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Interruption of the MnO2 oxidative process on dopamine and L-dopa by the action of S2O3(2-)
W J Barreto1, S R Barreto, M A Santos
1Laboratório de Físico-Química Ambiental, Departamento de Química, Universidade Estadual de Londrina, PR, Brazil. barreto@uel.br
Abstract:
The oxidation effects of Mn2+, Mn3+ or MnO2 on dopamine can be studied in vitro and, therefore, this offers a model of the auto-oxidation process that appears naturally in neurons causing Parkinson's disease. The use of MnO, as an oxidizer in aqueous solution at pH 7 causes the oxidation of catecholamines (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 mechanisms. 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 approximately/= 4 x 10(3) M(-1) cm(-1)) and at ca. 336, 557 (sh) nm, and an intense band at 597 nm (epsilon approximately 6 x 10(3) M(-1) cm(-1)), respectively. The latter transitions were assigned to d(pi)-->pi*-SQ. Noradrenaline and adrenaline do not form this blue complex in solution, but generate soluble oxidized compounds. The resonance Raman spectra of these complexes in solution showed bands at 950, 1006, 1258, 1378, 1508 and 1603 cm(-1) for the complex derivation of L-dopa and at 948, 1010, 1255, 1373, 1510 and 1603 cm(-1) for the dopamine-derived compound. The most intense Raman band at ca. 1378 cm(-1) was assigned to C-O stretching with major C1-C2 characteristics and indicated that dopamine and L-dopa do not occur complexed with manganese in the catecholate or quinone form, but suggests an intermediate compound such as an anionic o-semiquinone (SQ-), forming a complex such as [Mn(II)(SQ-)3]-. All enhanced Raman frequencies are characteristic of the benzenic ring without the participation of the aminic nitrogen. A mechanism is proposed for the formation of the dopamine and L-dopa complexes and a computational simulation was performed to support it.
Insights
This study explores manganese dioxide (MnO2) oxidation of catecholamines, revealing a novel mechanism involving anionic o-semiquinone complexes. This research models Parkinson
Area of Science:
- Biochemistry
- Neuroscience
- Inorganic Chemistry
Background:
- Manganese (Mn2+, Mn3+, MnO2) oxidation of dopamine in vitro serves as a model for Parkinson's disease-related neuronal auto-oxidation.
- Manganese dioxide (MnO2) typically oxidizes catecholamines to melanin in neutral aqueous solutions.
- Existing models do not fully explain the interaction of MnO2 with catecholamines under specific conditions.
Purpose of the Study:
- To investigate the oxidation mechanism of catecholamines (dopamine, L-dopa, noradrenaline, adrenaline) by MnO2 in the presence of sodium thiosulphate (Na2S2O3) at pH 6-7.
- To characterize the complexes formed between MnO2 and catecholamines using UV-Vis and resonance Raman spectroscopy.
- To propose a mechanism for the formation of these complexes and support it with computational simulation.
Main Methods:
- Spectroscopic analysis (UV-Vis and resonance Raman) of reactions between MnO2 and catecholamines in aqueous solutions at pH 6-7 with sodium thiosulphate.
- Formation of MLCT complexes with dopamine and L-dopa, characterized by specific absorption bands.
- Computational simulation to support the proposed reaction mechanism.
Main Results:
- Dopamine and L-dopa formed distinct MLCT complexes with MnO2, exhibiting characteristic absorption bands and resonance Raman spectra.
- Resonance Raman spectra indicated the formation of an intermediate anionic o-semiquinone (SQ-) complex, [Mn(II)(SQ-)3]-.
- Noradrenaline and adrenaline did not form these blue complexes but yielded soluble oxidized products.
Conclusions:
- The oxidation of dopamine and L-dopa by MnO2 in the presence of sodium thiosulphate proceeds via a novel mechanism involving anionic o-semiquinone intermediates, not catecholate or quinone forms.
- The observed spectroscopic data and computational simulations support the formation of [Mn(II)(SQ-)3]- complexes.
- This finding provides new insights into the redox chemistry of manganese and catecholamines, relevant to neurodegenerative disease models.