BETA-ALANINE AND BETA-BENDS - X-RAY-DIFFRACTION STRUCTURES OF 3 LINEAR OLIGOPEPTIDES

A crystal-state structural analysis of t-Boc-L-Ala-beta-Ala-NHMe, t-Boc-Aib-beta-Ala-NHMe, and t-Boc-Aib-Aib-beta-Ala-NHMe has been performed by X-ray diffraction. While the conformation adopted by t-Boc-L-Ala-beta-Ala-NHMe and t-Boc-Aib-beta-Ala-NHMe is essentially extended, t-Boc-Aib-Aib-beta-Ala-NHMe is folded into two consecutive intramolecularly hydrogen-bonded structures of the i + 3 --> i type (beta-bends), with Aib(1)-Aib(2) and Aib(2)-beta-Ala(3), respectively, as corner residues. Owing to the presence of the beta-aminoacid, the latter beta-bend is characterized by an unusual C-11 hydrogen-bonded ring. These results indicate that: (i) a beta-aminoacid may be incorporated into a beta-bend without a major perturbation of the overall geometry of this folded conformation, and (ii) the propensity of the beta-Ala residue for beta-bend formation is rather low, unless other conformational constraints (e.g. a preceding beta-bend) are present in the linear peptide molecule.