Fragmentations and proton-transfer mechanisms of gaseous radical-cationic tryptophan: A theoretical study

The fragmentations of radical-cationic gaseous tryptophan, which involve the Cα-Cβ and N-Cα bond cleavages, are studied using the B3LYP/6-31+G(d) method. Cleavage of the Cα-Cβ bond and dissociation with the limiting barrier of 20.2 kcal/mol result in loss of cationic side chain. Elimination of NH3 occurs via 1,4 H-shift from the Cβ to the carbonyl oxygen with the limiting barrier of 24.4 kcal/mol followed by the low-energy O-H…NH2 1,4 H-shift and the N-Cα bond cleavage. The entropy factor makes alternative proton transfer pathways competitive. For elimination of a neutral side chain, the mechanism involving the Cα-Cβ bond cleavage and proton abstraction from NH of indole is proposed.