Investigation of the protonation site in the dialanine peptide by infrared multiphoton dissociation spectroscopy

Protonated dialanine cations have been isolated in a Fourier transform ion cyclotron resonance mass-spectrometer (FT-ICR-MS) and subjected to infrared multiphoton dissociation (IRMPD) at the free electron laser facility CLIO in Orsay (France). The spectral dependence of the IR induced fragmentation pattern in the mid-infrared region (800–2000 cm−1) is interpreted with the help of structure and vibrational spectrum calculations of the different protonated conformers. This comparison allows for the assignment of the proton on the terminal amino group, as the most favourable proton site, the neighbouring amide bond being in the trans conformation.