Phenoxazinone synthase functional mimic activity of mixed-ligand copper(II) complexes of L-proline and diimine ligands conjugated with chitosan

Abstract Phenoxazinone synthase (PHS) is a copper-containing bacterial enzyme that is involved in vital biological oxidation processes. Mimicking these enzymatic functions using 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 of the general formula [Cu(L)(bpy)(H2O)]2+ (1) and [Cu(L)(phen)(H2O)]2+ (2), where L represents chitosan-conjugated-proline polymer, and diimine ligands bpy and phen represent 2,2’-bipyridine and 1,10-phenanthroline, respectively. Fourier-transform infrared, scanning electron microscopy, powder X-ray diffraction, diffuse reflectance UV-visible (UV-vis) spectroscopy, thermogravimetric and differential thermal analyses and various spectroscopic methods were used to characterize the ligands and complexes. Inductively coupled plasma optical emission spectrometry data revealed that complexes 1 and 2 contain 10.72% and 9.81% of copper, respectively. The oxidation of o-aminophenol (OAP), a model substrate, monitored by UV-vis spectrophotometry, provides valuable kinetic and mechanistic insights into substrate interactions. Interestingly, among the complexes, 2 exhibited the highest catalytic activity, featuring phenanthroline as a co-ligand, followed by 1 and then ligand (L), highlighting the influence of the co-ligands on the oxidation of o-aminophenol to phenoxazinone, a functional mimic of phenoxazinone enzyme. Notably, complex 2 demonstrates 72% conversion of OAP to 2-aminophenoxazine-3-one. 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.