Interaction of uronic acids with hypervalent chromium

GONZÁLEZ, JUAN C.; GARCÍA, SILVIA I.; BELLÚ, SEBASTIÁN; ATRIA, ANA MARÍA; SALA, LUIS F.; SIGNORELLA, SANDRA

Los Cocos

Congreso; 9th Latin American Conference on Physical Organic Chemistry; 2007

UNC

Compounds of CrVI represent a potential environmental hazard because of their mammalian toxicity and carcinogenicity. The observation of CrV and CrIV intermediates in the selective oxidation of organic by CrVI and their implication in the mechanism of Cr-induced cancers has generated a considerable amount of interest in the chemistry and biochemistry of this element. Pectic substances are a group of polysaccharides present in all plant, and D-galacturonic acid (Galur) and D-glucuronic acid (Glucur), are present in these biopolymers. The objective of this work was to compare the redox chemistry of different uronic acids with hypervalent chromium in acidic medium and the ability of these sacharides to coordinate CrV, CrIV and CrIII. Kinetic measurements were performed on a reaction mixture of substrate and CrVI in 1.0-0.2 M HClO4 following the time-evolution of the absorption spectra in the 350-500 nm range. Rate constants were calculated from the absorbance data obtained at 350 nm, for five different [substrate] and [H+]. The decrease in absorbance with time was not monophasic in all uronic acid CrVI systems studied. Differential UV/vis spectra of mixtures of CrVI and Glucur exhibited an absorption band with lmax = 377 nm consistent with that ascribed to CrVI oxy esters. The same was observed in mixtures of CrVI with Galur. EPR spectroscopy revealed the presence of long-lived CrV-uronic species. In the range of pH 1-4, the EPR spectra showed two triplets at giso = 1.9781 and 1.9783 and two doublets at giso = 1.9753 and 1.9763 in the CrVI- Glucur system. The giso values for the triplets correspond to those calculated for five-coordinate oxo-CrV bischelates with two carboxilate and two alcoholate ligands. Doublets at giso = 1.9754 and 1.9763 correspond to those calculated for five-coordinated oxo-CrV monochelates with one carboxilate and one alcoholate ligands, and two alcoholate ligands, respectively. In the CrVI-Galur system only five-coordinate oxo-CrV bischelates are formed. The same occurs in the CrVI-oligogalacturonic acid system. A periodic scanning of the O2-saturated solution of a CrVI + uronic acid reaction mixture in 2.0M HClO4 showed two absorption bands at 290 nm and 247 nm characteristic of CrO22+. These spectroscopic results reveal that CrII forms in the redox reaction, and can be taken as evidence that CrIV is implied in the redox mechanism. The reaction between uronic acids and CrIV was studied by in situ generation of (H2O)5CrO2+.  Free radicals formed in the CrIV ® CrIII path were detected by the induced polymerization of acrylonitrile. The redox reaction with CrVI is proposed to proceed through a mechanism combining CrVI ® CrIV ® CrII and CrVI ® CrIV ® CrIII pathways. Comparing the reactivities of these saccharides against CrVI, oligogalacturonic acids are more reactive than Galur or Glucur.