The fragmentations of radical-cationic gaseous tryptophan, which involve the C α –C β and N–C α bond cleavages, are studied using the B3LYP/6-31+G(d) method. Cleavage of the C α –C β bond and dissociation with the limiting barrier of 20.2kcal/mol result in loss of cationic side chain. Elimination of NH 3 occurs via 1,4 H-shift from the C β to the carbonyl oxygen with the limiting barrier of 24.4kcal/mol followed by the low-energy O–H…NH 2 1,4 H-shift and the N–C α bond cleavage. The entropy factor makes alternative proton transfer pathways competitive. For elimination of a neutral side chain, the mechanism involving the C α –C β bond cleavage and proton abstraction from NH of indole is proposed.