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