Shape of 4(S)- and 4(R)-Hydroxyproline in Gas Phase

• Published in: Journal of the American Chemical Society 2005-03, Vol.127 (8), p.2572-2579
• Main Authors: Lesarri, Alberto, Cocinero, Emilio J, López, Juan C, Alonso, José L
• Format: Article
• Language: English
• Subjects: Atomic and molecular physics; Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics); Exact sciences and technology; Molecular properties and interactions with photons; Physics; Properties of molecules and molecular ions
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja045955m

The α-amino acids 4(S)-hydroxyproline and 4(R)-hydroxyproline have been studied under isolation conditions in gas phase using laser-ablation molecular-beam Fourier transform microwave spectroscopy. Two conformers of each molecule have been detected in the jet-cooled rotational spectrum. The most stable conformer in both molecules exhibits an intramolecular N···H−O hydrogen bond (configuration 1) between the hydrogen atom of the carboxylic group and the nitrogen atom. The second conformer is characterized by an intramolecular N−H···OC hydrogen bond (configuration 2). The conformers of 4(R)-hydroxyproline adopt a Cγ-exo puckering, while those of 4(S)-hydroxyproline present a Cγ-endo ring conformation. These ring conformations, which show the same propensity observed in collagen-like peptides, are stabilized by additional intramolecular hydrogen bonds involving the 4-hydroxyl group, with the exception of the most stable form of 4(S)-hydroxyproline for which a n−π* interaction between the oxygen atom of the 4-hydroxyl group and the carboxyl group carbon seems to be established. A gauche effect could be also contributing to stabilize the observed conformers.