IR–UV Ion Dip Spectroscopy of Capped Phenylated Polyalanines in the Gas Phase

The gas-phase structures of capped phenylated polyalanine peptides, Ac-Ala-Ala-Phe-Ala-NH₂ (AAFA) and Ac-Ala-Ala-Phe-Ala-Ala-NH₂ (AAFAA), were investigated using conformer-selective IR-UV ion dip spectroscopy employing the IR light of the FELIX free electron laser. IR absorption spectra were measured in the wide 300-1900 cm^-1 range and additionally in the 3200-3600 cm^-1 region, complemented by extensive quantum-chemical calculations. The AAFA peptide was found to adopt a single dominant conformer with a β-hairpin structure stabilized by four hydrogen bonds, whose predicted spectrum closely matches the experimental data. In contrast, no conformer of AAFAA matches the experimental spectrum, despite generating over 200,000 conformers across multiple search strategies, suggesting that the true structure was not found. Additionally, computations of the molecules with and without the phenyl group reveal an induced alteration of the conformational landscape.