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 Zn²⁺. These results provide conclusive evidence for the DA oxidation mechanism, which consists of proton coupled electron transfer and sequential proton-electron transfer steps.