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| - Shape of 4(S)- and 4(R)-Hydroxyproline in Gas Phase
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| - The α-amino acids 4(S)-hydroxyproline and 4(R)-hydroxyproline have been studied under isolationconditions 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 moststable 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 ischaracterized by an intramolecular N−H···OC 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 ringconformation. 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 exceptionof the most stable form of 4(S)-hydroxyproline for which a n−π* interaction between the oxygen atom ofthe 4-hydroxyl group and the carboxyl group carbon seems to be established. A gauche effect could bealso contributing to stabilize the observed conformers.
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