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

Abstract

On reaction of e_aq^– with protonated dopamine (D) a transient optical absorption band [formed at λ_max= 355 nm (ε= 5100 dm³ mol^–1 cm^–1)] has been assigned to an ew_aq adduct of the dopamine ring (D˙^–)(the first site of attack of e_aq^–), which subsequently goes to the amine site to cleave the C–N bond. The bimolecular rate constant for the reaction of e_aq^– with protonated dopamine has been determined to be 2.5 × 10⁸ dm³ mol^–1 s^–1. The transient optical absorption band (λ_max= 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˙)(λ_max= 290 nm, ε= 7750 dm³ mol^–1 cm^–1).