title: Experimental and computational X-ray emission spectroscopy as a direct probe of protonation states in oxo-bridged Mn(IV) dimers relevant to redox-active metalloproteins.
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 Mn(IV) 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 Kβ(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 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 Mn(IV) dimer series are discussed.
authors.affiliation: Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley , California 94720, United States.
authors.forename: Benedikt
authors.initials: B
authors.lastname: Lassalle-Kaiser
authors.forename: Thaddeus T
authors.initials: TT
authors.lastname: Boron
authors.forename: Vera
authors.initials: V
authors.lastname: Krewald
authors.forename: Jan
authors.initials: J
authors.lastname: Kern
authors.forename: Martha A
authors.initials: MA
authors.lastname: Beckwith
authors.forename: Mario U
authors.initials: MU
authors.lastname: Delgado-Jaime
authors.forename: Henning
authors.initials: H
authors.lastname: Schroeder
authors.forename: Roberto
authors.initials: R
authors.lastname: Alonso-Mori
authors.forename: Dennis
authors.initials: D
authors.lastname: Nordlund
authors.forename: Tsu-Chien
authors.initials: TC
authors.lastname: Weng
authors.forename: Dimosthenis
authors.initials: D
authors.lastname: Sokaras
authors.forename: Frank
authors.initials: F
authors.lastname: Neese
authors.forename: Uwe
authors.initials: U
authors.lastname: Bergmann
authors.forename: Vittal K
authors.initials: VK
authors.lastname: Yachandra
authors.forename: Serena
authors.initials: S
authors.lastname: DeBeer
authors.forename: Vincent L
authors.initials: VL
authors.lastname: Pecoraro
authors.forename: Junko
authors.initials: J
authors.lastname: Yano
mesh_terms: D056831:Coordination Complexes / Q000737:chemistry*; D008345:Manganese / Q000737:chemistry*; D008667:Metalloproteins / Q000737:chemistry*; D008958:Models, Molecular; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D045332:Photosystem II Protein Complex / Q000737:chemistry; D011522:Protons; D013052:Spectrometry, X-Ray Emission
chemical_list: D056831:Coordination Complexes; D008667:Metalloproteins; D045332:Photosystem II Protein Complex; D011522:Protons; D008345:Manganese; D010100:Oxygen