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

The conformational preferences and infrared and ultraviolet spectral signatures of two model β-peptides, Ac-β³-hPhe- β³-hAla-NHMe (1) and Ac-β³-hAla- β³-hPhe-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 substituents (37200-37800 cm^-1). UV-UV hole-burning spectroscopy was used to determine the ultraviolet spectral signatures of five conformational isomers of both 1 and 2. Transitions due to two conformers (labeled A and B) dominate the R2PI spectra of each molecule, while the other three are minor conformers (C-E) with transitions a factor of 3-5 smaller. Resonant ion-dip infrared spectroscopy was used to obtain single-conformation infrared spectra in the 3300-3700 cm ^-1 region. The infrared spectra showed patterns of NH stretch transitions characteristic of the number and type of intramolecular H-bonds present in the β-peptide backbone. For comparison with experiment, full optimizations of low-lying minima of both molecules were carried out at DFT B3LYP/6-31+G*, followed by single point MP2/6-31+G* and selected MP2/aug-cc-pVDZ calculations at the DFT optimized geometries. Calculated harmonic vibrational frequencies and infrared intensities for the amide NH stretch vibrations were used to determine the β-peptide backbone structures for nine of the ten observed conformers. Conformers 1B, 1D, and 2A were assigned to double ring structures containing two C6 H-bonded rings (C6a/C6a), conformers 1A and 2B are C10 single H-bonded rings, conformers 1C and 2D are double ring structures composed of two C8 H-bonded rings (C8/C8), and conformers 1E and 2E are double ring/double acceptor structures in which two NH groups H-bond to the same C=O group, thereby weakening both H-bonds. Both 1E and 2E are tentatively assigned to C6/C8 double ring/double acceptor structures, although C8/C12 structures cannot be ruled out unequivocally. Finally, no firm conformational assignment has been made for conformer 2C whose unusual infrared spectrum contains one very strong H-bond with NH stretch frequency at 3309 cm^-1, a second H-bonded NH stretch fundamental of more typical value (3399 cm^-1), and a third fundamental at 3440 cm^-1, below that typical of a branched-chain free NH. The single conformation spectra provide characteristic wavenumber ranges for the amide NH stretch fundamentals ascribed to C6 (3378-3415 cm^-1), C8 (3339-3369 cm^-1), and C10 (3381-3390 cm^-1) H-bonded rings.