Development and Mechanistic Characterization of High-Activity Catalase Mimics that Rely on Second-Sphere Metal-Ligand Interactions

In this project, funded by the Chemical Structure, Dynamics & Mechanisms B Program of the Chemistry Division, Professor Christian R. Goldsmith of the Department of Chemistry at Auburn University is developing new coordination complexes that functionally mimic the antioxidant enzyme catalase. Catalases have a prominent role in the detoxification of hydrogen peroxide within the body. Functional small molecule mimics of catalase enzymes have the potential to supplement the body’s defenses against oxidative stress, which is believed to be a component of many neurological, cardiovascular, and inflammatory disorders. The proposed catalase mimics are manganese, iron, and zinc complexes with redox-active organic ligands; correspondingly, the research lies at the interface of biochemistry, organic chemistry, and inorganic chemistry. The interdisciplinary nature of the work makes it well suited for the education of scientists at all levels. The outreach portion of the project will encourage and enable more undergraduate students in the East Alabama/West Georgia area to participate in scientific research through interactions with a recently established local chemistry symposium and a pre-existing Research Experiences for Undergraduates program. Manganese, iron, and zinc complexes with cyclams with appended quinol groups have proven to be highly water-stable and highly active catalysts for the dismutation of hydrogen peroxide to water and oxygen. More efficient catalase activity is hypothesized to result when the quinol portion of the organic ligand is in the second-sphere, rather than the inner-sphere, of the metal ion. The proposed compounds will test this hypothesis by featuring synthetic modifications that should weaken the binding affinities of the quinols to the metal ions while keeping them close enough to react with metal-bound molecules of hydrogen peroxide. Specific modifications that will be investigated include A) lengthening the linker between the quinol and the macrocycle, B) installing steric bulk near the metal-binding oxygen atom on the quinol, C) replacing the cyclam with a macrocycle that will tilt the quinols farther away from the metal, and D) installing electron-donating substituents on the quinol. Successful catalase mimics will be mechanistic probed through spectroscopic experiments and calculations to determine the relative contribution of metal- and ligand-based redox couples to the reactivity. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.