Secondary structure binding motifs of the jet cooled tetrapeptide model Ac-Leu-Val-Tyr(Me)-NHMe

In this paper the structure of the isolated tetrapeptide model Ac-Leu-Val-Tyr(Me)-NHMe (Leu = leucine, Val = valine, Tyr tyrosine) is investigated by mass- and isomer-selective IR/UV double resonance spectroscopy. Two isomers of this peptide are observed and in combination with force field, ab initio, and DFT calculations these structures are assigned to folded arrangements presenting two different secondary structure binding motifs: (a) a combined gamma-turn/beta-turn structure and (b) a triple gamma-turn structure, which is described for the first time for an isolated model system in the gas phase.