Active Site Models for Galactose Oxidase. Electronic Effect of the Thioether Group in the Novel Organic Cofactor

The electronic effect of the thioether linkage between Tyr 272 and Cys 228 (the novel organic cofactor) of galactose oxidase has been examined by using model compounds, 2-(methylthio)-p-cresol (1H), 2-(methylthio)-4,6-dimethylphenol (2H), and 2-(methylthio)-4-methyl-6-[[bis[2-(2-pyridyl)ethyl]amino]methyl]phenol (3H), the physicochemical properties of which are compared to those of 2-[[bis[2-(2-pyridyl)ethyl]amino]methyl]-4-methylphenol (4H) and p-cresol (5H). ¹H NMR and electrochemical studies indicate that the methylthio group has essentially an electron-donating nature. On the other hand, the lower pK_a values of 1H and 2H as compared to that of 5H suggest that the methylthio group also has a 2pπ-3dπ electron conjugative effect, stabilizing the negative charge on the phenolate oxygen. Furthermore, the electron-sharing conjugative effect of the substituent in the radical state has been clearly demonstrated by ESR studies and semiempirical molecular orbital calculations. Dimer copper(II) complexes [Cu^II₂(3^-)₂](PF₆) ₂ (7) and [Cu^II₂(4^-)₂](PF₆) ₂ (8) were prepared, and the crystal structures were determined by the X-ray diffraction method. Electrochemical analyses of the monomeric species [Cu^II(3^-)(py)](PF₆) (9) and [Cu^II(4^-)(py)](PF₆) (10) generated in situ by adding an external ligand such as pyridine (py) reveal that the methylthio substituent in the copper complex shows electronic effects similar to those of the free ligand stabilizing the phenoxyl radical state of the cofactor moiety in the Cu(II) complex.