ELECTRONIC-STRUCTURE AND SPECTROSCOPY OF MANGANESE CATALASE AND DI-MU-OXO [MN(III)MN(IV)] MODEL COMPLEXES

Electronic absorption and magnetic circular dichroism (MCD) spectra are presented for a series of structurally perturbed di-mu-oxo [Mn(III)Mn(IV)] dimers and for superoxidized L. plantarum manganese catalase and azide perturbed manganese catalase. The study of this series of dimers allows the characteristics (energy, sign, bandshape, and intensity) of each observed spectral feature to be determined and correlated throughout the series, leading to assignment of 10 transitions in the optical spectra. Resonance Raman enhancement profiles on Mn-oxo stretching vibrations in the models are also presented which assist in the assignment of the observed features. The similarities of the MCD spectra in this series indicate that the observed spectral features below 30 000 cm-1 are all characteristic of the di-mu-oxo [Mn(III)Mn(IV)] structural unit, and absorption intensity in these dimers is attributable largely to oxo-to-Mn(IV) charge transfer and Mn(IV) d-d transitions. Spectral variations within the series are associated with structural perturbations involving ancillary and additional bridging acetate ligation. These spectrostructural correlations provide insight into various aspects of bonding in these dimers. A near-IR MCD band with little corresponding absorption intensity is identified as the transition between Jahn-Teller split e(g) orbitals of Mn(III). Its energy reflects the magnitude of Mn(III) tetragonal distortion and provides insight into structural contributions to the redox properties of these dimers. A decrease in spectral intensity is observed upon the addition of bridging acetate ligation and is associated with the concomitant core bending of approximately 20-degrees. A molecular orbital description is presented to explain this intensity variation with core bending and additionally provides insight into the orbital nature of di-mu-oxo dimer superexchange interactions. MCD allows a detailed spectral comparison of the models with the protein data and reveals a one-to-one correlation in band energies and signs with reduced absorption and MCD intensity in the energy region below approximately 19 000 cm-1 being the predominant difference. From the spectrostructural correlations developed in the model studies, these results indicate the presence of a bent and likely tribridged di-mu-oxo [Mn(III)Mn(IV)] dimeric core in superoxidized manganese catalase. The application of MCD to superoxidized manganese catalase and a series of model complexes has allowed for electronic and geometric insight into this metalloenzyme and the di-mu-oxo [Mn(III)Mn(IV)] structural unit and provides a basis for future studies on this and other Mn cluster active sites.