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Laser flash absorption spectroscopy has been used to investigate the kinetics of electron transfer from reduced cytochrome c₆ and plastocyanin to photooxidized P700 in Photosystem I (PS I) particles from the cyanobacterium Synechocystis PCC 6803. Data analysis yields second-order rate constants of 1.3 · 10⁷ M⁻¹ s⁻¹ and 1.0 · 10⁷ M⁻¹ s⁻¹ for the heme- and copper-proteins, respectively. With the two donor proteins, the observed rate constants (k_obs) present a linear protein-concentration dependence, thus suggesting an apparent one-step bimolecular collisional mechanism. At neutral pH, the k_obs values monotonically increase with increasing NaCl or MgCl₂ concentration, which is ascribed to the involvement of repulsive electrostatic interactions between the donor proteins and PS I. The difference in the effective concentration at which MgCl₂ has its maximum effect as compared with that of NaCl is attributed …