Model complexes of the active site of galactose oxidase. Effects of the metal ion binding sites

Model compounds of the active site of galactose oxidase have been developed by using new cofactor model ligands, L1H (2-methylthio-4-tert-butyl-6-[{bis(pyridin-2-ylmethyl)amino}methyl]phenol) and L2H (2-methylthio-4-tert-butyl-6-[{bis(6-methylpyridin-2-ylmethyl)amino}methyl]phenol). Treatment of the ligands with copper(II) and zinc(II) perchlorate in the presence of triethylamine followed by anion exchange reaction with NaPF6 or NaBPh4 provided the corresponding copper(II) and zinc(II) complexes, the crystal structures of which have been determined by X-ray crystallographic analysis. All the copper(II) and zinc(II) complexes have been isolated as a dimeric form in which the phenolate oxygen of each ligand acts as the bridging ligand to form a rhombic M2(OAr)2 core (M=Cu or Zn). The dimeric complexes can be converted into the corresponding monomer complexes by the treatment with exogenous ligand such as acetate ion. The redox potential and the spectroscopic features of the monomer complexes have also been examined. Furthermore, the copper(II)- and zinc(II)-complexes of the phenoxyl radical species of the ligands have been generated in situ by the oxidation of the phenolate complexes with (NH4)2[CeIV(NO3)6] (CAN) in CH3CN, and their spectroscopic features have been explored. The structures and physicochemical properties of the phenolate and phenoxyl radical complexes of L1 and L2 have been compared to those of the previously reported copper(II) and zinc(II) complexes of L3 (2-methylthio-4-tert-butyl-6-[{bis(2-pyridin-2-ylethyl)amino}methyl]phenol) in order to get insights into the interaction between the metal ions and the organic cofactor moiety.