Gas-phase models of gamma-turns: Effect of side-chain/backbone interactions investigated by IR/UV spectroscopy and quantum chemistry
Résumé
The conformations of laser-desorbed jet-cooled short peptide chains Ac–Phe–Xxx–NH2 (Xxx=Gly, Ala, Val, and Pro) have been investigated by IR/UV double resonance spectroscopy and density-functional-theory (DFT) quantum chemistry calculations. Singly -folded backbone conformations (L-) are systematically observed as the most stable conformers, showing that in these two-residue peptide chains, the local conformational preference of each residue is retained (L for Phe and turn for Xxx). Besides, turns are also spontaneously formed but appear as minor conformers. The theoretical analysis suggests negligible inter-residue interactions of the main conformers, which enables us to consider these species as good models of turns. In the case of valine, two similar types of turns, differing by the strength of their hydrogen bond, have been found both experimentally and theoretically. This observation provides evidence for a strong flexibility of the peptide chain, whose minimum-energy structures are controlled by side-chain/backbone interactions. The qualitative conformational difference between the present species and the reversed sequence Ac–Xxx–Phe–NH2 is also discussed.