Experimental and computational X-ray emission spectroscopy as a direct probe of protonation states in oxo-bridged MnIV dimers relevant to redox-active metalloproteins

Benedikt Lassalle-Kaiser, Thaddeus T. Boron, 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

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37 Citations (Scopus)

Abstract

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-OH 2) 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.

Original languageEnglish
Pages (from-to)12915-12922
Number of pages8
JournalInorganic Chemistry
Volume52
Issue number22
DOIs
Publication statusPublished - 18 Nov 2013

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