Special Joint EFRC and Bioenergy Center Seminar 5/20/14

20 May 2014

"The Structure of Nature’s Water Splitting Catalyst Prior to O-O Bond Formation: Water Binding and Water Splitting in Photosynthesis"

 

  Nicholas Cox

 Max Planck Institute for Chemical Energy Conversion, Mülheim/Ruhr, Germany

   

WHEN: May 20, 2014 at 11:00 AM

WHERE: Physical Sci C-101

 

 Abstract

 EPR spectroscopy is a versatile technique for the study of transition metal cofactors, providing chemical information on the geometric and electronic structure of the complex itself and its interaction with the substrate. Experiments performed with isotopically labelled water (H217O/ H218O) provide a means to identify the two substrate binding sites of nature’s water splitting catalyst, a pentaoxygen tetramanganese-calcium cofactor. Earlier membrane-inlet mass spectrometry (MIMS) results, which monitor the uptake of H218O into the product O2 molecule, have demonstrated that the complex contains two chemically different substrate sites: an early (Ws) and late (Wf) binding substrate site, both of which exchange with bulk water in all catalytic states (S-states). Owing to the relatively slow rate of exchange of Ws ( ≈ 1 s–1), and its dependence on the oxidation state of the Mn tetramer, Ws is usually considered to be an oxygen ligand of one of the manganese ions. By using water labelled with the magnetic isotope (17O, I = 5/2) the same substrate binding site (Ws) can be characterized spectroscopically, using the EPR technique ELDOR-detected NMR (EDNMR). These measurements identify a unique, exchangeable μ-oxo bridge as a potential candidate for Ws. It is noted that the unusually fast rate of exchange of O5, as compared to that of μ-oxo bridges in simple synthetic model systems is likely due to the unique flexibility of O5’s metal coordination; μ-oxo bridge lability appears to be a feature of new heterometallic models of the biological cofactor.

These results are complemented by recent multi-frequency, multi-resonance (X-, Q-, W-band) pulse EPR data obtained for the last metastable intermediate of catalyst reaction cycle, the S3 state. It is observed that in this state all four Mn ions are structurally and electronically similar: they all have the same formal oxidation state of IV+ and an octahedral local geometry (3tg0eg). These results are interpreted with the aid of density functional theory calculations from models developed from the recent X-ray crystal structure. It is shown that only one structural model is consistent with all magnetic resonance data. This model requires the binding of an additional water molecule, possibly the second substrate water to the manganese cofactor during the formation of the last intermediate1,2 and assign its binding position.

Together these experimental results resolve the mechanism of the biological water splitting reaction, with O-O bond formation occurring between two manganese bound oxygens in the transition state, most likely an oxo-bridge and an oxyl radical. It is demonstrated that structural flexibility is important for second substrate inclusion5 and that oxygen-oxygen coupling is facilitated by the spin topology of the cofactor.

 

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