Kinetic Studies and Free-Radical Mechanisms of the Alkaline Ferricyanide Oxidation of L-Arginine

International Journal of Chemistry | Vol. 11, No. 11, November 2020 | pp. 81–88

DOI: 10.46882/2020/IJC/000145

Article Type: Original Research Paper

Title: Kinetic Studies and Free-Radical Mechanisms of the Alkaline Ferricyanide Oxidation of L-Arginine

Names of Authors: T. M. Usman¹, F. M. Al-Rasheed²*

Authors’ Affiliations:
¹Department of Chemistry, Bayero University, Kano, Nigeria.
²Department of Chemistry, King Saud University, Riyadh, 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-arginine by ferricyanide ions ([Fe(CN)₆]³⁻) was investigated spectrophotometrically in an aqueous sodium hydroxide medium at a constant ionic strength of 0.20 M (NaClO₄). The reaction progress was monitored under pseudo-first-order conditions by following the absorbance decay of [Fe(CN)₆]³⁻ at its absorption maximum of 420 nm. The empirical rate law showed a first-order dependence on [ferricyanide] and a fractional-first-order dependence on [L-arginine]. The reaction rate increased with rising hydroxyl ion concentration, revealing a base-catalyzed pathway driven by the active unprotonated amine species. Variations in ionic strength produced positive kinetic shifts, indicating a rate-determining step involving two similarly charged ionic species. Stoichiometric determinations confirmed that 1 mole of L-arginine consumed 2 moles of ferricyanide, producing 4-guanidinobutanal and ferrocyanide 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 45.8 kJ/mol and an entropy of activation (delta S*) of -112.4 J/mol K, supporting an outer-sphere mechanism.

Keywords: Reaction kinetics; Spectrophotometry; Ferricyanide oxidation; L-arginine; Activation parameters; Outer-sphere mechanism

Manuscript Timeline: Received: April 02, 2020; Revised: May 22, 2020; Accepted: June 15, 2020; Published: November 02, 2020.

Citation: Usman, T. M., & Al-Rasheed, F. M. (2020). Kinetic Studies and Free-Radical Mechanisms of the Alkaline Ferricyanide Oxidation of L-Arginine. International Journal of Chemistry, 11(11), 81–88.

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