Heteronuclear Mn-Ca/Sr complexes, and Ca/Sr EXAFS spectral comparisons with the oxygen-evolving complex of photosystem II

Abstract

Heterometallic Mn-Ca and Mn-Sr complexes have been prepared and employed as model complexes for Ca and Sr EXAFS spectral comparisons with the Oxygen-Evolving Complex (OEC) of Photosystem II (PS II); these have revealed similarities that support the presence of at least one O atom bridge between the Mn and Ca/Sr in the OEC.

Fig. 1

The structures of complexes 1 (A) and 2 (B), with only the ipso C atoms of benzoate rings shown for clarity. Color code: blue, Mn^IV; cyan, Mn^III; purple, Mn^II; yellow, Ca; green, Sr; red, O; dark green, N; gray, C. H atoms have been omitted for clarity.

Fig. 2

(A) The centrosymmetric core structure of complex 1. (B) The core structure of complex 2. Color code: blue, Mn^IV; cyan, Mn^III; purple, Mn^II; yellow, Ca; green, Sr; red, O.

Fig. 3

Selected Mn–Ca and Mn–Sr sub-units present within the structures of complexes 1 and 2.

Fig. 4

(A) Ca EXAFS spectra of the Mn₁₃Ca₂ complex 1 (red) and PS II S₁ state (black), and (B) Sr EXAFS spectra of the Mn₁₄Sr complex 2 (red) and Sr-reconstituted PS II S₁ state (black).

Fig. 5

(A) Fourier transforms of the Ca EXAFS spectra of the Mn₁₃Ca₂ complex 1 (red) and PS II S₁ state (black), and (B) Fourier transforms of the Sr EXAFS spectra of the Mn₁₄Sr complex 2 (red) and Sr-reconstituted PS II S₁ state (black). Note that there are two Ca per PS II, including one Ca extraneous to the OEC. Also, note that the apparent distances (R′) of the FT peaks are shorter by ∼0.5 Å compared to the actual distances due to a phase shift induced by the potentials of the absorber–backscatterer pair on the photo-electron. The distances in the inset are from the X-ray crystal structures (XRD).