Raman spectra and normal coordinate analyses of low-frequency vibrations of oxo-bridged manganese complexes

摘要

The active sites of certain metalloenzymes involved in oxygen metabolism, such as manganese catalase and the oxygen-evolving complex of photosystem II, contain μ-oxo-bridged Mn clusters with ligands that include H_2O and μ_(1,3)-carboxylato bridges provided by protein side chains. In order to understand better the vibrational spectra of such clusters, the low-frequency resonance Raman spectra of a series of structurally characterized Mn-oxo model complexes were examined. The series includes complexes of the type [Mn_2(O)(OAC)_2(bpy)_2(L)_2] and [Mn_2(O_2)(OAc)(bpy)_2(L)_2], where bpy = 2,2'-bipyridine, OAc = acetate and L = H_2O or Cl~-. Complexes containing the isotopomers OAc-d_3 and D_2O, as well as those containing both isotopomers, were also examined. Normal coordinate analyses (NCA) were performed on the various complexes using the GF matrix method. Selected vibrational modes in the 200-600 cm~(-1) region were assigned based on the spectra and DCA, which identify vibration modes arising from the metal-ligand bonds. These results will be useful in interpreting the vibrational spectra obtained from metalloproteins containing Mn-oxo complexes in their active sites.