Kinetics of the degradative oxidation of sugar-type ligands catalyzed by copper(II) ions

The autoxidation of gluconate (Gluc) and glucuronate (GlcA) catalyzed by co pper(II) ions was studied in alkaline medium (pH 13), at 30.0 +/- 0.1 degre es C, by monitoring the oxygen uptake and the oxidative degradation of the ligands. Different complexes of copper with gluconate were obtained, and th eir reactivities were compared. The compound [Cu(Gluc)(2)] is commercially available, while the compound Na[Cu(Gluc)(OH)] was isolated at pH 11.5. A t hird complex, the dimeric Na-2[Cu-2(Gluc)(2)(OH)(2)], was obtained at pH 5. 0. The most active complex was Na[Cu(Gluc)(OH)], showing a first-order depe ndence on both complex and gluconate concentrations. A saturation effect wa s observed when a large excess of gluconate was added to the reaction solut ion. Induction periods of ca. 90 min were also observed in the presence of excess gluconate. A second-order rate constant of k = (1.44 +/- 0.09) x 10( -3) mol(-1) dm(3) s(-1) was determined, based on manometric measurements of the consumed oxygen. Comparative studies revealed a faster oxidation of gl ucuronate in the presence of copper(II) ions, with k = (2.5 +/- 0.3) mol(-1 ) dm(3) s(-1) Free radicals were detected as reactive intermediates in thes e catalyzed oxidations by EPR spectroscopy, using DMPO or POBN as spin trap s. Evidence of the degradative oxidation of both ligands was obtained by th e formation of formate, glycolate and carbonate ions, resulting from carbon -carbon bond cleavage, monitored by capillary electrophoresis. (C) 1999 Els evier Science Ltd. All rights reserved.