Kinetics and mechanism of the reduction of Cr(VI) and Cr(V) by D-lactobionic acid

V. ROLDÁN; MABEL SANTORO; JUAN CARLOS GONZÁLEZ; JUAN MANUEL SALAS-PEREGRIN; SANDRA SIGNORELLA; LUIS F. SALA

JOURNAL OF INORGANIC BIOCHEMISTRY

Elsevier

Año: 2004 vol. 98 p. 347 - 357

0162-0134

The oxidation of D-lactobionic acid by CrVI yields the 2-ketoaldobionic acid and Cr3+ as final products when a 20-times or higher excess of the aldobionic acid over CrVI is used. The redox reaction takes place through a complex multistep mechanism which involves the formation of intermediate CrIV and CrV species. CrIV reacts with lactobionic acid much faster than CrV and CrVI do, and can not be directly detected. However, the formation of CrO22+, observed by the first time for an acid saccharide/CrVI system, provides indirect evidence for the intermediacy of CrIV in the reaction path. CrVI and the intermediate CrV react with lactobionic acid at comparable rates, being the complete rate laws for the CrVI and CrV consumption expressed by: -d[CrVI]/dt = {kI + kII [H+]}[lactobionic acid][CrVI], where kI = (4.1 ± 0.1) x 10-3 M-1 s-1 and kII = (2.1 ± 0.1) x 10-2 M-2 s-1; and -d[CrV]/dt = {kIII [H+] + kIV [lactobionic acid]}[CrV], where kIII = (1.8 ± 0.1) x 10-3 M-1 s-1 and kIV = (1.1 ± 0.1) x 10-2 M-1 s-1, at 33 °C. The EPR spectra show that five-co-ordinate oxo-CrV bischelates are formed at pH 1-5 with the aldobionic acid bound to CrV through the a-hydroxyacid group.