Conformational
Properties of Oxazole-Amino Acids:
Effect of the Intramolecular N–H···N Hydrogen
Bond
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Abstract
Oxazole ring occurs in numerous natural
peptides, but conformational
properties of the amino acid residue containing the oxazole ring in
place of the C-terminal amide bond are poorly recognized. A series
of model compounds constituted by the oxazole-amino acids occurring
in nature, that is, oxazole-alanine (l-Ala-Ozl), oxazole-dehydroalanine
(ΔAla-Ozl), and oxazole-dehydrobutyrine ((<i>Z</i>)-ΔAbu-Ozl), was investigated using theoretical calculations
supported by FTIR and NMR spectra and single-crystal X-ray diffraction.
It was found that the main feature of the studied oxazole-amino acids
is the stable conformation β2 with the torsion angles φ
and ψ of −150°, −10° for l-Ala-Ozl,
−180°, 0° for ΔAla-Ozl, and −120°,
0° for (<i>Z</i>)-ΔAbu-Ozl, respectively. The
conformation β2 is stabilized by the intramolecular N–H···N
hydrogen bond and predominates in the low polar environment. In the
case of the oxazole-dehydroamino acids, the π-electron conjugation
that is spread on the oxazole ring and C<sup>α</sup>C<sup>β</sup> double bond is an additional stabilizing interaction.
The tendency to adopt the conformation β2 clearly decreases
with increasing the polarity of environment, but still the oxazole-dehydroamino
acids are considered to be more rigid and resistant to conformational
changes