Oxidation of 2-amino-2-deoxy-D-glucopyranose by hypervalent chromium: Kinetics and mechanism

Amino sugars have been reported to occur in plants, microorganisms, and crustaceans and are an important constituent of the soil nitrogen; even, they are protected from decomposition in soils by being incorporated into the humic acid fraction. In order to obtain a more complete insight into the interaction of metal ions with these compounds a spectroscopic and kinetic study interaction of hypervalent chromium with 2-amino-2-deoxy-D-glucopyranose (Nglc) in aqueous solution was undertaken. Kinetic studies results afforded that the oxidation of Nglc by Cr-VI in acid media proceeded through a mechanism combining one- and two-electron pathways for the reduction of intermediate Cr-IV by the organic substrate: Cr-VI -> Cr-IV -> Cr-II and Cr-VI -> Cr-IV -> Cr-III. The proposed mechanism is supported by the observation of Cr-VI esters, free radicals, CrO22+ (superoxoCr(III) ion) and oxo-Cr-V complexes as reaction intermediates. Intermediacy of free radicals and Cr-II in the reaction was proved by the observation of induced polymerization of acrylamide and detection of CrO22+ respectively. The experimental results indicated that Cr-IV was a highly reactive intermediate since its disappearance rate was much faster than Cr-VI and Cr-V. Even at low [H+], Cr-IV intermediate was involved in fast steps and did not accumulate in the redox reaction. Oxo-Cr-V-Nglc species were detected by EPR spectroscopy at low [H+]. In 0.40-1.00 M HClO4, Cr-V rapidly decomposed affording the reaction products, while, at pH 4.0 and 7.5, where the redox processes were very slow, five-coordinate oxo-Cr-V bis-chelates and mono-chelates of Nglc remained in solution and could be detected by EPR spectroscopy at room temperature. Kinetics results indicated that replacement of C2-OH by C2-NH2 retarded the oxidation reaction of Nglc by Cr-VI/IV thus evidencing the role of -NH2 group at C2 of the sugar frame. (C) 2011 Elsevier Ltd. All rights reserved.