ION CHEMISTRY OF PROTONATED LYSINE DERIVATIVES

Protonated lysine fragments primarily by elimination of the epsilon-am ino group as ammonia to form an ion of m/z 130 and to a minor extent b y elimination of H2O to form an ion of m/z 129. Protonated lysine deri vatives such as lysine beta-naphthylamide and H-Lys-Gly-OH show more p ronounced 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-i nduced 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 deri vatives or the glycine residue for the N-glycyl derivatives. Although the m/z 129 ion is nominally an acylium ion, its metastable ion charac teristics and collision-induced dissociation mass spectrum are very si milar to those of protonated alpha-amino-epsilon-caprolactam. It is co ncluded that this lactam is formed from the lysine derivatives by inte raction of the amino group of the lysine side-chain with the lysine ca rbonyl function as HX departs. Protonated N-epsilon-methyllysine and N -epsilon-dimethyllysine fragment exclusively by elimination of CH3NH2 and (CH3)(2)NH, respectively. Evidence is presented that the stable st ructure of the m/z 130 ion so formed is protonated pipecolic acid. Bot h the protonated alpha-amino-epsilon-caprolactam and protonated pipeco lic acid ions fragment further primarily to [C5H10N](+) (m/z 84), a lo w mass ion commonly observed in the spectra of lysine-containing pepti des.