High-energy collision-activated and electron-transfer dissociation of gas-phase complexes of tryptophan with Na+, K+, and Ca2+

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_3⁺ 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_3⁺ 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.