Mass spectrometric observation on free radicals during electrooxidation of dopamine.

It has been challenging to directly observe the o-semiquinone radicals and transient intermediates produced during the oxidation of dopamine (DA). To achieve this goal, we developed an electrochemistry-neutral reionization-mass spectrometry (EC-NR-MS) technique for online studying the electrooxidation process of DA. The EC-NR device mainly composed by a self-designed EC flow cell, a sonic spray ionization source, a heating tube, an ion deflector and an electrospray ionization source. By precisely controlling the oxidation potential at 0.55 V, a series of reaction products include o-quinone (DAQ), Leukodopaminochrome (LDAC), Dopaminochrome (DAC), 5,6-dihydroxyindole (DHI) and DA dimer clearly appeared in the MS spectrum. Based on the ion deflector of EC-NR setup, the neutral o-semiquinone radical DA and neutral Leukodopaminochrome radical LDAC were successfully extracted from these ionic products and allowed to be detected by MS. Such finding was further confirmed by spin trapping experiment. The formation of o-semiquinone radical anion (DA) has been identified as its metal complex with Zn. These results provide conclusive evidence for the DA oxidation mechanism, which consists of proton coupled electron transfer and sequential proton-electron transfer steps.