REDOX REACTIONS OF DOPAMINE TRANSIENTS IN AQUEOUS-SOLUTION - A PULSE-RADIOLYSIS STUDY

On reaction of e(aq)(-) with protonated dopamine (D) a transient optic al absorption band [formed at lambda(max) = 355 nm (epsilon = 5100 dm( 3) mol(-1) cm-(1))] has been assigned to an e(aq)(-) adduct of the dop amine ring (D-.-) (the first site of attack of e(aq)(-)). which subseq uently goes to the amine site to cleave the C-N bond. The bimolecular rate constant for the reaction of e(aq)(-) with protonated dopamine ha s been determined to be 2.5 x 10(8) dm(3) mol(-1) s(-1). The transient optical absorption band (lambda(max) = 355 nm, epsilon = 3760 dm(3) m ol(-1) cm(-1)) formed on reaction of H atoms with protonated dopamine has been assigned to a H-adduct of dopamine. The bimolecular rate cons tant for the reaction has been determined to be 4.2 x 10(9) dm(3) 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 radica l is unable to undergo a one electron transfer reaction with dopamine. whereas an equilibrium is established between the benzamide radical a nion 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 x 10(10) dm(3) mol(-1) s(-1). Speci fic one electron oxidants are able to undergo one electron transfer re actions forming a dopamine semiquinone radical (DSQ(.)) (lambda(max) = 290 nm, epsilon = 7750 dm(3) mol(-1) cm(-1)).