Bioinspired design of a Cu-Zn-imidazolate mesoporous silica catalyst system for superoxide dismutation

An imidazolate-bridged Cu(II)-Zn(II) model compound, Cu(II)- diethylenetriamino-μ-imidazolato-Zn(II)-tris(aminoethyl)amine perchlorate (CZS), was synthesized and encapsulated into various mesoporous silicas (MPSs) to mimic the structure and functionalities of copper zinc superoxide dismutase (CuZnSOD). Two encapsulation methods were studied: covalent bonding and ionic exchange. The encapsulated CZS yielded good thermal stability and enhanced superoxide disproportionation activity in various designed MPS solids mimicking the environment and functionality of native CuZnSOD enzyme. Ionic exchange method generally gave better SOD-like catalytic activities than covalent bonding because the local attraction of SiO^- to Cu(II) opens up the active axial position. We employed several spectroscopic techniques: UV-vis, EPR, and EXAFS to characterize the active site-Cu(II) ion of the immobilized CZS and to obtain the structural information of Cu(II) and Zn(II) centers confined in MPS. In the analyses of EPR and EXAFS spectra, we established the geometry of confined copper center changed to a more distorted square planar symmetry than free-form CZS, which can accommodate superoxide anion radical (O ₂^-•) coordinating to copper center in the proper manner. As the result of confinement in MPS, the active site of Cu(II) center gives rise to much increased SOD activities. Inspired by the local environments of guanidinium group in the native CuZnSOD enzyme in facilitating the superoxide dismutation, we further modified the silanol surface of MPS with N-trimethoxysilylpropyl-N,N,N-trimethyl-ammonium chloride (TMAC) preparing MPS-N⁺ to facilitate the transport of superoxide anion radicals. Optimized activities are associated with fine-tuning of pore size, surface acidity, and positively charged functional group. The spectroscopic studies allow us to establish the structure-reactivity relationship between the nanostructure of MPS and the efficacy of the superoxide dismutation in MPS and to elucidate the mechanism of the SOD-like activity of immobilized model compound in MPS.