Single-conformation ultraviolet and infrared spectroscopy of model synthetic foldamers: β-peptides Ac-β³-hPhe-NHMe and Ac-β³-hTyr-NHMe

The conformational preferences and infrared and ultraviolet spectral signatures of two model β-peptides, Ac-β³-hPhe-NHMe (1) and Ac-β³-hTyr-NHMe (2), have been explored under jet-cooled, isolated molecule conditions. The mass-resolved, resonant two-photon ionization spectra of the two molecules were recorded in the region of the S ₀-S₁ origin of the phenyl or phenol ring substituents, respectively. UV-UV hole-burning spectroscopy was used to determine that two conformations of 1 are present, with the transitions due to conformer A, with S₀-S₁ origin at 34431 cm^-1, being almost 20 times larger than those due to conformer B, with S₀-S₁ origin at 34404 cm^-1. Only one conformation of 2 was observed. Resonant ion-dip infrared spectroscopy provided single-conformation infrared spectra in the 3300-3700 cm^-1 region. The spectra of conformer A of both molecules have H-bonded and free amide NH stretch infrared transitions at 3400 and 3488 cm^-1, respectively, while conformer B of 1 possesses bands at 3417 and 3454 cm^-1. For comparison with experiment, full optimizations of all low-lying minima of 1 were carried out at the DFT B3LYP/6-31+G* and RIMP2/aug-cc-pVDZ levels of theory, and single point MP2/6-31+G* calculations at the DFT geometries. On the basis of the comparison with previous studies in solution and the calculated results, conformer A of 1 and 2 were assigned to a C6 conformer, while conformer B of 1 was assigned to a unique C8 structure with a weak intramolecular H-bond. The reasons for the preference for C6 over C8 structures and the presence of only two conformations in the jet-cooled spectrum are discussed in light of the predictions from calculations.