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 (Î μ = 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, Î μ = 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, Î μ = 7750 dm³ mol^-1 cm^-1).