Supersonic expansion of model peptides : conformational cooling, structures and excited states dynamics studied by Monte-Carlo methods, laser spectroscopy and quantum chemistry

The very good spectral resolution of laser spectroscopy achieved in the gas phase is a powerful tool to study the folding properties and the hydrogen bonding network of flexible molecules such as small peptide chains. The experimental strategy used in this work to determine the structural properties of these systems is based on IR-UV double resonance spectroscopy and combines laser vaporisation with a supersonic expansion. The final assignment then requires a comparison between experimental spectra and DFT-D calculations. The conformational selectivity brought by gas phase laser spectroscopy also makes it possible to study the dependence of the dynamics of relaxation of electronic excited states of model peptides with their secondary structure by using pump-probe methods or fluorescence detection. The issue of the conformational population of flexible molecules cooled in a supersonic expansion is a difficult issue, often disregarded due to the nonequilibrium processes that control the distributions experimentally observed. A statistical model was developed in order to describe this collisional cooling and the isomerizations experienced by one molecule during the expansion. These calculations were consistent with the experimental trends in the population ratios between conformations, they have provided orders of magnitude for the different processes involved (number of collision, trajectory in the expansion after desorption, final temperatures) and a better understanding of the cooling processes and the conformational relaxation. The conformational studies have been applied to two model systems selected to investigate structural interactions involved in proteins: protein-solvent interactions and hydrophobic interactions. The microhydrated protected phenylalanines (Ac­Phe­NH₂ : H₂O) and (Ac­Phe­NHMe : H₂O) were used to locate the solvation sites preferentially occupied by a water molecule, which then helped to propose a mechanism for the formation of hydrates in the supersonic expansion. The strong structuring properties of hydrophobic interactions between aromatic side chains has been revealed by studying two model peptides containing a sequence of phenylalanine amino acids: Ac­Phe­Phe­NH₂ and Ac­Phe­Phe­Phe­NH₂. A comparative study of the relaxation dynamics of the first ππ* excited state performed on various model peptides has demonstrated the existence of a strong conformational effect. TDDFT and CC2 calculations carried out on the protected phenylalanines have shown that this effect could be explained by an excitation transfer from the aromatic ring of a side chain toward a peptide bond of the backbone. Finally, adding a water molecule to the protected phenylalanine is also found to open new ultrafast channels of nonradiative deactivation.