Conformational Properties of Oxazole-Amino Acids: Effect of the Intramolecular N-H...N Hydrogen Bond

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 ( Delta Ala-Ozl), and oxazole-dehydrobutyrine ((Z)- Delta Abu-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 beta 2 with the torsion angles phi and psi of -150 degree , -10 degree for l-Ala-Ozl, -180 degree , 0 degree for Delta Ala-Ozl, and -120 degree , 0 degree for (Z)- Delta Abu-Ozl, respectively. The conformation beta 2 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 pi -electron conjugation that is spread on the oxazole ring and C super( alpha )=C super( beta ) double bond is an additional stabilizing interaction. The tendency to adopt the conformation beta 2 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.