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

The autoxidation of gluconate (Gluc) and glucuronate (GlcA) catalyzed by copper(II) ions was studied in alkaline medium (pH 13), at 30.0±0.1 °C, by monitoring the oxygen uptake and the oxidative degradation of the ligands. Different complexes of copper with gluconate were obtained, and their 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 third complex, the dimeric Na2[Cu2(Gluc)2(OH)2], was obtained at pH 5.0. The most active complex was Na[Cu(Gluc)(OH)], showing a first-order dependence on both complex and gluconate concentrations. A saturation effect was observed when a large excess of gluconate was added to the reaction solution. 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)×10−3 mol−1 dm3 s−1 was determined, based on manometric measurements of the consumed oxygen. Comparative studies revealed a faster oxidation of glucuronate in the presence of copper(II) ions, with k=(2.5±0.3) mol−1 dm3 s−1. Free radicals were detected as reactive intermediates in these catalyzed oxidations by EPR spectroscopy, using DMPO or POBN as spin traps. Evidence of the degradative oxidation of both ligands was obtained by the formation of formate, glycolate and carbonate ions, resulting from carbon–carbon bond cleavage, monitored by capillary electrophoresis.