Add time:08/19/2019         Source:sciencedirect.com

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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