Anaerobic oxidation of dopamine by iron(III)

Abstract

Iron(III) [in the form of Fe(OH) ²⁺ ] reacted reversibly in acid aqueous solution with dopamine, 2-(3,4-dihydroxyphenyl)ethylamine (H ₂ LH ⁺ , in which the phenolic protons are written to the left of L) to give the complex ion Fe(LH) ²⁺ . This species then decomposed to yield iron(II) and a semiquinone, which in turn is oxidised further to a quinone. The latter cyclised to form leucodopaminochrome (indoline-5,6-diol), which was finally oxidised by iron(III) to pink dopaminochrome (6-hydroxy-3H-indol-5-one), presumably via another semiquinone. The rate of appearance and disappearance of the complex and of the ortho-quinone were separately followed by stopped-flow photometric methods. Mechanisms are proposed for the various steps and these are supported by measurements at varying ionic strengths. Rate constants for the reversible formation of the iron–dopamine complex have been evaluated [k ₁ = (2.09 ± 0.05) × 10 ³ and k _-1 = 23 ± 2 dm ³ mol ^-1 s ^-1 ]. The rate of decomposition of the protonated complex to yield iron(II) and the semiquinone was established as k ₂ = 0.23 ± 0.02 s ^-1 and K _M ^H = 33 ± 0.9 dm ³ mol ^-1 [for the protonation of Fe(LH) ^{2 +} ]. The stability constant of the Fe(LH) ²⁺ complex has been calculated (log K ₁ ^M = 21.14) and ε _max is 1260 dm ³ mol ^-1 cm ^-1 at 700 nm. The effect of chloride on the rate of complex formation at low pH has been explained by the fact that FeCl ²⁺ also reacts with dopamine (k _Cl = 148 ± 7 dm ³ mol ^-1 s ^-1 ) to form the complex but that this is predominantly reversible via the non-chloride route at low pH values. The stability constant for FeCl ²⁺ formation (a constant not readily accessible by standard methods) was extracted from the data (log K ₁ ^Cl = 1.53). The rate of disappearance of the quinone enabled the ring-closure reaction (i.e. the formation of the indole) to be followed and the mechanism established. All measurements were carried out at 25 °C in solutions of ionic strength 0.10 mol dm ^-3 (KNO ₃ ) except for ionic strength dependence studies.