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

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• 作者：Benedikt Lassalle-Kaiser ; Thaddeus T. Boron III ; Vera Krewald ; Jan Kern ; Martha A. Beckwith ; Mario U. Delgado-Jaime ; Henning Schroeder ; Roberto Alonso-Mori ; Dennis Nordlund ; Tsu-Chien Weng ; Dimosthenis Sokaras ; Frank Neese ; Uwe Bergmann ; Vittal K. Yachandra ; Serena DeBeer ; Vincent L. Pecoraro ; Junko Yano
• 刊名：Inorganic Chemistry
• 出版年：2013
• 出版时间：November 18, 2013
• 年：2013
• 卷：52
• 期：22
• 页码：12915-12922
• 全文大小：440K
• 年卷期：v.52,no.22(November 18, 2013)
• ISSN：1520-510X

文摘

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-渭-oxo-bridged Mn^IV complexes in different protonation states was studied by K尾 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 K尾_2,5 peaks arise primarily from the ligand 2p to Mn 1s transitions, with a characteristic low energy shoulder appearing upon oxo-bridge protonation. The satellite K尾鈥?peak provides a more direct signature of the protonation state change, since the transitions originating from the 2s orbitals of protonated and unprotonated 渭-oxo bridges dominate this spectral region. The energies of the K尾鈥?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 (渭-O)(渭-OH₂) motif can be distinguished from a symmetric (渭-OH)₂ one. The results reported here highlight the sensitivity of K尾 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 Mn^IV dimer series are discussed.