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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.