Primary charge separation in Photosystem II.

In this Minireview, we discuss a number of issues on the primary photosynthetic 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 that these forms may reflect the single-site spectrum with dominant contributions from the zero-phonon line and a pronounced approximately 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 which there is no 'special pair' and in which for each realization of the disorder the excitation may be dynamically localized on basically any combination of neighbouring chlorins. The key features of our model for the primary reactions in PS II include ultrafast (<500 fs) energy transfer processes within the multimer, 'slow' ( approximately 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' chlorophyll to a 'special pair' chlorophyll and/or charge separation starting from this 'special pair' chlorophyll ( approximately 8 ps), and slow relaxation ( approximately 50 ps) of the radical pair by conformational changes of the protein. The charge separation in the PS II RC can probably not be described as a simple trap-limited or diffusion-limited process, while for the PS II core and larger complexes the transfer of the excitation energy to the PS II RC may be rate limiting.