Eur. Phys. J. D (2014) 68: 273
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

Abstract

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.