FAST-ATOM-BOMBARDMENT AND TANDEM MASS-SPECTROMETRY OF QUATERNARY PYRIDINIUM SALT-TYPE TRYPTOPHAN DERIVATIVES

Fast atom bombardment and collision-induced dissociation tandem mass spectrometry were used to study the fragmentation of quaternary pyridinium salt-type amides of tryptophan (alpha-amino-3-indolepropionic acid) esters and their analogs which incorporate the alpha-nitrogen into the quaternary pyridinium structure. By cleavage directly at the pyridine nitrogen, the 1-alkyl-substituted nicotinamides decompose exlcusively to a carbocation, which then becomes the intermediate to further fragments. Rearrangement of the 3-indolepropionate-2-yl carbocations may involve a five- to seven-membered ring expansion, which generates alternative fragmentation pathways; the formation of an even-electron and a radical cation, respectively. In trigonellyl amide-type tryptophan derivatives, fragmentation of the pyridinium ion proceeds on multiple pathways induced by the positive charge which may not be localized on the quaternary nitrogen, and isomerization to a dihydropyridinyl structure is probably involved. Besides the formation of protonated nicotinamide and alkene from tryptophan amides that contain methylene or ethylene units between the amino and the quaternary pyridinium nitrogens, a fragmentation route leading to the carbocation identical with that of the 1-alkyl-substituted nicotinamides has also been revealed.