Gas-Phase Zwitterion Stabilization by a Metal Dication

Although solution-phase amino acids normally exist as zwitterions, this is not the case under gas-phase conditions, where the canonical structure is favored. Complexation to a metal ion can increase the relative zwitterion stability, but even then, the zwitterion (salt bridge, SB) form is not the most stable form of such singly charged complexes except for basic amino acids. Computation suggests enhanced SB stability for doubly charged complexes of weakly binding metal ions, but this has not hitherto been verified experimentally. Using infrared-spectroscopic characterization of the ion structure by multiple-photon infrared dissociation by the FELIX free electron laser, the Ba²⁺ complex of Trp has been shown to have the SB structure, and the presence of the nonzwitterionic (charge-solvated) form has been ruled out. The principal spectroscopic signatures of the SB structure are the appearance of the antisymmetric CO stretch of the carboxylate group at 1600 cm^-1 and the umbrella mode of NH₃ at 1400 cm^-1.