Antioxidant Pathways and One‐Electron Oxidation of Dopamine and Cysteine in Electrospray and On‐Line Electrochemistry Electrospray Ionization Mass Spectrometry

Abstract Dopamine [DA] + ( m / z 154), DA dimer [2DA‐H] + ( m / z 307) and DA quinone [DAQ] + ( m / z 152) are detected in positive ion mode electrospray ionization mass spectrometry (ESI MS) of dopamine in 50/1/49 (vol%) water/acetic acid/methanol. H/D exchange experiments support a covalent structure of DA dimer. Thus, ESI of DA may involve 1e − , 1H + oxidation processes followed by rapid radical dimerization. The DA quinone signal is low in ESI MS, which indicates a low efficiency of the 2e − , 2H + oxidation reaction. On‐line electrochemistry ESI MS (EC/ESI MS) with low electrochemical cell voltage floated on high ES voltage increases electrospray current and improves sensitivity for DA. The DA quinone signal increases and DA dimer signal decreases. A new configuration of the ESI MS instrument with a cone‐shaped capillary inlet significantly enhanced sensitivity of ESI and EC/ESI MS measurements. A DA quinone‐cysteine adduct [DAQ+Cys] + was detected in solutions of DA with cysteine (Cys). ESI MS and EC/ESI MS indicate formation of the DA quinone‐cysteine adduct by 1e − pathway. Oxidation pathways in ESI MS are relevant to biological reactivity of DA and Cys.