Visualizing Catalytic Oxidation of Tryptophan by Nanoceria via an Oligonuclear Cerium Oxo-Complex Model

Metal oxide species interact with biologically relevant molecules, which are crucial to the life cycle of plants and animals. Metal oxides can also act as catalysts in various reactions required for proper plant development. In this study, we investigated the hydrolysis of inorganic Ce(IV) precursors in the presence of carboxylic acids, leading to the formation of oligonuclear cerium oxo-complexes. The structure of the species was obtained by X-ray single-crystal studies and found to be hexanuclear, with the composition Ce6O4(OH)4(H2O)2(NO3)3(C7H5O2)9(C3H7NO)4 (Ce-BA-DMF). The catalytic properties of these complexes on the oxidation of amino acids have been investigated, aiming to establish a transformation mechanism providing insights into both molecular and surface interactions. A redox feature assigned to the CeIV/III couple in the cerium oxo-complex was observed by cyclic voltammetry and found to be sufficiently positive to oxidize tryptophan directly, without the need for intermediate generation of reactive oxygen species. Our findings provide new insights into the possible molecular mechanisms and open the door for more targeted applications of ceria nanoparticles in agriculture and biomedicine.