The structure of proline in [proline + K](+) has been investigated in the gas phase using high level DFT and MP2 calculations and infrared photo dissociation spectroscopy with a free electron laser (FELIX). The respective FELIX spectrum of [proline + K](+) matches convincingly the calculated spectra of two structurally closely related and nearly iso-energetic zwitterionic salt bridge (SB) structures. An additional unresolved band at similar to 1725 cm(-1) matching with the characteristic CO stretching mode of charge solvation (CS) structures points toward the presence of a minor population of these conformers of proline in [proline + K](+). However, theory predicts a significant energy gap of 18.9 kJmol(-1) (B3LYP/6-311++G(2d,2p)) or 15.6 kJ mol(-1) (MP2) between the lowest CS conformer of proline and the clearly favored SB structure.