PS II include ultrafast (< 500 fs) energy transfer processes wi thin the multimer, 'slow' (similar to 20 ps) energy transfer processes from peripheral RC chlorophylls to the RC multimer, ultrafast charge separation (< 500 fs) with a low yield starting from the singlet-excited 'accessory' chlorophyll of the active branch, cation transfer from this 'accessory' chl orophyll to a 'special pair' chlorophyll and/or charge separation starting from this 'special pair' chlorophyll (similar to8 ps), and slow relaxation (similar to 50 ps) of the radical pair by conformational changes of the pro tein. The charge separation in the PS II RC can probably not be described a s a simple trap-limited or diffusion-limited process, while for the PS II c ore and larger complexes the transfer of the excitation energy to the PS II RC may be rate limiting.