Ion Chemistry of Protonated Lysine Derivatives

Protonated lysine fragments primarily by elimination of the ε‐amino group as ammonia to form an ion of m / z 130 and to a minor extent by elimination of H 2 O to form an ion of m / z 129. Protonated lysine derivatives such as lysine β‐naphthylamide and H‐Lys‐Gly‐OH show more pronounced formation of m / z 129 while protonated derivatives such as N α ‐Ac‐Lys‐X (X = OH, OMe, NHMe) and H‐Gly‐Lys‐X (X = OH, NHCH 2 COOH) 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 α‐amino‐ε‐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 ε ‐methyllysine and N ε ‐dimethyllysine fragment exclusively by elimination of CH 3 NH 2 and (CH 3 ) 2 NH, respectively. Evidence is presented that the stable structure of the m / z 130 ion so formed is protonated pipecolic acid. Both the protonated α‐amino‐ε‐caprolactam and protonated pipecolic acid ions fragment further primarily to [C 5 H 10 N] + ( m / z 84), a low mass ion commonly observed in the spectra of lysine‐containing peptides.