https://doi.org/10.1140/epjd/e2014-50302-5
Regular Article
High-energy collision-activated and electron-transfer dissociation of gas-phase complexes of tryptophan with Na+, K+, and Ca2+
1 Department of Chemistry, Graduate
School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, 599-8531
Osaka,
Japan
2 Project Research Center for
Fundamental Sciences, Graduate School of Science, Osaka University,
1-1 Machikaneyama, Toyonaka,
560-0043
Osaka,
Japan
a
e-mail: fujihara@c.s.osakafu-u.ac.jp
Received:
16
April
2014
Received in final form:
23
June
2014
Published online:
1
October
2014
The structure and reactivity of gas-phase complexes of tryptophan (Trp) with
Na+,
K+, and
Ca2+ were
examined by high-energy collision-activated dissociation (CAD) and electron transfer
dissociation (ETD) using alkali metal targets. In the CAD spectra of M+Trp (M = Na and K), neutral Trp loss was the
primary dissociation pathway, and the product ion of collision-induced intracomplex
electron transfer from the indole π ring of Trp to the alkali metal ion was observed,
indicating a charge-solvated structure in which Trp is non-zwitterionic. The
NH3 loss observed
in the CAD spectrum of Ca2+Trp2 is ascribed to a CZ (mixed
charge-solvated/zwitterionic)-type structure, in which one Trp is non-zwitterionic and the
other Trp adopts a zwitterionic structure with an NH moiety. The H atom and NH3 losses observed in the ETD
spectrum of Ca2+Trp2 indicate the formation of a hypervalent radical in the
complex, R–NH3, via
electron transfer from the alkali metal target to the NH
group of the CZ-type structure. Ca2+ attachment to Trp cluster
induces the zwitterionic structure of Trp in the gas phase, and an electron transfer to
the zwitterionic Trp forms the hypervalent radical as a reaction intermediate.
Key words: Molecular Physics and Chemical Physics
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2014