β-Alanine and β-bends. X-ray diffraction structures of three linear oligopeptides

A crystal-state structural analysis of t-Boc-L-Ala-β-Ala-NHMe, t-Boc-Aib-β-Ala-NHMe, and t-Boc-Aib-Aib-β-Ala-NHMe has been performed by X-ray diffraction. While the conformation adopted by t-Boc-L-Ala-β-Ala- NHMe and t-Boc-Aib-β-Ala-NHMe is essentially extended, f-Boc-Aib-Aib- β-Ala-NHMe is folded into two consecutive intramolecularly hydrogen-bonded structures of the i + 3 → i type (β-bends), with Aib(1)-Aib(2) and Aib(2)-β-Ala(3), respectively, as corner residues. Owing to the presence of the β-aminoacid, the latter β-bend is characterized by an unusual C11 hydrogen-bonded ring. These results indicate that: (i) a β-aminoacid may be incorporated into a β-bend without a major perturbation of the overall geometry of this folded conformation, and (ii) the propensity of the β-Ala residue for β-bend formation is rather low, unless other conformational constraints (e.g. a preceding β-bend) are present in the linear peptide molecule.