Kinetic Studies and Mechanistic Pathway of the Chromic Acid Oxidation of L-Methionine in Aqueous Acidic Medium

International Journal of Chemistry | Vol. 8, No. 6, June 2017 | pp. 41–48

DOI: 10.46882/2017/IJC/000105

Article Type: Original Research Paper

Title: Kinetic Studies and Mechanistic Pathway of the Chromic Acid Oxidation of L-Methionine in Aqueous Acidic Medium

Names of Authors: T. M. Usman¹, F. A. Al-Otaibi²*

Authors’ Affiliations:
¹Department of Chemistry, Bayero University, Kano, Nigeria.
²Department of Chemistry, King Abdulaziz University, Jeddah, Saudi Arabia.

Abstract: Investigating the kinetics of transition metal electron transfer processes involving sulfur-containing amino acids provides vital structural indicators required to map metabolic oxidation pathways. The oxidation of L-methionine by chromic acid (H₂CrO₄) was investigated spectrophotometrically in an aqueous perchloric acid medium at a constant ionic strength of 0.40 M (NaClO₄). The reaction progress was monitored under pseudo-first-order conditions by following the absorbance decay of Cr(VI) at its wavelength maximum of 350 nm. The empirical rate law showed a first-order dependence on [chromic acid] and a fractional-first-order dependence on [L-methionine]. The reaction rate increased with rising hydronium ion concentration, revealing an acid-catalyzed pathway governed by the active protonated oxidant species, [HCrO₃⁺]. Variations in the dielectric constant of the solvent medium produced significant kinetic shifts, confirming a rate-determining step involving two polar molecular species. Stoichiometric determinations confirmed that 3 moles of L-methionine consumed 2 moles of chromic acid, producing methionine sulfoxide and Cr(III) ions as the primary end products. Thermodynamic activation constants calculated from temperature-dependence datasets using the Eyring equation yielded an enthalpy of activation (delta H*) of 48.5 kJ/mol and an entropy of activation (delta S*) of -104.2 J/mol K, supporting an inner-sphere mechanism.

Keywords: Reaction kinetics; Spectrophotometry; Chromic acid oxidation; L-methionine; Activation parameters; Inner-sphere mechanism

Manuscript Timeline: Received: July 10, 2016; Revised: August 20, 2016; Accepted: September 15, 2016; Published: June 03, 2017.

Citation: Usman, T. M., & Al-Otaibi, F. A. (2017). Kinetic Studies and Mechanistic Pathway of the Chromic Acid Oxidation of L-Methionine in Aqueous Acidic Medium. International Journal of Chemistry, 8(6), 41–48.

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