Oxidative Degradation ofl-Isoleucine by Au3+Complexes in Weakly Acid Medium: A Kinetic and Mechanistic Investigation

Abstract

Oxidative degradation of l-isoleucine (Ileu) by Au3+ complexes has been studied spectrophotometrically in weakly acid medium (acetic acid–sodium acetate buffer, pH range 3.72–4.80) in the temperature range 288–308 K. The reaction is first order with respect to AuIII but complex order (<1) with respect to isoleucine. Ionic strength has no significant effect on the reaction kinetics. Both H+ and Cl− ions have been found to show inhibiting effect on the reaction rate. Decreasing solvent polarity exerts an adverse effect on the reaction. Au3+ complexes react with the zwitterion form of isoleucine in a one-step two-electron transfer redox process. The reaction passes through intermediate formation of iminic cation, which hydrolyzes to produce 2-methyl butanal, identified by 1H NMR. The activation parameters ΔH≠ and ΔS≠ related to the rate-limiting step of the reaction are evaluated. The derived rate law is in excellent agreement with the experimental results. The kinetic and activation parameters of this investigation have been compared and analyzed with those of the oxidation of l-leucine by gold(III).