The role of metal cation in electron-induced dissociation of tryptophan

The fragmentation of tryptophan (Trp) - metal complexes [Trp+M](+), where M = Cs, K, Na, Li and Ag, induced by 22 eV energy electrons was compared to [Trp+H](+). Additional insights were obtained through the study of collision-induced dissociation (CID) of [Trp+M](+) and through deuterium labelling. The electron-induced dissociation (EID) of [Trp+M](+) resulted in the formation of radical cations via the following pathways: (i) loss of M to form Trp(+aEuro cent), (ii) loss of an H atom to form [(Trp-H)+M](+aEuro cent), and (iii) bond homolysis to form C2H4NO2M+aEuro cent. Deuterium labelling suggests that H atom loss can occur from heteroatom and/or C-H positions. Other types of fragment ions observed include: C9H7NM+, C9H8N+, M+, C2H3NO2M+, CO2M+, C10H11N2M+, C10H9NOM+. Formation of C2H4NO2M+aEuro cent and C9H7NM+ cations suggests that the metal interacts with both the backbone and aromatic side chain, thus implicating pi-interactions for all M. CID of [Trp+M](+) resulted in: loss of metal cation (for M = Cs and K); successive loss of NH3 and CO as the dominant channel for M = Na, Li and Ag; formation of C2H3NO2M+. Preliminary DFT calculations were carried out on [Trp+Na](+) and [(Trp-H)+Na](+aEuro cent) which reveal that: the most stable conformation involves chelation by the backbone together with a Pi-CH of the side chain is thermodynamically favoured over losses from other positions, with the resultant radical cation maintaining a (N, O, ring) chelated structure which is stabilized by conjugation.