Conformational landscape of small model peptides in the gas phase: UV/IR spectroscopy and theory

The progress achieved in gas-phase laser spectroscopy makes it now possible to study complex and flexible molecules such as small peptide chains. Thanks to the very good spectral resolution achieved in the gas phase, this work aims at characterising the structural intrinsic properties of model peptides containing two and three residues in terms of folding properties and H-bonding. The experimental strategy combining a laser vaporisation method with a supersonic jet is based on IR-UV double resonance laser spectroscopy. UV spectroscopy enables one to detect the most stable conformers in the jet, while spectroscopy in the near IR region (NH and CO stretches, and NH bends) gives access to their H-bonding. The final assignment is based on the comparison with DFT calculated spectra of structures of which H-bonding network is compatible with experimental data, thereby assessing or not the corresponding quantum chemistry calculations. Such a synergy between experiment and theory has thus led us to evidence a competition between two main conformational families showing specific spectroscopic signatures. The first family consists of a succession of local conformational preferences on each residue, in which H-bonds link close CO-NH amide groups in the sequence. The second family is composed of secondary structures stabilised by H-bonds linking more remote amide groups. By isolating and characterising them for the first time in the gas phase, formation of secondary structures of biology such as beta-turns and 3-10 helices has been shown to be an intrinsic property of these molecules. (author)