Study of the neurotransmitter dopamine and the neurotoxin 6-hydroxydopamine by electrospray ionization coupled with tandem mass spectrometry.

Electrospray ionization combined with tandem mass spectrometry has been applied to a study of dopamine and 6-hydroxydopamine, an important neurotransmitter and a well-known neurotoxin, respectively. Both protonated and deprotonated molecules were observed for the two compounds. Upon collision-induced dissociation of protonated and deprotonated 6-hydroxydopamine molecules, the number of fragmentation pathways observed was greater than that observed with protonated and deprotonated dopamine molecules; the greater proclivity to fragment of the former is due to the 6-substituted hydroxyl group, which is para to the 3-OH group and ortho to the CH2CH2NH2 group. Furthermore, 6-hydroxydopamine showed a greater propensity to oxidize than did dopamine when sample solutions were kept uncovered in the air for 24 h prior to mass spectrometric examination. Radical structures of the four main oxidation products of 6-hydroxydopamine have been suggested on the basis of their product ion mass spectra; one or more of these oxidation products may be responsible for the cytotoxic property of 6-hydroxydopamine. Copyright 2002 John Wiley & Sons, Ltd.