Conformational properties of oxazoline-amino acids

• Oxazoline-amino acids occur in natural peptides.
• Conformations properties of model oxazoline-alanine and valine were studied.
• Conformation β2 (ϕ,ψ ∼ −161°, −6°) is main feature of the studied structural motif.
• The conformational changes with increase of the environment polarity were observed.

Oxazoline-amino acids (Xaa-Ozn) occur in natural peptides of potentially important bioactivity. The conformations of the model compounds: Ac-(S)-Ala-Ozn(4R-Me), Ac-(S)-Ala-Ozn(4S–Me), and (gauche+, gauche−, anti) Ac-(S)-Val-Ozn(4R-Me) were studied at meta-hybrid M06-2X/6–311++G(d,p) method including solvent effect. Boc-l-Ala-l-Ozn-4-COOMe and Boc-l-Val-l-Ozn-4-COOMe were synthesized and studied by FT-IR and NMR-NOE methods. The conformations in crystal state were gathered from the Cambridge Structural Data Base. The main conformational feature of the oxazoline amino acids is the conformation β2 (ϕ,ψ ∼ −161°, −6°), which predominates in weakly polar environment and still is accessible in polar surrounding. The changes of the conformational preferences towards the conformations αR (ϕ,ψ ∼ −70°, −15°) and then β (ϕ,ψ ∼ −57°, −155°) are observed with increase of the environment polarity.

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