A Raman and UV-Vis study of catecholamines oxidized with Mn(III)

A UV-Vis and Raman spectroscopy study of three aminochromes generated throu gh Mn3+ oxidation of the dopamine, L-dopa and adrenaline molecules at physi ological pH was performed. The UV-Vis spectra of the catecholamines oxidize d using Mn3+ in buffer solution at pH 7.2 show a band at ca. 300 nm, formed by two transitions at 280 nm and 300 nm assigned to an L-a and L-b transit ion respectively, and other at ca. 470 nm assigned to an n-pi* transition l ocalized in the carbonyl group. This assignment is suggested by the UV-Vis and Raman spectra of ortho-aminoquinone generated by MnO2 oxidation of a do pamine aqueous acidic solution. The resonance Raman spectra of the three ch romes at buffer pH 7.2 show a very similar feature and the most intense ban ds are observed in the spectral range 1100-1800 cm(-1). The band around 168 0 cm(-1) for the three compounds is assigned to a nu(C=O) stretching vibrat ion, 1630 cm(-1) to the nu(C=C) ring mode, two bands at 1423, 1439 cm(-1); 1427, 1438 cm(-1) and 1456, 1475 cm(-1) are assigned to a nu(C=N+) vibratio n, for aminochrome, dopachrome and adrenochrome, respectively. The excitati on profiles for the most intense bands for aminochrome and adrenochrome wer e obtained. The band assigned to the nu(C=N+) present a red shift with resp ect to the visible band peak, however the band in adrenochrome at 1475 cm(- 1) shows a profile similar to nu(C=O) and nu(CC) modes that reflects the me thyl group effect on mixing this mode more effectively with the nu(CC) ring mode. (C) 1999 Elsevier Science B.V. All rights reserved.