文摘
The protonation state of oxo bridges in nature is of profound importance for a variety of enzymes, including the Mnub>4ub>CaOub>5ub> cluster of photosystem II and the Mnub>2ub>Oub>2ub> cluster in Mn catalase. A set of dinuclear bis-渭-oxo-bridged Mnup>IVup> 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ub>2,5ub> 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-OHub>2ub>) motif can be distinguished from a symmetric (渭-OH)ub>2ub> 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 Mnup>IVup> dimer series are discussed.