UCLA

Biological systems can often drive complex chemical transformations under mild conditions (e.g., aqueous solution, physiological pH, room temperature and atmospheric pressure), which is difficult to achieve in conventional chemical reactions. This unique ability is generally empowered by a series of synergistic protein catalysts that can facilitate reaction cascades through complex metabolic pathways. There is significant interest in exploring molecular assemblies and/or conjugated catalytic systems as analogs to the functional proteins that can facilitate chemical transformation under biologically mild conditions. Hemin, the catalytic center for many protein families including cytochromes, peroxidases, myoglobins and hemoglobins, can catalyze a variety of oxidation reactions like peroxidase enzymes. However, direct application of hemin as an oxidation catalyst is a significant challenge because of its molecular aggregation in aqueous solution to form catalytic inactive dimers and oxidative self-destruction in the oxidizing media, which causes passivation of its catalytic activity. In the first part of my thesis, we employed the graphene supported hemin as a high efficient peroxidase-mimic catalyst, showing with exceptionally high catalytic activity (kcat) and substrate binding affinity (KM) approaching that of natural enzymes. The second part of my thesis, we conjugate enzymatic catalyst glucose oxidase (GOx) to the graphene-hemin conjugate. The graphene-hemin-GOx catalyst can readily enable the continuous generation of nitroxyl, an antithrombotic species, from physiologically abundant glucose and L-arginine. We also demonstrate the conjugates can be embedded within polyurethane to create a novel, long-lasting antithrombotic coating for blood contacting biomedical devices. The last part of my thesis, we developed palladium particles with specially engineered ligands to mimic the catalytic cycle of hemin using oxygen as oxidant for ketone dehydrogenative oxidation reaction. By varying the ligands, yield was optimized to 93%, much higher than 1.5% reported previously, exhibiting good heterogeneity and recyclability. Systematic mechanistic studies demonstrate that the ligands can significantly impact on the reaction pathway to modify the catalytic activity and stability. The resulted heterogeneous catalysts also present also have far better activity and stability than homogenous Pd(II) catalysts in high turnover conditions. The novel design on the above biomimetic catalysts have contributed to advancing the synthesis methods and fundamental understandings of heterogeneous catalysts, impacting many applications such as biomedical devices and green chemistry.