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4270037 
Journal Article 
Synthetic manganese-calcium oxides mimic the water-oxidizing complex of photosynthesis functionally and structurally 
Zaharieva, I; Najafpour, MM; Wiechen, M; Haumann, M; Kurz, P; Dau, H 
2011 
Yes 
Energy and Environmental Science
ISSN: 1754-5692
EISSN: 1754-5706 
4 
7 
2400-2408 
WOS:000292205100008 
In the worldwide search for sustainable energy technologies, water oxidation by abundant low-cost materials is of key importance. In nature, this process is efficiently catalyzed by an intricate manganese-calcium (Mn(4)Ca) complex bound to the proteins of photosystem II (PSII). Recently synthetic manganese-calcium oxides were found to be active catalysts of water oxidation but at the atomic level their structure has remained elusive. To investigate these amorphous catalysts, extended-range X-ray absorption spectroscopy (XAS) at the K-edges of both manganese and calcium was performed. The XAS results reveal striking similarities between the synthetic material and the natural Mn(4)Ca complex. The oxidation state of manganese in the active oxides was found to be close to +4, but Mn(III) ions are present as well at a level of about 20%. Neighboring Mn ions are extensively interconnected by two bridging oxygens, a characteristic feature of layered manganese oxides. However, the oxides do not exhibit long-range order, as opposed to canonical, but catalytically inactive Mn(III)- or Mn(IV)-oxides. Two different Ca-containing motifs were identified. One of them results in the formation of Mn(3)CaO(4) cubes, as also proposed for the natural paragon in PSII. Other calcium ions likely interconnect oxide-layer fragments. We conclude that these readily synthesized manganese-calcium oxides are the closest structural and functional analogs to the native PSII catalyst found so far. Evolutionary implications are considered. From the differences to inactive manganese oxides, we infer structural features facilitating the catalysis of water oxidation in both the protein-bound Mn(4)Ca complex of PSII and in the synthetic oxides.