posted on 2006-07-13, 00:00authored byP. Y. Iris Shek, Junfang Zhao, Yuyong Ke, K. W. Michael Siu, Alan C. Hopkinson
The fragmentation pathways of protonated arginine, protonated Nα,Nα-dimethylarginine, the Nα,Nα,Nα-trimethylarginine ion, three protonated Nε,Nε-dimethyllysines, and three permanent lysine ions in which the
charge is fixed by trimethylation are reported. Ion assignment was facilitated by 15N-labeling and deuterium
substitution. The chemistries are dominated by charge-induced elimination of the amino groups as neutrals,
including dimethylamine, trimethylamine and guanidine. Competitive losses of the α-amino and side-chain
amino groups were observed; these losses led to intermediates that had different structures and different
subsequent dissociation reactions. Concomitant losses of CO or CO2 with these amines were also commonly
observed. However, the ionic products of amine losses did not subsequently lose CO or CO2, suggesting
strongly that in these concomitant eliminations, it is the CO or CO2 that was first eliminated, followed
immediately by the loss of the amine. Results of density functional theory calculations on protonated arginine
and protonated Nα,Nα-dimethylarginine reveal that, in such concomitant eliminations, the dissociating complex
is vibrationally hot and the intermediate ion formed by losing CO or CO2 can immediately dissociate to
eliminate the amine.