Vibrational Signature of Charge Solvation vs Salt Bridge Isomers of Sodiated Amino Acids in the Gas Phase

The vibrational spectra of the gaseous sodium complexes of glycine (Gly-Na⁺) and proline (Pro-Na⁺) have been recorded in the spectral range 1150-2000 cm^-1. The complexes were formed by matrix-assisted laser desorption-ionization, introduced in the cell of a Fourier transform ion cyclotron resonance mass spectrometer, and their infrared spectra were recorded using photons of variable energy emitted by a free electron laser. Photon absorption was probed by the diminished intensity of the parent ion, due to its infrared-induced dissociation into bare sodium cation and the free amino acid and the appearance of Na⁺. The observed absorption bands are assigned using ab initio computations of the IR spectra of the lowest energy isomers in each case. They provide the first experimental evidence that the salt bridge isomer is formed in the case of Pro-Na ⁺. In contrast, charge solvation by chelation of Na⁺ between nitrogen and the carbonyl oxygen seems to be most favorable for Gly-Na⁺, but a mixture of isomers cannot be ruled out in this case.