Anaerobic oxidation of dopamine by iron(III)

Iron(III) [in the form of Fe(OH)(2+)] reacted reversibly in acid aqueous solution with dopamine, 2-(3,4-dihydroxyphenyl)ethylamine (H2LH+, 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) x 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 epsilon(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 FeCl2+ also reacts with dopamine (k(Cl)= 148 +/- 7 dm(3) moel(-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 FeCl2+ 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 degrees C in solutions of ionic strength 0.10 mol dm(-3) (KNO3) except for ionic strength dependence studies.