Excitation transfer and energy transfer in the photosystem I reaction centre

We have observed absorption transients associated with electron transfer through the primary electron acceptor A(O) in the photosystem I (PST) reaction centre from spinach under non-reducing conditions. The antenna subtraction method was used to obtain the difference spectra and decays of electron transferring species P700(+) and A(O)(-). Analysis of antenna absorption transients excited at 625nm indicates that the intrinsic rate constant forming the primary radical pair P700(+) A(O)(-) cannot be measured directly. Even upon direct excitation of WOO at 708nm, this intrinsic rate constant probably cannot be extracted because of rapid energy transfer from P700* into the antenna. Using a model of energy migration based on the xray structure of the antenna we find that the energy migration in the antenna is fast and quenching is approaching 'trap limited' behaviour. Limits on the intrinsic rate constant for P700* --> A(O)(-) of 0.8 to 7ps are deduced.