Eur. Phys. J. D 60, 11-20 (2010)
https://doi.org/10.1140/epjd/e2010-00019-6

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

¹ ARC Centre of Excellence for Free Radical Chemistry and Biotechnology, School of Chemistry and Bio21 Institute of Molecular Science and Biotechnology, The University of Melbourne, 30, Flemington Road, Victoria, 3010, Australia
² Department of Chemistry, National University of Singapore, Science Drive 3, 117543 Singapore, Singapore

Corresponding authors: ^a lfe@unimelb.edu.au - ^b rohair@unimelb.edu.au

Received: 13 November 2009
Revised: 16 December 2009
Published online: 26 January 2010

Abstract

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 -interaction with the indole side chain; loss of H atom from -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.