Characterization of a Photosystem I Core Containing P700 and Intermediate Electron Acceptor A₁

A new photosystem I core has been isolated that is devoid of the bound iron-sulfur clusters but preserves electron flow from P700 to the intermediate electron acceptor A₁. The particle is prepared by incubation of a Synechococcus sp. PCC 6301 photosystem I core protein (which contains electron acceptors A₀, A₁, and F_x) with 3 M urea and 5 mM K₃Fe(CN)₆ to oxidatively denature the F_x iron-sulfur cluster to the level of zero-valence sulfur. In this apo-F_x preparation, over 90% of the flash-induced absorption change at 820 nm decays with a 10-µs half-time characteristic of the decay of the P700 triplet state formed from the backreaction of P700⁺ with an acceptor earlier than F_x. Chemical reduction at high pH values with aminoiminomethanesulfinic acid results in kinetics identical with those seen in the P700 chlorophyll a protein prepared with sodium dodecyl sulfate (SDS-CP1, which contains only electron acceptor A₀); the flash-induced absorption change decays primarily with a 25-ns half-time characteristic of the backreaction between P700⁺ and A₀−, and the magnitude of the total absorption change is larger than can be accounted for by the P700 content alone. Addition of oxygen results in a reversion to the 10-µs kinetic decay component attributed to the decay of the P700 triplet state. At 77 K, the optical transient in the apo-F_x preparation decays with a 200-µs half-time characteristic of the backreaction between P700⁺ and A₁−. In contrast, an SDS-CP1 particle shows an absorption change of equal magnitude but is followed by a 1.3-ms transient characteristic of the decay of the P700 triplet state formed from the P700⁺ A₁− backreaction. The high efficiency of P700 triplet formation at room temperature, the ability to be chemically reduced, and the 200-µs backreaction at 77 K indicate that charge separation in the apo-F_x preparation occurs between P700 and intermediate electron acceptor