Phenoxazinone Synthase Functional Mimic Activity of Mixed-ligand Copper(II) Complexes of L-proline and Diimine Ligands Conjugated with Chitosan

Phenoxazinone synthase (PHS) is a copper-containing bacterial enzyme that is involved in vital biological oxidation processes. Mimicking these enzymatic functions using synthetic copper complexes provides valuable insights and potential applications in green catalysis. In this work, chitosan-conjugated ligands were synthesized and anchored with Cu(II) to yield mixed-ligand complexes ( 1 and 2 ) of the general formula [Cu( L )(bpy)(H ₂ O)] ²⁺ (1) , and [Cu( L )(phen)(H ₂ O)] ²⁺ (2) , where L represents chitosan-conjugated-proline polymer, and diimine ligands bpy and phen represent 2,2’-bipyridine and 1,10-phenanthroline, respectively. SEM, PXRD, FTIR, and various spectroscopic methods were used to characterize the ligands and complexes. ICP-OES data revealed that complexes 1 and 2 contain 10.72% and 9.81% of copper, respectively. The oxidation of o-aminophenol (OAP), used as a model substrate, monitored via UV-Visible spectrophotometry, provides valuable kinetics and mechanistic insights into substrate interactions. Interestingly, among all the complexes, 2 exhibited the highest catalytic activity, which features phenanthroline as a co-ligand, followed by 1 and then the free ligand ( L ), highlighting the influence of the co-ligand in the oxidation of o-aminophenol to phenoxazinone as the functional mimic of phenoxazinone enzyme. Notably, the same complex 2 demonstrates 72% conversion of o-aminophenol (OAP) to 2-aminophenoxazine-3-one (APX). Kinetic analysis revealed pseudo-first-order rate constants consistent with efficient oxidase mimics. These results demonstrate the potential of Cu-chitosan hybrids as sustainable, bioinspired catalysts.