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 ar e presented for a series of structurally perturbed di-mu-oxo [Mn(III)M n(IV)] dimers and for superoxidized L. plantarum manganese catalase an d azide perturbed manganese catalase. The study of this series of dime rs allows the characteristics (energy, sign, bandshape, and intensity) of each observed spectral feature to be determined and correlated thr oughout the series, leading to assignment of 10 transitions in the opt ical spectra. Resonance Raman enhancement profiles on Mn-oxo stretchin g vibrations in the models are also presented which assist in the assi gnment of the observed features. The similarities of the MCD spectra i n this series indicate that the observed spectral features below 30 00 0 cm-1 are all characteristic of the di-mu-oxo [Mn(III)Mn(IV)] structu ral unit, and absorption intensity in these dimers is attributable lar gely to oxo-to-Mn(IV) charge transfer and Mn(IV) d-d transitions. Spec tral variations within the series are associated with structural pertu rbations involving ancillary and additional bridging acetate ligation. These spectrostructural correlations provide insight into various asp ects of bonding in these dimers. A near-IR MCD band with little corres ponding absorption intensity is identified as the transition between J ahn-Teller split e(g) orbitals of Mn(III). Its energy reflects the mag nitude of Mn(III) tetragonal distortion and provides insight into stru ctural contributions to the redox properties of these dimers. A decrea se in spectral intensity is observed upon the addition of bridging ace tate ligation and is associated with the concomitant core bending of a pproximately 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 superexch ange interactions. MCD allows a detailed spectral comparison of the mo dels with the protein data and reveals a one-to-one correlation in ban d energies and signs with reduced absorption and MCD intensity in the energy region below approximately 19 000 cm-1 being the predominant di fference. From the spectrostructural correlations developed in the mod el studies, these results indicate the presence of a bent and likely t ribridged di-mu-oxo [Mn(III)Mn(IV)] dimeric core in superoxidized mang anese catalase. The application of MCD to superoxidized manganese cata lase and a series of model complexes has allowed for electronic and ge ometric insight into this metalloenzyme and the di-mu-oxo [Mn(III)Mn(I V)] structural unit and provides a basis for future studies on this an d other Mn cluster active sites.