The reactivity of dopamine precursors and metabolites towards ABTS(center dot-): An experimental and theoretical study

The antiradical activity of L-3,4-dihydroxyphenylalanine (L-DOPA), dihydroxyphenylacetic acid (DOPAC), homovanillic acid and tyrosine towards 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) diammonium salt radical (ABTS(center dot-)) was investigated experimentally and theoretically by UV-Vis spectroscopy and the DFT theory. The importance of the catechol moiety for this reaction was proven due to the formation of intramolecular hydrogen bond in the formed anions and radicals. The results indicated L-DOPA and DOPAC were more potent radical scavengers than homovanillic acid and tyrosine just because of intramolecular hydrogen bond formation. Based on experimental spectra, it was proved that electron transfer led to the reduction of ABTS(center dot-). The values of thermodynamic parameters were used to predict the preferred mechanism. The reaction rates were calculated for the electron transfer processes and it was shown that these were both kinetically and thermodynamically driven processes. Based on the reaction rate values, thermodynamic parameters, and present species, as determined by electronic spectra, it was concluded that single proton loss electron transfer (SPLET) is the dominant reaction mechanism in the investigated system.