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
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.
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2010