Enzyme-Mimicking of Copper-Sites in Metal-Organic Frameworks for Oxidative Degradation of Phenolic Compounds

Through the sequential assembly of simple repetitive units, biology has built up complex three-dimensional scaffolds of metalloproteins, which deposit functional amino acid residues able to coordinate the metal catalytic site in specific spatial configurations. The chemical arrangement of the amino acid residues that conforms the first and second coordination environment of the metal-sites plays a key role in defining the activity and selectivity of metalloenzymes. Thus, an alteration of any of the amino acid sequence into the heart of the metalloenzyme sites, affects drastically their efficiency, selectivity and function. An important example is the case of copper metalloenzymes, where the coordination of copper ions is usually completed by histidine, cysteine and carboxyl amino acid residues. Depending on the copper-biocatalyst, Cu^II/I ions are stabilized as isolated and/or clustered sites able to carry out oxidative catalytic reactions. The overall selectivity and oxidoreductive efficiency of copper-enzymes over a specific substrate is partially defined by the coordination modes of the copper sites. Furthermore, their oxidative capacity has important technological implications in water remediation, since it can be adapted to degrade phenolic pollutants via Catalytic Wet Peroxide Oxidation (CWPO).<br/><br/>Extensive research is being carried out to replicate and expand the metalloenzymes’ pockets and catalytic activity to robust porous materials. In this regard, the chemical encoding of bio-catalytic like sites into a particular class of porous ordered materials called Metal-Organic Frameworks (MOFs) holds an enormous promise. MOFs are crystalline solids built from metal ions or clusters that are connected by organic linkers to form extended, ordered, and highly porous networks. Thanks to their (i) impressive and ordered porosity metrics and (ii) their versatility to be encoded with functionalities placed surgically at specific positions of their frameworks, MOFs have been successfully employed as matrixes to install amino acids and peptides. Once the pore space has been decorated with specific amino acid moieties, metal-sites can be easily installed by adsorbing them from aqueous or non-aqueous solutions.<br/><br/>In this work, we have decorated the mesopore space of the MOF-808 with amino acid and carboxylic acid molecules, to later, stabilize coppers ions with varied coordination modes and clustering degrees. In a second step, we have unraveled the impact of the characteristics of the metal-sites on the catalytic wet oxidation (CWPO) of phenolic compounds. The features of the copper-sites installed within the framework have been studied by a combination of Infrared (IR), Raman, X-ray photoelectron (XPS), electron paramagnetic (EPR) and inelastic neutron scattering (INS) spectroscopies. Local structural models for the copper sites have been proposed after a crystallochemical analysis of the copper coordination modes with (amino)acid molecules found in the Cambridge Structural Database. The efficiency and selectivity of our system to oxidize phenolic pollutants through CWPO have been duly assessed, confirming that both the coordination modes and the clustering degree of the copper sites modulate its efficiency and selectivity to oxidize phenols through the hydrogen peroxide activation. The results presented in this work represent a milestone for the installation of metal-amino catalytic sites into the robust architectural backbone of MOFs, which successfully mimic those oxidative functions of metalloenzymes.<br/><br/><b>Acknowledgments: </b>The authors thank the Spanish Agencia Estatal de Investigación (AEI) (EVOLMOF PID2021-122940OB-C31 (AEI/FEDER, UE), ENZYMOF (TED2021-130621B-C42) and Tailing23Green-ERAMIN). MSCA-RISE-2017 INDESMOF (No 778412), and H2020-RIA-4AirCraft (No 101022633) are also acknowledged. Basque Government Industry and Education Departments under the IKUR, ELKARTEK and IT1554-22 programs are also acknowledged.