Charge Recombination Between the Reduced Iron-Sulphur Clusters and P700+

Photosystem I (PS I) is a membrane-bound pigment-protein complex, consisting of at least 11 subunits (1). The cofactors which are involved in the electron transfer are located in the PsaA and PsaB subunits (P700, a chlorophyll a dimer, A0 a monomeric chlorophyll a, Al, a phylloquinone and FX, a [4Fe-4S] cluster) and in the PsaC subunit (two additional [4Fe-4S] clusters, FA and FB) (for a recent review see (2)). In this work, the recombination of the flash-induced charge separated pair was studied by transient absorbance spectroscopy under conditions of oxidized and stepwise prereduced [4Fe-4S] cluster to clarify the kinetics of the different recombination reactions. Measurements of the amplitudes of the recombination phases as a function of the potential offer the possibility to determine the midpoint potentials of the first and second reduction of the iron-sulphur clusters at room temperature. In the past, midpoint potentials were mostly measured by EPR at cryogenic temperatures (e.g. 3,4). In addition, the activation energy of the charge recombination between P700+ and (FAFB) has been measured since values described in the literature are contradictory (200 meV in (5) and 460 meV in (6)).