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 (α‐amino‐3‐indolepropionic acid) esters and their analogs which incorporate the α‐nitrogen into the quaternary pyridinium structure. By cleavage directly at the pyridine nitrogen, the 1‐alkyl‐substituted nicotinamides decompose exclusively 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.