Redox reactions of dopamine transients in aqueous solution: a pulse radiolysis study

Abstract

On reaction of eaq– with protonated dopamine (D) a transient optical absorption band [formed at λmax= 355 nm (ε= 5100 dm3 mol–1 cm–1)] has been assigned to an ewaq adduct of the dopamine ring (D˙–)(the first site of attack of eaq–), which subsequently goes to the amine site to cleave the C–N bond. The bimolecular rate constant for the reaction of eaq– with protonated dopamine has been determined to be 2.5 × 108 dm3 mol–1 s–1. The transient optical absorption band (λmax= 355 nm, ε= 3760 dm3 mol–1 cm–1) formed on reaction of H atoms with protonated dopamine has been assigned to a H-adduct of dopamine. The bimolecular rate constant for the reaction has been determined to be 4.2 × 109 dm3 mol–1 s–1 from the build-up kinetics of the 355 nm band. The transient H-adduct decays with the formation of ammonia. The isopropanol radical is unable to undergo a one electron transfer reaction with dopamine, whereas an equilibrium is established between the benzamide radical anion and dopamine from which the redox potential for the D/D˙– couple has been determined to be –1.91 V. The dopamine radical anion (D˙–) is a strong reducing agent and is able to reduce methyl viologen with a bimolecular rate constant of 3.3 × 1010 dm3 mol–1 s–1. Specific one electron oxidants are able to undergo one electron transfer reactions forming a dopamine semiquinone radical (DSQ˙)(λmax= 290 nm, ε= 7750 dm3 mol–1 cm–1).