A matrix isolation study on Ac-l-Pro-NH2: a frequent structural element of β- and γ-turns of peptides and proteins

The fine tuned flexibility of peptides and proteins is one of their key features to achieve full bioactivity. Proline diamides are inherently the most rigid natural peptide models, but they still have a potential to adopt several backbone and side-chain conformers. It has been tried to spell out the residual conformational flexibility of Ac-l-Pro-NH2 using matrix isolation IR and VCD spectroscopy in Ar and Kr matrices. Spectra were analyzed by the help of quantum chemical calculations. The spectra are dominated by the transitions of the tγL+ conformer, furthermore trace amounts of the cαL+ conformer are also present. Another low-energy conformer, tγL−, cannot be identified in the spectra, which was interpreted as it converts to the lowest energy tγL+ form through a low-energy barrier during the deposition of the sample onto the cold window. Our results confirm that proline can act as a conformational lock, since the backbone predominantly adapts to the tγL structure.