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).
When excess uronic acid over Cr(VI) is used, the oxidation of D-glucuronic acid (Glucur) by Cr(VI) yields D-glucaric acid (Glucar) and Cr(III) as final products. The redox reaction involves the formation of intermediate Cr(IV) and Cr(V) species, with Cr(VI) and Cr(V) reacting with Glucur at comparable rates. The rate of disappearance of Cr(VI), and Cr(V) increases with [H(+)] and [substrate]. The experimental results indicated that Cr(IV) is a very reactive intermediate since its disappearance rate is much faster than Cr(VI)/Cr(V) and decreases when [H(+)] rises. Even at high [H(+)] Cr(IV) intermediate was involved in fast steps and does not accumulate in the reaction. Kinetic studies show that the redox reaction between Glucur and Cr(VI) proceeds 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 mechanism is supported by the observation of free radicals, CrO(2)(2+) (superoxoCr(III) ion) and Cr(V) as reaction intermediates. The EPR spectra show that five-co-ordinate oxo-Cr(V) bischelates are formed at pH < or = 4 with the uronic acid bound to Cr(V) through the carboxylate and the alpha-OH group of the furanose form. Five-co-ordinated oxo-Cr(V) monochelates are observed as minor species in addition to the major five-co-ordinated oxo-Cr(V) bischelates. At pH 7.5 the EPR spectra show the formation of a Cr(V) complex where the cis-diol groups of Glucur participate in the bonding to Cr(V). In vitro, our studies on the chemistry of Cr(V) complexes can provide information on the nature of the species that are likely to be stabilized in vivo. In particular, the EPR pattern of Glucur-Cr(V) species can be used as a finger print to identify Cr(V) complexes formed in biological systems.
When a 60‐times or higher excess of D‐glycero‐D‐gulo‐heptono‐1,4‐lactone (GHL) over CrVI is used, reaction yields D‐gluconic acid, formic acid and CrIII as final products. The redox reaction involves formation of intermediates, CrIV and CrV species, reacting with GHL at comparable rates. CrIV is a very reactive intermediate and does not accumulate during this reaction; its rate of disappearance is 2.0 × 104 and 4.0 × 103 times higher than CrVI or CrV reaction with GHL, respectively. Kinetic studies show that the redox reaction proceeds through a mechanism combining CrVI → CrIV → CrII and CrVI → CrIV → CrIII pathways. This mechanism is supported by the observation of free radicals, superoxoCrIII (CrO) and oxo‐CrV as intermediates species. Complete rate laws for the GHL/chromium redox reaction are described in the present work. EPR spectra show that five‐coordinate oxo‐CrV bischelates (giso1 = 1.9802; giso2 = 1.9803) are formed at pH ≤ 4 where the OH and O‐ring groups of GHL participate in the bonding to oxo‐CrV. Penta‐coordinated oxo‐CrV monochelates are observed as minor species (giso3 = 1.9866; giso4 = 1.9879) in addition to the major penta‐coordinated oxo‐CrV bischelates. Copyright © 2010 John Wiley & Sons, Ltd.
The equilibrium reactions between deprotonated D-lactobionic acid (4-O-β-D-galactopyranosyl-D-gluconic acid) and cobalt(II), nickel(II), copper(II), zinc(II), cadmium(II), and mercury(II) have been studied by potentiometric and spectrophotometric methods in aqueous solution. All measurements have been carried out at a temperature of 20.0 ± 0.1 °C and at an ionic strength of 0.100 M (NaNO3) with the corresponding stability constants calculated by applying computational methods. The interactions between the proposed cations with deprotonated D-lactobionic acid were compared with those corresponding to D-gluconic acid. Compounds of type: Co(C12H21O12)2•2H2O•C2H5OH, Ni(C12H21O12)2•2H2O•C2H5OH, Cu(C12H21O12)2•2H2O•C2H5OH, Zn(C12H21O12)2•2H2O•C2H5OH, and Cd(C12H21O12)2•2H2O•0.5C2H5OH have been isolated. These metal–sugar salts were characterized by elemental, thermogravimetric, and susceptibility analyses and FT-IR, UV–visible absorption, diffuse reflectance, and 13C NMR spectroscopies. Keywords: D-lactobionate, metal complexes, equilibrium constants, potentiometry, solid state studies.
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