Synthesis,Characterization And Reactivity Study Of Polyphenol Oxidase Model Compounds

Twelve polyphenoloxidase (PPO) model complexes containing mono-, di- and polynuclear metal centres have been synthesized, and characterized by elemental analysis, molar conductivity, ultraviolet-visible spectra(UV-Vis), infrared spectra(IR), electron spin resonance(ESR) ,cyclic voltammetry(CV) and X-ray crystallography.Besides these complexes' structures detected by X-ray diffractometer, two crystal structures of ligands BBA(1, 2-bis(benzotriazolylethyl) amine) and DTPB(1,1,4,7,7-pentakis(1H-benzimidazol-2-ylmethyl)-1,4,7-triazaheptane) have been obtained. Mononuclear complexes including five ligand systems amount to 9, such as [Cu(IDB)Cl₂]·MeOH(I1) , [Cu(EDTB)]SO₄·2HOCH₂CH₂OH·H₂O(ED1), Cu(CTB) (NO₃)₂·EtOH (C1), [Cu(BBA)₂ (NO₃) ] NO₃ (B1), [Ni(DTPB)Cl](NO₃)₃·5H₂O (D2). Among them, Bl is the first example of crystal structure in BBA system. It crystallized in triclinic crystal system, P(?) space group. Cu(II) in the complex adopts a distorted square pyramid coordination geometry. A benzotriazol ring is flicked away due to steric hindrance. [Cu ₂(EGTB)(NO₃)₂(DMF)₂](NO₃)₂·2DMF ( EG1 ) and [C(?)₂(EGTB)(NO₃)₂(DMF)₂] (NO₃)₂·2DMF (EG2) are two dinuclear complexes, which have a general formula.The coordination geometry around each metal ion can be described as a distorted octahedron, consisting of two benzimidazolyl nitrogens, one amine nitrogen from ligand EGTB, two oxygens of bidentate ligand NO₃^- and one DMF carbonyl oxygen. The Cu…Cu and Co…Co distance are 11.391A and 11.804 A, respectively. They may be considered as two mononuclear compounds.[Cu₄ (DTPB)₂Cl₄](NO₃)₄ 5H₂O MeOH (D1) is a polynuclear (tetranuclear) complex, whose stacking figure has an infinitely outspread zigzag structure owing to strongΠ-Πstacking in the adjacent benzimidazole rings of DTPB. In the crystal structure of D1, it has four Cu(II) ions and two Cu-Cl bridges. The coordination structure around each Cu(II) can be divided into two categories: one is distorted five-coordinated triangle bipyramid, the other is distorted square pyramid.The PPO activity of each model complex has been studied by UV-Vis, using catechol and pyrogallol as substrates. The results show that all complexes' activities increase with increases of reaction temperatures and pH values when pH and the temperature are changed at 7.60—8.50 and 20—50℃. While kept the conditions above unchanged, the reaction rates of each complex obey the first order reaction kinetics. The binuclear complexes EG1 and EG2 catalyzed pyrogallol in different conditions are emphasized. It was found that their kinetic behaviors are in line with the Michaelis-Menten equation.The final oxidative product of catechol is purpurogallinoid. Based on electrospray ionization mass spectra in positive and negative ion modes ESI-MS(±) and ESR spin-trapping data, possibly catalyzed mechanisms of pyrogallol are proposed. Pyrogallol was firstly bound to a metal center to form a semiquinone radical, then it was oxidized purpurogallin and purpurogallinoid as well as a series of their phenol-quinone tautomers. There seems to exist simultaneously intradiol and extradiol mechanisms of pyrogallol oxidation. The products of polyphenol can further be hydroxylated via an active oxygen radical.