Kinetic Studies and Inner-Sphere Mechanisms of the Ceric Sulfate Oxidation of L-Cysteine in Acidic Media

International Journal of Chemistry | Vol. 13, No. 6, June 2022 | pp. 41–48

DOI: 10.46882/2022/IJC/000164

Article Type: Original Research Paper

Title: Kinetic Studies and Inner-Sphere Mechanisms of the Ceric Sulfate Oxidation of L-Cysteine in Acidic Media

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: The kinetics of transition metal electron transfer reactions involving basic amino acids yield essential data required to map biochemical oxidation paths and structural intermediate transformations. The oxidation of L-cysteine by ceric sulfate [Ce(SO₄)₂] 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 absorption maximum of 320 nm. The empirical rate law showed a first-order dependence on [ceric sulfate] and a fractional-first-order dependence on [L-cysteine]. The reaction rate increased with rising hydronium ion concentration, revealing an acid-catalyzed pathway driven by the active protonated species. Variations in the dielectric constant of the solvent medium produced significant kinetic shifts, indicating a rate-determining step involving two polar molecular species. Stoichiometric determinations confirmed that 2 moles of L-cysteine consumed 1 mole of ceric sulfate, producing cystine and cerium(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; Ceric sulfate; L-cysteine; Activation parameters; Inner-sphere mechanism

Manuscript Timeline: Received: July 10, 2021; Revised: August 20, 2021; Accepted: September 15, 2021; Published: June 03, 2022.

Citation: Usman, T. M., & Al-Otaibi, F. A. (2022). Kinetic Studies and Inner-Sphere Mechanisms of the Ceric Sulfate Oxidation of L-Cysteine in Acidic Media. International Journal of Chemistry, 13(6), 41–48.

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