Models for the active site in galactose oxidase: Structure, spectra and redox of copper(II) complexes of certain phenolate ligands

Galactose oxidase (GOase) is a fungal enzyme which is unusual among metallo enzymes in appearing to catalyse the two electron oxidation of primary alco hols to aldehydes and H2O2. The crystal structure of the enzyme reveals tha t the coordination geometry of mononuclear copper(II) ion is square pyramid al, with two histidine imidazoles. a tyrosinate, and either H2O (pH 7.0) or acetate (from buffer, pH 4.5) in the equatorial sites and a tyrosinate lig and weakly bound in the axial position. This paper summarizes the results o f our studies on the structure, spectral and redox properties of certain no vel models for the active site of the inactive form of GOase. The monopheno lato Cu(II) complexes of the type [Cu(L1)X][H(L1) = 2-(bis(pyrid-2-ylmethyl )aminomethyl)-4-nitrophenol and X- = Cl- 1, NCS- 2, CH3COO- 3, ClO4- 4] rev eal a distorted square pyramidal geometry around Cu(II) with an unusual axi al coordination of phenolate moiety. The coordination geometry of 3 is remi niscent of the active site of GOase with an axial phenolate and equatorial CH3COO- ligands. All the present complexes exhibit several electronic and E PR spectral features which are also similar to the enzyme. Further, to esta blish the structural and spectroscopic consequences of the coordination of two tyrosinates in GOase enzyme, we studied the monomeric copper(II) comple xes containing two phenolates and imidazole/pyridine donors as closer struc tural models for GOase. N,N-wdimethylethylenediamine and N,N'-dimethylethyl enediamine have been used as starting materials to obtain a variety of 2,4- disubstituted phenolate ligands. The X-ray crystal structures of the comple xes [Cu(L5)(py)], (8) [H-2(L5) = N,N-dimethyl-N',N'-bis(2-hydroxy-4-nitrobe nzyl) ethylenediamine, py = pyridine] and [Cu(L8)(H2O)] (11), [H-2(L8) = N, N'-dimethyl-N,N'-bis(2-hydroxy-4-nitrobenzyl)ethylenediamine] reveal distor ted square pyramidal geometries around Cu(II) with the axial tertiary amine nitrogen and water coordination respectively. Interestingly, for the latte r complex there are two different molecules present in the same unit cell c ontaining the methyl groups of the ethylenediamine fragment cia to each oth er in one molecule and trans to each other in the other. The ligand field a nd EPR spectra of the model complexes reveal square-based geometries even i n solution. The electrochemical and chemical means of generating novel radi cal species of the model complexes, analogous to the active form of the enz yme is presently under investigation.