Interruption of the MnO2 oxidative process on dopamine and L-dopa by the action of S2O32-

The oxidation effects of Mn2+, Mn3- or MnO2 on dopamine can be studied in v itro and, therefore, this offers a model of the auto-oxidation process that appears naturally in neurons causing Parkinsons disease. The use of MnO2 a s an oxidizer in aqueous solution at pH 7 causes the oxidation of catechola mines (L-dopa, dopamine, noradrenaline and adrenaline) to melanin, However, this work shows that, in water at pH 6-7. the oxidation of catecholamines by MnO2 in the presence of sodium thiosulphate (Na2S2O3) occurs by other me chanisms. For dopamine and L-dopa, MLCT complexes were formed with bands at 312, 350 (sh), 554 (sh) nm, and an intense band at 597 nm (epsilon congrue nt to 4 X 10(3) M-1 cm(-1)) and at ca. 336, 557 (sh) nm, and an intense ban d at 597 nm (epsilon approximate to 6 X 10(3) M-1 cm(-1)), respectively. Th e latter transitions were assigned to d(pi)-->pi*-SQ. Noradrenaline and adr enaline do not form this blue complex in solution, but generate soluble oxi dized compounds. The resonance Raman spectra of these complexes in solution showed bands at 950, 1006, 1258, 1378, 1508 and 1603 cm(-1) for the comple x derivation of L-dopa and at 948, 1010, 1255, 1373, 1510 and 1603 cm(-1) f or the dopamine-derived compound. The most intense Raman band at ca. 1378 c m(-1) was assigned to C-O stretching with major C-1-C-2 characteristics and indicated that dopamine and L-dopa do not occur complexed with manganese i n the catecholate or quinone form, but suggests an intermediate compound su ch as an anionic o-semiquinone (SQ(-)), forming a complex such as [Mn(II)(S Q(-))(3)](-). All enhanced Raman frequencies are characteristic of the benz enic ring without the participation of the aminic nitrogen. A mechanism is proposed for the Formation of the dopamine and L-dopa complexes and a compu tational simulation was performed to support it. (C) 2001 Elsevier Science B.V. All rights reserved.