A Ce-MOF as an alkaline phosphatase mimic: Ce-OH2 sites in catalytic dephosphorylation

Abstract

The field of biomimetic catalysis poses several challenges, including selective dephosphorylation of enzymes. Lanthanide-based metal–organic frameworks (Ln-MOFs) are widely used as hydrolytic enzymes due to their well-controlled structural motifs and having comparable enzyme cofactors, making them ideal for biomimetic catalysis. In this work, we have synthesized and structurally characterized a Ce–OH₂–Ce motif-containing MOF (Ce-MOF) to mimic the active sites of alkaline phosphatase. Single crystal X-ray diffraction (SXRD) analysis illustrates that the Ce-MOF has a robust ladder-like supramolecular network that is stable in a wide range of solvents and basic aqueous solutions, which is confirmed through independent powder XRD (PXRD) analysis. The catalytic activity of the Ce-MOF was investigated by UV-visible spectroscopy by hydrolyzing the model substrate, the disodium salt of 4-nitrophenyl phosphate (4-NPP), taken in an N-methylmorpholine buffer aqueous solution. Experimental studies reveal that the Ce-MOF has the highest catalytic activity for the hydrolysis of the P–O bond of 4-NPP at pH 9.0. To the best of our knowledge, this is the first time a cerium-MOF has been used as a catalyst in an alkaline medium to mimic phosphatase enzymes. Importantly, the high catalytic activity of the Ce-MOF towards 4-NPP hydrolysis was found owing to the synergistic effect of the Ce(III) ion, which reinforces the PO bond with the metal, and metal hydroxide activation under basic circumstances. The calculated turnover number (k_cat) for 4-nitrophenyl phosphate (4-NPP) hydrolysis was 7.42 × 10⁻³ min⁻¹. The formation of phosphate ions during the hydrolytic reaction has been monitored through time-dependent ³¹P-NMR spectroscopy and this provides very crucial information on the possible mechanistic pathway.