Resonant infrared multiphoton dissociation spectroscopy of gas-phase protonated peptides. Experiments and Car-Parrinello dynamics at 300 K.

The gas-phase structures of protonated peptides are studied by means of resonant infrared multiphoton dissociation spectroscopy (R-IRMPD) performed with a free electron laser. The peptide structures and protonation sites are obtained through comparison between experimental IR spectra and their prediction from quantum chemistry calculations. Two different analyses are conducted. It is first supposed that only well-defined conformations, sufficiently populated according to a Boltzmann distribution, contribute to the observed spectra. On the contrary, DFT-based Car-Parrinello molecular dynamics simulations show that at 300 K protonated peptides no longer possess well-defined structures, but rather dynamically explore the set of conformations considered in the first conventional approach.