Observation of the Reduction and Reoxidation of the Primary Electron Acceptor in Photosystem I

Femtosecond transient absorption spectroscopy has been used to investigate the primary charge separation in a photosystem II deletion mutant from the cyanobacterium Synechocystis sp. PCC 6803. These cells contain only the photosystem I reaction center and have a pigment content of ∼ 100 chlorophylls per P700. Utilizing relatively high excitation intensities, the difference spectrum for the reduction of primary electron acceptor [(A₀⁻ – A₀) difference spectrum] was obtained from experiments performed under both reducing and oxidizing conditions. Both approaches yield very similar results with the (A₀⁻ – A₀) difference spectrum displaying a maximum bleaching at 687 nm. The shape of the difference spectrum suggests that the primary electron acceptor in photosystem I may be a chlorophyll a molecule. The observed rate of primary radical pair formation depends on the overall rate of decay of excitations in the antenna; the radical pair state forms as the antenna decays. The decay of the primary radical pair state is characterized by a 21 -ps time constant. Under conditions that avoid annihilation effects, the mean lifetime for excitations in the antenna is 28 ps [Hastings, G., Kleinherenbrink, F. A. M., Lin, S., & Blankenship, R. E. (1994) Biochemistry (preceding paper in this issue)]. This indicates that the reduced acceptor decays faster than it forms. Therefore, only a low concentration of the reduced acceptor will accumulate under most conditions.