Silver(I) and Copper(II) Catalysis for Oxidation of Histidine by Cerium(IV) in Acid Medium: A Comparative Kinetic Study

Abstract

The catalytic effect of silver(I) and copper(II) ions on the oxidation of histidine by cerium(IV) in aqueous sulfuric acid solutions was studied spectrophotometrically at a constant ionic strength of 3.0 mol dm−3 and at 25°C. In both uncatalyzed and metal ions‐catalyzed paths, the reactions exhibited first‐order kinetics with respect to [Ce(IV)] and [catalyst], and fractional first‐order dependences with respect to [His] and [H+]. The oxidation rates increased as the ionic strength and dielectric constant of the reactions media increased. The catalytic efficiency of Ag(I) was higher than that of Cu(II). Plausible mechanistic schemes for both uncatalyzed and catalyzed reactions were proposed, and the rate laws associated with the suggested mechanisms were derived. In both cases, the final oxidation products of histidine were identified as 2‐imidazole acetaldehyde, ammonium ion, and carbon dioxide. The activation parameters associated with the second‐order rate constants were evaluated.

Redox Activity of Ce(IV)-Substituted Polyoxometalates toward Amino Acids and Peptides

“…Another indication of complex formation between Ce IV K and Cys was the faster reduction of Ce IV K observed upon increasing the ionic strength of the solution by adding 0.05 M NaClO 4 . Increasing the ionic strength helps in shielding the electrostatic repulsion between the negatively charged carboxylate group of the Cys and POM surface, facilitating the reagent approximation interaction and thereby accelerating the reaction. ,,, …”

“…Increasing the ionic strength helps in shielding the electrostatic repulsion between the negatively charged carboxylate group of the Cys and POM surface, facilitating the reagent approximation interaction and thereby accelerating the reaction. 62,29,63,64 Redox Activity of Ce IV K toward Aromatic Amino Acids (Trp, Tyr, Phe, and His). In contrast to the fast reaction between Ce IV K and Cys yielding Ce III K and cystine, the reduction of Ce IV K in the presence of the redox-active aromatic amino acids Trp, Tyr, Phe, and His was much slower, and no products of amino acid oxidation could be detected.…”