TY - JOUR
T1 - P700
T2 - The primary electron donor of photosystem I
AU - Webber, Andrew
AU - Lubitz, Wolfgang
N1 - Funding Information:
A.W. acknowledges USDA and DOE for financial support, Scott Bingham and Hyeonmoo Lee for construction of mutants, and Robert Blankenship and Alexander Melkozernov for collaborations on ultrafast optical spectroscopy experiments. W.L. is grateful to his collaborators who contributed to this work. Heike Witt, Ludwig Krabben, and Matthias Kuhn generated the mutant PSI. Hanno Käss, Friedhelm Lendzian, Stephan Zech, Wulf Hofbauer, Robert Bittl, Rafael Jordan, and Eberhard Schlodder did the EPR/ENDOR/ESEEM and optical experiments, and Martin Plato and Matthias Stein performed the so far unpublished MO calculations on P700 +⋅ . Norbert Krauss, Patrick Jordan and Wolfram Saenger (FU Berlin) are acknowledged for communicating details on the PSI crystal structure to us prior to publication. The PSI single crystals for the EPR experiments were obtained from Petra Fromme and Horst T. Witt who are gratefully acknowledged. We thank the group of Giovanni Giacometti/Donatella Carbonera (University of Padua) for a fruitful collaboration on the triplet states. This work was supported by the DFG (Sfb 312 and Sfb 498) and by Fonds der Chemischen Industrie (Frankfurt/Main), and EU (FMRX-CT98-0214) to W.L.
PY - 2001/10/30
Y1 - 2001/10/30
N2 - 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 precise 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 residues provided by protein subunits PsaA and PsaB, respectively. The Chl a′, but not the Chl a, is also H-bonded to the protein. The H-bonding is likely responsible 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 specifically stabilize the Chl a′-side of the dimer, suggesting that the unpaired electron would predominantly reside on the Chl a. This is supported by results of specific mutagenesis of the PsaA and PsaB axial His residues, which show that only mutations of the PsaB subunit significantly alter the hyperfine coupling constants associated with a single Chl molecule. The PsaB mutants also alter the microwave induced triplet-minus-singlet spectrum indicating that the triplet state is localized on the same Chl. Excitonic coupling between the two Chl a of P700 is weak due to the distance and overlap of the porphyrin planes. Evidence of excitonic coupling is found in PsaB mutants which show a new bleaching band at 665 nm that likely represents an increased intensity of the upper exciton band of P700. Additional properties of P700 that may give rise to its unusually low potential are discussed.
AB - 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 precise 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 residues provided by protein subunits PsaA and PsaB, respectively. The Chl a′, but not the Chl a, is also H-bonded to the protein. The H-bonding is likely responsible 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 specifically stabilize the Chl a′-side of the dimer, suggesting that the unpaired electron would predominantly reside on the Chl a. This is supported by results of specific mutagenesis of the PsaA and PsaB axial His residues, which show that only mutations of the PsaB subunit significantly alter the hyperfine coupling constants associated with a single Chl molecule. The PsaB mutants also alter the microwave induced triplet-minus-singlet spectrum indicating that the triplet state is localized on the same Chl. Excitonic coupling between the two Chl a of P700 is weak due to the distance and overlap of the porphyrin planes. Evidence of excitonic coupling is found in PsaB mutants which show a new bleaching band at 665 nm that likely represents an increased intensity of the upper exciton band of P700. Additional properties of P700 that may give rise to its unusually low potential are discussed.
KW - Electron nuclear double resonance
KW - P700
KW - Photosystem I
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U2 - 10.1016/S0005-2728(01)00198-0
DO - 10.1016/S0005-2728(01)00198-0
M3 - Review article
C2 - 11687208
AN - SCOPUS:0035976006
SN - 0005-2728
VL - 1507
SP - 61
EP - 79
JO - Biochimica et Biophysica Acta - Bioenergetics
JF - Biochimica et Biophysica Acta - Bioenergetics
IS - 1-3
ER -