P700: the primary electron donor of photosystem I

The primary electron donor of photosystem I, P700, is a chlorophyll species that in its excited state has a potential of approximately -1.2 V. The pre cise chemical composition and electronic structure of P700 is still unknown . Recent evidence indicates that P700 is a dimer of one chlorophyll (Chl) a and one Chl a'. The Chl a' and Chl a are axially coordinated by His residu es provided by protein subunits PsaA and PsaB, respectively. The Chl a', bu t not the Chl a, is also H-bonded to the protein. The H-bonding is likely r esponsible for selective insertion of Chl a' into the reaction center. EPR studies of P700(+.) in frozen solution and single crystals indicate a large asymmetry in the electron spin and charge distribution towards one Chl of the dimer. Molecular orbital calculations indicate that H-bonding will spec ifically stabilize the Chl a'-side of the dimer, suggesting that the unpair ed electron would predominantly reside on the Chl a. This is supported by r esults of specific mutagenesis of the PsaA and PsaB axial His residues, whi ch show that only mutations of the PsaB subunit significantly alter the hyp erfine coupling constants associated with a single Chl molecule. The PsaB m utants also alter the microwave induced triplet-minus-singlet spectrum indi cating that the triplet state is localized on the same Chl. Excitonic coupl ing between the two Chl a of P700 is weak due to the distance and overlap o f the porphyrin planes. Evidence of excitonic. coupling is found in PsaB mu tants which show a new bleaching band at 665 nin that likely represents an increased intensity of the upper exciton band of P700. Additional propertie s of P700 that may give rise to its unusually low potential are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.