Figure 9.

(A) Comparison of the experimental Mn EXAFS dichroism from single crystals of PS II with the polarized spectrum calculated from the proposed structures from XRD studies. There are clear differences between the experimental spectra shown in the bottom panel and those calculated from the XRD coordinates for the Mn₄Ca cluster at 3.5 Å resolution (top panel)²¹ and 3.0 Å resolution (middle panel).²² The intensity and the dichroism pattern of FT peaks I–III (3.5 Å structure) and of FT peaks II and III (3.0 Å structure) are significantly different from the experimental data (bottom panel). For the 3.0 Å resolution structure, the μ-oxo bridges were not resolved in the electron density. Hence, only FT peaks II and III, with contributions from Mn and/or Ca, are shown. (B) Comparison of model II for the Mn₄Ca cluster derived from polarized Mn EXAFS with the models derived from the X-ray crystal structures at 3.0²² and 3.5 Ų¹ resolution. The spheres represent Mn (red), Ca (green), and the bridging oxygen ligand atoms (gray) for model II from polarized EXAFS of single crystals and Mn (violet), Ca (cyan), and the bridging oxygen ligand atoms (yellow) for the X-ray crystal structures. (a and b) Comparison of model II with the 3.0-Å-resolution structure for the Mn₄Ca cluster. a: The view is along the membrane plane. b: The view is onto the membrane plane from the cytoplasmic (stromal) side. (c and d) Comparison of model II with the 3.5-Å-resolution X-ray structure for the Mn₄Ca cluster. c: The view is along the membrane plane. d: The view is onto the membrane plane from the cytoplasmic side.