Antioxidant pathways and one-electron oxidation of dopamine and cysteine in electrospray and on-line electrochemistry electrospray ionization mass spectrometry

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.