Structure of Protonated Heterodimer of Proline and Phenylalanine: Revealed by Infrared Multiphoton Dissociation Spectroscopy and Theoretical Calculations^†

The infrared multiphoton dissociation (IRMPD) spectrum of the protonated heterodimer of ProPheH ^+ , in the range of 2700-3700 cm ^-1 , has been obtained with a Fourier-transform ion cyclotron mass spectrometer combined with an IR OPO laser. The experimental spectrum shows one peak at 3565 cm ^-1 corresponding to the free carboxyl O-H stretching vibration, and two broad peaks centered at 2935 and 3195 cm ^-1 . Theoretical calculations were performed on the level of M062X/6-311++G(d, p). Results show that the most stable isomer is characterized by a charge-solvated structure in which the proton is bound to the unit of proline. Its predicted spectrum is in good agreement with the experimental one, although the coexistence of salt-bridged structures cannot be entirely excluded.