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

Fast atom bombardment and collision-induced dissociation tandem mass s pectrometry were used to study the fragmentation of quaternary pyridin ium salt-type amides of tryptophan (alpha-amino-3-indolepropionic acid ) esters and their analogs which incorporate the alpha-nitrogen into t he quaternary pyridinium structure. By cleavage directly at the pyridi ne nitrogen, the 1-alkyl-substituted nicotinamides decompose exlcusive ly to a carbocation, which then becomes the intermediate to further fr agments. Rearrangement of the 3-indolepropionate-2-yl carbocations may involve a five- to seven-membered ring expansion, which generates alt ernative 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 struc ture is probably involved. Besides the formation of protonated nicotin amide and alkene from tryptophan amides that contain methylene or ethy lene units between the amino and the quaternary pyridinium nitrogens, a fragmentation route leading to the carbocation identical with that o f the 1-alkyl-substituted nicotinamides has also been revealed.