Fragmentations of Protonated Arginine, Lysine and Their Methylated Derivatives: Concomitant Losses of Carbon Monoxide or Carbon Dioxide and an Amine†

The fragmentation pathways of protonated arginine, protonated NR,NR-dimethylarginine, the NR,NR,NR-trimethylarginine ion, three protonated N,N-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 R-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 NR,NR-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