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.