SYNTHETIC MODELS OF THE INACTIVE COPPER(II)-TYROSINATE AND ACTIVE COPPER(II)-TYROSYL RADICAL FORMS OF GALACTOSE AND GLYOXAL OXIDASES

A series of Cu-II and Zn-II complexes with new ligands having either o ne or two substituted phenolates appended to the 1,4,7-triazacyclonona ne frame were prepared and characterized by optical absorption, EPR, N MR, and/or resonance Raman spectroscopy, cyclic voltammetry, and, in e ight cases, X-ray crystallography. Features of the active site geometr ies of the Cu-II-tyrosinate forms of galactose and glyoxal oxidases (G AO and GLO) were modeled by these complexes, including the binding of a redox-active phenolate and an exogenous ligand (Cl-, CH3CO2-, or CH3 CN) in a cis-equatorial position of a square pyramidal metal ion. The role of the unique ortho S-C covalent bond between a cysteine (C228) a nd the equatorial tyrosinate (Y272) in the proteins was probed through an examination of the optical absorption and electrochemical properti es of sets of similar complexes comprised of phenolate ligands with di ffering ortho substituents, including thioether groups. The o-alkylthi o unit influences the PhO- --> Cu-II LMCT transition and the M-II-phen olate/M-II-phenoxyl radical redox potential, but to a relatively small degree. Electrochemical and chemical one-electron oxidations of the C u-II and Z(II) complexes of ligands having tert-butyl protecting group s on the phenolates yielded new species that were identified as novel M-II-phenoxyl radical compounds analogous to the active Cu-II-tyrosyl radical forms of GAO and GLO. The M-II-phenoxyl radical species were c haracterized by optical absorption, EPR, and resonance Raman spectrosc opy, as well as by their stoichiometry of formation and chemical reduc tion. Notable features of the Cu-II-phenoxyl radical compounds that ar e similar to their protein counterparts include EPR silence indicative of magnetic coupling between the Cu-II ion and the bound radical, a b and with lambda(max) approximate to 410 nm (epsilon x 3900 M-1 cm(-1)) in UV-vis spectra diagnostic for the phenoxyl radical, and a feature attributable to the phenoxyl radical C-O vibration (nu(7a)) in resonan ce Raman spectra. Similar Raman spectra and electrochemical behavior f or the Zn-II analogs, as well as an isotropic signal at g = 2.00 in th eir X-band EPR spectra, further corroborate the formulations of the M- II-phenoxyl radical species.