Probing the γ-turn in a short proline dipeptide chain.

The small peptide derived from proline, N-acetyl-prolinamide (Ac-Pro-NH2), has been investigated using a combination of Fourier transform microwave spectroscopy with laser ablation. Spectral signatures belonging to only one conformer have been detected in the supersonic expansion. Rotational constants and nuclear quadrupole coupling constants of the two (14)N nuclei have been used in the characterization of a γ-turn structure in the gas phase, which is stabilized by a CO···HN intramolecular hydrogen bond closing a seven-membered ring. A methyl group internal rotation barrier of 354 cm(-1) has been determined from the analysis of the A-E splittings.