Bimetallic Ce/Zr UiO-66 Metal–Organic Framework Nanostructures as Peptidase and Oxidase Nanozymes

The catalytic activity of metal–organic frameworks (MOFs) toward peptides and proteins provides an attractive route for the development of nanozymes for applications in biotechnology and proteomics, particularly in the field of protein identification using mass spectrometry. Here, we report that carefully tuning the Ce/Zr metal ratio is a promising strategy to overcome structural limitations that originate from the high connectivity of the Zr6 node and also increase the peptidase activity of the MOF while preserving the material's nano-topology and stability. A series of bimetallic Ce/Zr-UiO-66 MOFs, in which the amount of Ce was systematically varied from 28 to 87 mol%, have been shown to efficiently catalyze peptide bond hydrolysis in a large variety of peptides with different functional groups, demonstrating their nanozyme potential. Detailed kinetic analysis of the hydrolysis of peptide bonds with a range of Ce/Zr MOFs suggests that among the different metallic clusters present in UiO-66, the Ce6 clusters have superior reactivity compared to the CeZr5 sites. In addition to increasing the catalytic potency of the MOF toward peptide bond hydrolysis, the introduction of Ce(IV) also broadens the reaction scope of MOF catalysts. Selective oxidation of the thiol sidechains and the formation of disulfide bridges have been observed at physiological pH both in cysteine and in glutathione tripeptide as substrates. The rate of oxidation is directly proportional to the amount of Ce present in the MOF, demonstrating that the introduction of Ce into these nanomaterials is a promising strategy to introduce oxidase activity toward biologically relevant substrates. In addition to this, adsorption of dipeptides onto MOF nanomaterials has been studied for the first time. These studies revealed a close link between the nature of peptide side chains and the extent of their adsorption, which has a direct influence on their ability to act as substrates in MOF-catalyzed reactions.