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^+?, (ii) loss of an H atom to form (Trp-H)+M]^+?, and (iii) bond homolysis to form C₂H₄NO₂M^+?. 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^+? and C₉H₇NM⁺ cations suggests that the metal interacts with both the backbone and aromatic side chain, thus implicating π-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]^+? which reveal that: the most stable conformation involves chelation by the backbone together with a $\pi $ -interaction with the indole side chain; loss of H atom from $\alpha $ -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.