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Guanqi Wang

High oxidation state of galactose oxidase-like copper complexes for biomimetic alcohol oxidation

Published on 12 December 2023

Thesis presented December 12, 2023

Galactose Oxidase (GOase), a copper-containing metallo-enzyme, is one of the most studied biocatalysts for the enzymatic oxidation of carbohydrates. The consensus mechanism involves the key oxidized form (GOaseox), in which an the carbohydrate substrate (galactose unit) binds to the equatorial (free) site and is subsequently deprotonated. It undergoes hydrogen atom abstraction by the radical and further electron transfer to give the final product aldehyde and the reduced form of the GOase. Due to the potential for highly selective catalytic oxidations, the development of small-molecular models of the GOase active site has been carried​ out. Notably, sterically hindered schiff bases, which stand as one of the most representative mimics, have garnered significant attention. This biomimetic approach has extended to encompass other strategies. Within this framework, a range of Cu(II)-phenol complexes, serving as pre-catalysts, have been synthesized, subsequently undergoing one-electron oxidation to yield the "active" catalyst form. A central question then arises: What factors determine whether the oxidation pathway proceeds toward the ligand, resulting in the formation of Cu(II)-phenoxyl radicals, or toward the metal, giving rise to the Cu(III)-phenolate species? Despite substantial efforts, a definitive answer to this question has yet to be obtained.

The aim of this thesis is to develop redox-active ligands aimed at understanding the factors affecting the oxidation state of copper, able to catalyze the oxidation of an alcohol into an aldehyde. The strategy is to include chemical functions that can stabilize either valence tautomer (Cu(II)-phenoxyl radical and Cu(III)-phenolate) and study their effect. For that purpose, several complexes were synthesized and characterized by different ways to understand their properties. The catalytic activities were also tested against different families of substrates comprising hydroxyl functions. Finally, quantum chemistry (DFT) calculations have been carried to help understand the characteristics of different complexes and elucidate of the catalytic mechanisms at work.

High Oxidation State of Copper, Catalysis,mechanism, Galactose oxidase like complexes