Experimental and Computational X‑ray Emission Spectroscopy as a Direct Probe of Protonation States in Oxo-Bridged MnIV Dimers Relevant to Redox-Active Metalloproteins

• Published in: Inorganic chemistry Vol. 52; no. 22; pp. 12915 - 12922
• Main Authors: Lassalle-Kaiser, Benedikt, Boron, Thaddeus T, Krewald, Vera, Kern, Jan, Beckwith, Martha A, Delgado-Jaime, Mario U, Schroeder, Henning, Alonso-Mori, Roberto, Nordlund, Dennis, Weng, Tsu-Chien, Sokaras, Dimosthenis, Neese, Frank, Bergmann, Uwe, Yachandra, Vittal K, DeBeer, Serena, Pecoraro, Vincent L, Yano, Junko
• Format: Journal Article
• Language: English
• Published: American Chemical Society 18.11.2013
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic400821g

The protonation state of oxo bridges in nature is of profound importance for a variety of enzymes, including the Mn4CaO5 cluster of photosystem II and the Mn2O2 cluster in Mn catalase. A set of dinuclear bis-μ-oxo-bridged MnIV complexes in different protonation states was studied by Kβ emission spectroscopy to form the foundation for unraveling the protonation states in the native complex. The valence-to-core regions (valence-to-core XES) of the spectra show significant changes in intensity and peak position upon protonation. DFT calculations were performed to simulate the valence-to-core XES spectra and to assign the spectral features to specific transitions. The Kβ2,5 peaks arise primarily from the ligand 2p to Mn 1s transitions, with a characteristic low energy shoulder appearing upon oxo-bridge protonation. The satellite Kβ″ peak provides a more direct signature of the protonation state change, since the transitions originating from the 2s orbitals of protonated and unprotonated μ-oxo bridges dominate this spectral region. The energies of the Kβ″ features differ by ∼3 eV and thus are well resolved in the experimental spectra. Additionally, our work explores the chemical resolution limits of the method, namely, whether a mixed (μ-O)(μ-OH2) motif can be distinguished from a symmetric (μ-OH)2 one. The results reported here highlight the sensitivity of Kβ valence-to-core XES to single protonation state changes of bridging ligands, and form the basis for further studies of oxo-bridged polymetallic complexes and metalloenzyme active sites. In a complementary paper, the results from X-ray absorption spectroscopy of the same MnIV dimer series are discussed.