Primary Charge Separation and Energy Transfer in the Photosystem I Reaction Center of Higher Plants

Using low intensity femtosecond duration laser pulses at 708 nm, we have observed absorption transients associated with electron transfer through the primary electron acceptor A 0 in the photosystem I (PSI) reaction center from spinach under nonreducing conditions. At this wavelength the electron donor P 700 is excited directly, although some antenna chlorophylls are also excited. Using a nanosecond duration preflash of 690 nm to oxidize P 700, and then measuring the absorption transients from the antenna alone, it is possible by subtraction to isolate the absorption transients arising from electron transfer. We discuss this method critically. The spectrum of A 0 - − A 0 does not appear promptly but takes ∼3 ps to reach maximum intensity and resembles those spectra previously obtained from higher plants, with a maximum bleaching at 685 ± 2 nm and a shoulder in the region 670−675 nm. The decay time of the primary radical pair P 700 + A 0 - is calculated as 20 ps. Analysis of absorption transients indicates that the intrinsic rate constant forming the primary radical pair P 700 + A 0 - cannot be measured directly because energy migration in the antenna is fast and quenching is approaching “trap limited” behavior. With use of a detailed model of the antenna energy migration based on the X-ray structure, the intrinsic rate constant for electron transfer is estimated as k 1 ∼ 0.7 ps -1 . The implications of these findings on energy and electron transfer are discussed.