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 osemiquinone radical DA center dot and neutral Leukodopaminochrome radical LDAC center dot 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 center dot-) has been identified as its metal complex with Zn2 thorn . These results provide conclusive evidence for the DA oxidation mechanism, which consists of proton coupled electron transfer and sequential proton-electron transfer steps. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

The newly developed EC-NR-MS technique successfully allowed for online investigation of the electrooxidation process of dopamine, confirming the formation of various reaction products and demonstrating their presence in the mass spectrum.