Structure of Protonated Heterodimer of Proline and Phenylalanine: Revealed by Infrared Multiphoton Dissociation (IRMPD) Spectroscopy and Theoretical Calculations

The 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 3560 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 Pro. Its predicted spectrum is in good agreement with the experimental one, although the coexistence of salt-bridged structures cannot be entirely excluded.