Gas phase double-resonance IR/UV spectroscopy of an alanine dipeptide analogue using a non-covalently bound UV-tag: observation of a folded peptide conformation in the Ac-Ala-NH2–toluene complex

Abstract

The conformation-selective IR spectroscopy of the alanine dipeptide analogue Ac-Ala-NH2, a molecule deprived of a UV chromophore, has been obtained by laser double-resonance IR/UV spectroscopy, using a UV-tag (toluene) non-covalently bound to the peptide. The Ac-Ala-NH2–toluene complex, as detected through a resonant multiphoton ionisation process, is composed of a folded peptide monomer (C7 conformation) attached to the toluene molecule through an NH–π H-bond. Quantum chemistry calculations suggest that the structure of this complexed folded peptide is very similar to that of the isolated peptide, legitimating this experimental strategy for investigating the peptide structure. The population of exclusively folded peptides, in contrast to the C5/C7 conformational mixture observed on isolated monomers measured with other techniques, suggest that conformational changes might not only be induced by complexation of a single polar solvent molecule as previously reported, but also by an apolar, weakly interacting molecule like toluene.