title: New insights on the mechanism of oxidation of D-galacturonic acid by hypervalent chromium.
abstract: The pollutant Cr(VI) is known to be very carcinogenic. In conditions of excess of Cr(VI), oxidation of D-galacturonic acid (Galur), the major metabolite of pectin, yields d-galactaric acid (Galar) and Cr(III). The redox reaction takes place through a multistep mechanism involving formation of intermediate Cr(II/IV) and Cr(V) species. The mechanism combines one- and two-electron pathways for the reduction of Cr(IV) by the organic substrate: Cr(VI)→ Cr(IV)→ Cr(II) and Cr(VI)→ Cr(IV)→ Cr(III). This is supported by the observation of the optical absorption spectra of Cr(VI) esters, free radicals, CrO(2)(2+) (superoxoCr(III) ion) and oxo-Cr(V) complexes. Cr(IV) cannot be directly detected; however, formation of CrO(2)(2+) provides indirect evidence for the intermediacy of Cr(II/IV). Cr(IV) reacts with Galur much faster than Cr(V) and Cr(VI) do. The analysis of the reaction kinetics via optical absorption spectroscopy shows that the Cr(IV)-Galur reaction rate inversely depends on [H(+)]. Nevertheless, high [H(+)] still does not facilitate accumulation of Cr(IV) in the Cr(VI)-Galur mixture. Cr(VI) and the intermediate Cr(V) react with Galur at comparable rates; therefore the build-up and decay of Cr(V) accompany the decay of Cr(VI). The complete rate laws for the Cr(VI), Cr(V) and Cr(IV)-Galur redox reaction are here derived in detail. Furthermore, the nature of the five-co-ordinated oxo-Cr(V) bischelate complexes formed in Cr(VI)-Galur mixtures at pH 1-5 is investigated using continuous-wave and pulsed electron paramagnetic resonance (EPR) and density functional theory (DFT).
authors.affiliation: Departamento de Química, Facultad de Ciencias Bioquímicas y Farmacéuticas, UNR, Suipacha 531, 2000, Rosario, Argentina.
authors.forename: M F
authors.initials: MF
authors.lastname: Mangiameli
authors.forename: J C
authors.initials: JC
authors.lastname: González
authors.forename: S I
authors.initials: SI
authors.lastname: García
authors.forename: M I
authors.initials: MI
authors.lastname: Frascaroli
authors.forename: S
authors.initials: S
authors.lastname: Van Doorslaer
authors.forename: J M
authors.initials: JM
authors.lastname: Salas Peregrin
authors.forename: L F
authors.initials: LF
authors.lastname: Sala
mesh_terms: D002857:Chromium / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy; D006603:Hexuronic Acids / Q000737:chemistry*; D007700:Kinetics; D010084:Oxidation-Reduction; D011789:Quantum Theory
chemical_list: D006603:Hexuronic Acids; D002857:Chromium; C007819:galacturonic acid