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 Fx) with 3 M urea and 5 mM K3Fe(CN)6 to oxidatively denature the Fx iron-sulfur cluster to the level of zero-valence sulfur. In this apo-Fx 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 Fx. 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-Fx 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-Fx preparation occurs between P700 and intermediate electron acceptor © 1990, American Chemical Society. All rights reserved.