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 examin ed by using model compounds, 2-(methylthio)-p-cresol (1H), 2-(methylth io)-4,6-dimethylphenol (2H), and hyl-6-[[bis[2-(2-pyridyl)ethyl]amino] methyl]phenal (3H), the physicochemical properties of which are compar ed to those of 2-[[bis[2-(2-pyridyl)ethyl]amino]methyl] methylphenol ( 4H) and p-cresol (5H). H-1 NMR and electrochemical studies indicate th at the methylthio group has essentially an electron-donating nature. O n the other hand, the lower pK(a) values of 1H and 2H as compared to t hat of 5H suggest that the methylthio group also has a 2p pi-3d pi ele ctron conjugative effect, stabilizing the negative charge on the pheno late oxygen. Furthermore, the electron-sharing conjugative effect of t he substituent in the radical state has been clearly demonstrated by E SR studies and semiempirical molecular orbital calculations. Dimer cop per(II) complexes [Cu(II)2(3(-))(2)](PF6)(2) (7) and [Cu-2(II)(4-)(2)] (PF6)(2) (8) were prepared, and the crystal structures were determined by the X-ray diffraction method. Electrochemical analyses of the mono meric species [Cu-II(3(-))(py)](PF6) (9) and [Cu-II(4(-))(py)](PF6) (1 0) generated in situ by adding an external ligand such as pyridine (py ) reveal that the methylthio substituent in the copper complex shows e lectronic effects similar to those of the free ligand stabilizing the phenoxyl radical state of the cofactor moiety in the Cu(II) complex.