X-ray Absorption Spectroscopy of Calcium-Substituted Derivatives of the Oxygen-Evolving Complex of Phostosytem II

X-ray absorption spectroscopy (XAS) has been used to characterize the structural consequences of Ca<sup>2+</sup> replacement in the reaction center complex of the photosynthetic oxygen-evolving complex (OEC). EPR and activity measurements demonstrate that, in the absence of the 17 and 23 kDa extrinsic polypeptides, it is not necessary to use either low pH or Ca chelators to effect complete replacement of the active site Ca<sup>2+</sup> by Sr<sup>2+</sup>, Dy<sup>3+</sup>, or La<sup>3+</sup>. The extended X-ray absorption fine structure (EXAFS) spectra for the OEC show evidence for a Mn···M interaction at ca. 3.3 Å that could arise either from Mn···Mn scattering within the Mn cluster or Mn···Ca scattering between the Mn cluster and the inorganic Ca<sup>2+</sup> cofactor. There is no significant change in the either the amplitude or the phase of this feature when Ca<sup>2+</sup> is replaced by Sr<sup>2+</sup> or Dy<sup>3+</sup>, thus demonstrating that there is no EXAFS-detectable Mn···Ca contribution at ca. 3.3 Å in these samples. The only significant consequence of Ca<sup>2+</sup> replacement is a small change in the ca. 2.7 Å Mn···Mn distance. The average Mn···Mn distance decreases 0.014 Å when Ca<sup>2+</sup> is replaced by Sr<sup>2+</sup> and increases 0.012 Å when Ca<sup>2+</sup> is replaced by Dy<sup>3+</sup>. A structural model which can account both for the variation in Mn···Mn distance and for the known properties of Ca<sup>2+</sup>-substituted samples is one in which there is a hydrogen bond between a Ca<sup>2+</sup>-bound water and a Mn<sub>2</sub>(μ-O)<sub>2</sub> unit. This scheme suggests that an important role for the Ca<sup>2+</sup> may be to modulate the protonation state, and thus the redox potential, of the Mn cluster.