COMPLEX-FORMATION FOLLOWED BY INTERNAL ELECTRON-TRANSFER - THE REACTION BETWEEN L-DOPA AND IRON(III)

In anaerobic acid solutions dopa (L-3-(3,4-dihydroxyphenyl)-alanine, H2LH) reacts with iron(III) to form two complexes, FeLH+ and its protonated form FeHLH2+. These both decompose via an intramolecular electron transfer in FeHLH2+ to yield iron(II) and dopasemiquinone which in turn is oxidised by iron(III) to dopaquinone. The quinone then cyclises by an intramolecular Michael addition giving the (UV-transparent) leucodopachrome. These reactions have been studied by stopped-flow spectrophotometric, spectroscopic, and chronoamperometric methods, and a mechanism is proposed. Rate constants for the various steps have been evaluated. Spectral data for the complexes and the quinone have also been obtained: FeLH+:maxima at 442 and 700 nm, epsilon-700 = 1380 M-1 cm-1; FeHLH2+:maxima at 435 and 660 nm, epsilon-660 = 520 M-1 cm-1; dopaquinone:maximum at 385 nm, epsilon-385 = 1650 M-1 cm-1, shoulder at 458 nm).