Manganese is an essential trace element in living organisms to perform various redox transformations of dioxygen. A new program will be developed to explore fundamental questions of structure and reactivity at manganese active sites of biological systems by using spectroscopic methods, particularly resonance Raman (RR) spectroscopy. Initial targets will include the mononuclear Mn(III) site of superoxide dismutases(SODs) and dinuclear non-heme Mn-catalases for which several plausible structures have been proposed. Our approach will be to obtain high-quality RR spectra of proteins and of judiciously chosen model complexes, with the aid of low-temperature Raman techniques and variable excitation wavelengths, and to record as many frequencies and isotope shifts as possible. Normal mode calculations will then be carried out, based on structurally characterized Mn biomimetic model systems, with the aim of developing physically plausible force fields capable of reproducing the observed frequencies and isotope shifts accurately and to understand in detail the nature of vibrational modes being monitored. The RR and solution FT-IR spectroscopies will be used to elucidate the anion interaction modes at the mononuclear metal sites of superoxide dismutases by probing ligand vibrations of the Cu,Zn-, Fe-, and Mn-containing SOD-azide adducts. The effects of arginine residue (Arg 141) chemical modification and phosphate addition on azide-bound vibrational frequencies will be used to provide insights into the role of Arg 141 in the SOD catalytic process. An understanding of the nature of the vibrational modes being monitored will allow to pin down structural features of these important enzymes, and to establish protein structural changes associated with the chemistry of their active metal sites.
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