Electronic structure of the Mn4OxCa cluster in the S0 and S2 states of the oxygen-evolving complex of photosystem II based on pulse 55Mn-ENDOR and EPR spectroscopy.
The heart of the oxygen-evolving complex (OEC) of photosystem II is a Mn4OxCa cluster that cycles through five different oxidation states (S0 to S4) during the light-driven water-splitting reaction cycle. In this study we interpret the recently obtained 55Mn hyperfine coupling constants of the S0 and S2 states of the OEC [Kulik et al. J. Am. Chem. Soc. 2005, 127, 2392-2393] on the basis of Y-shaped spin-coupling schemes with up to four nonzero exchange coupling constants, J. This analysis rules out the presence of one or more Mn(II) ions in S0 in methanol (3%) containing samples and thereby establishes that the oxidation states of the manganese ions in S0 and S2 are, at 4 K, Mn4(III, III, III, IV) and Mn4(III, IV, IV, IV), respectively. By applying a "structure filter" that is based on the recently reported single-crystal EXAFS data on the Mn4OxCa cluster [Yano et al. Science 2006, 314, 821-825] we (i) show that this new structural model is fully consistent with EPR and 55Mn-ENDOR data, (ii) assign the Mn oxidation states to the individual Mn ions, and (iii) propose that the known shortening of one 2.85 A Mn-Mn distance in S0 to 2.75 A in S1 [Robblee et al. J. Am. Chem. Soc. 2002, 124, 7459-7471] corresponds to a deprotonation of a mu-hydroxo bridge between MnA and MnB, i.e., between the outer Mn and its neighboring Mn of the mu3-oxo bridged moiety of the cluster. We summarize our results in a molecular model for the S0 --> S1 and S1 --> S2 transitions.
Kulik
Leonid V
LV
Max Planck Institute for Bioinorganic Chemistry, Stiftstrasse 34-36, Mülheim an der Ruhr, Germany. chemphy@ns.kinetic.nsc.ru
Epel
Boris
B
Lubitz
Wolfgang
W
Messinger
Johannes
J
D002118:Calcium / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D004583:Electrons; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008345:Manganese / Q000737:chemistry*; D008954:Models, Biological; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D011868:Radioisotopes; D012680:Sensitivity and Specificity
D045332:Photosystem II Protein Complex; D011868:Radioisotopes; D008345:Manganese; D010100:Oxygen; D002118:Calcium