Oxidation of 3‐(3,4‐dihydroxy phenyl)‐l‐alanine (levodopa) and 3‐(3,4‐dihydroxy phenyl)‐2‐methyl‐l‐alanine (methyl dopa) by manganese(III) in pyrophosphate media: Kinetic and mechanistic study

Manganese(III) (Mn(III)) has been stabilized in weakly acidic solution by means of pyrophosphate and the nature of the complex was elucidated spectrophotometrically. Stoichiometry of Mn(III)‐oxidation of levodopa and methyl dopa in pyrophosphate medium was established in the pH range 2.5–4.0 by iodometric and spectrophotometric methods. The reaction shows a distinct variation in kinetic order with respect to [Mn(III)], a first‐order dependence in the pH range 1.9–2.6, decreasing to fractional order above pH 3. Other common features include first‐order dependence on [dopa], positive fractional order dependence on [H⁺], and inverse first‐order dependence on [Mn(III)] in the pH range studied. The effects of varying ionic strength and solvent composition were studied. Added ions such as SO₄^2? and ClO₄^? alter the reaction rate, probably due to the change in the formal redox potential of Mn(III)–Mn(II) couple because of the changes in coordination environment of the oxidizing species. Evidence for the transient existence of the free radical intermediate is given. Cyclic voltametric sensing of levodopa and methyl dopa has ruled out the formation of dopaquinones as oxidation products in the pH range studied. Activation parameters have been evaluated using the Arrhenius and Erying plots. Mechanisms consistent with the kinetic data have been proposed and discussed. These studies are expected to throw some light on dopa metabolism. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 449–457, 2001