Synthesis, Characterization And Catalytic Activity Of Polyphenol Oxidase Model Compounds

Polyphenol oxidase is a class of natural enzymes, which is the best catalyst to degradate phenolic compounds in industrial wastewater. The study of the structure and catalytic mechanism of these enzymes is the basis for their applications in degradation of the phenolic compounds in the industrial field. This work aims to synthesize model compounds of the polyphenol oxidase to mimic their catalytic reactions, to study the role of the central metal ion on the catalytic activity of model compounds for understanding theiy catalytic mechanism.Based on the structure of active site of catechol oxidase, we synthesized two types of model compounds:pyridine and benzimidazole-containing metal complexes. Pyridine-containing metal complexes include [Cu2(μ-Cl)2(hbpg)2] (1), [Cu2(μ-OH2)2(hbpg)2](NO3)2(H2O)2 (2), [Fe2(μ-Cl)2(hbpg)2]Cl2(H2O)2 (3) and [Mn2(μ-Cl)2(hbpg)2](H2O)2 (4); Benzimidazole- containing metal compounds include [Cu(bbaa)Cl](H2O)2 (5), [Fe2(bbaa)2Cl2] Cl2H2O (6), [Mn2(bbaa)2Cl2] (7), [Cu(tbcd)]ClH2O (8), [Fe(tbcd)]Cl2 (9), [Cu2(tbhd)(μ-Cl)Cl2](H2O)2 (10), [Fe2(tbhd) (μ-Cl)Cl2]Cl2H2O (11), [Mn2(tbhd)(μ-Cl)Cl2](H2O)2 (12), in which ligand hbpg=N-(2-hydroxybenzyl)-N-(2-picolyl) glycine, bbaa=N,N-bis(2-benzimidazolyl- methyl)amino acetic acid, tbcd = N,N,N',N'-Tetrakis-(2-benzimidazolylmethyl)-N'-(carbocylmethyl) diethylenetriamine, tbhdN,N,N',N'-Tetrakis(2-benzimidazolyl-methyl)-2-hydroxo-1,3-diaminopropane. The complexes were characterized by elemental analysis, UV, IR, NMR and other methods. Most of them were binuclear, similar to the active site of catechol oxidase.Comparing the catalytic activities of complexes 1-4, we found that with the same coordination environment, metal ions have a greater impact on the catalytic activity. The complex with iron ions is more active than that of copper and manganese ions containing complexes. The order of the catalytic activity of complexes 5-12 confirmed this conclusion, the complexes with iron ions had the highest catalytic activity. The results showed that the redox potential of metal ions is one of the key factors that affect the catalytic activity of complexes, providing a theoretical guidance for the design of enzyme model compounds and the development of their application in the degradation of phenolic compounds.In addition, the catalytic reactions with bisphenol substrates are generally faster than those with monophenol compounds. The study also found that organic solvents such as methanol, and H2O2 also could raise the catalytic reaction rate with these model compounds.