Ion Chemistry of Protonated Lysine Derivatives

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Protonated lysine fragments primarily by elimination of the epsilon-amino group as ammonia to form an ion of m/z 130 and to a minor extent by elimination of H2O to form an ion of m/z 129. Protonated lysine derivatives such as lysine beta-naphthylamide and H-Lys-Gly-OH show more pronounced formation of m/z 129 while protonated derivatives such as N alpha-Ac-Lys-X (X = OH, OMe, NHMe) and H-Gly-Lys-X (X = OH, NHCH2COOH) also show formation of m/z 129 in both metastable ion and collision-induced fragmentation. In both the latter systems m/z 129 is formed by sequential loss of HX followed by loss of ketene for the N-acetyl derivatives or the glycine residue for the N-glycyl derivatives. Although the m/z 129 ion is nominally an acylium ion, its metastable ion characteristics and collision-induced dissociation mass spectrum are very similar to those of protonated alpha-amino-epsilon-caprolactam. It is concluded that this lactam is formed from the lysine derivatives by interaction of the amino group of the lysine side-chain with the lysine carbonyl function as HX departs. Protonated N alpha-methyllysine and N alpha-dimethyllysine fragment exclusively by elimination of CH3NH2 and (CH3)2NH, respectively. Evidence is presented that the stable structure of the m/z 130 ion so formed is protonated pipecolic acid. Both the protonated alpha-amino-epsilon-caprolactam and protonated pipecolic acid ions fragment further primarily to [C5H10N]+ (m/z 84), a low mass ion commonly observed in the spectra of lysine-containing peptides.