The beta-bend ribbon spiral. Synthesis and conformational analysis in solution and in the crystal state of depsipeptides containing alpha-hydroxyisobutyric acid.

The synthesis and conformational analysis in solution (by FTIR absorption and 1H NMR) and in the crystal state (by X-ray diffraction) of three Hib-containing depsipeptides have been performed. In the crystal state Z-Aib-Hib-Aib-OMe is folded into a type-III beta-bend, while the conformation adopted by Z-Aib-Hib)2-Aib-OMe is a beta-bend ribbon spiral, characterized by two type-III beta-bends with Aib(1)-Hib(2) and Aib(3)-Hib(4) as corner residues, respectively. Both independent molecules in the asymmetric unit of t-Boc-L-Ala-Hib-L-Ala-OMe crystals are folded into a type-II beta-bend. For the Aib-Hib depsipeptides the conformation adopted in the crystal state is also that largely prevailing in solution, whereas for t-Boc-L-Ala-Hib-L-Ala-OMe the beta-bend conformation is significantly less populated in solution. A comparison is also made with: (i) the published crystal-state conformations of fully protected -(Aib)3-, -(Aib)5-, and -L-Ala-Aib-L-Ala- sequences and the beta-bend ribbon spiral generated by (Aib-L-Pro)n oligomers, and (ii) with the herewith described solution preferred conformation of Z-L-Ala-Aib-L-Ala-OMe. The possible use of Hib as an isosteric replacement for Aib in the design of conformationally constrained depsipeptides is briefly discussed.