Trishomocubane Amino Acid as a β-turn scaffold

The synthesis and X-ray structure of two diasteriomeric heptapeptides [Ac-Ala-Ala-Ala-(R/S)-Cage-Ala-Ala-Ala-NH₂] with a trishomocubane amino acid as a β-turn scaffold are reported. The amino acid was synthesized as a racemate and two diastereomeric peptides were obtained. The two peptides were separated by preparative high-pressure liquid chromatography and crystals suitable for X-ray analysis were grown for both diasteriomeric peptides. In general, both the peptides satisfy the criteria for β-turn conformations. Five of the six Ala residues of both cage peptide crystals satisfy the criteria for 3₁₀-helix characteristics and the cage amino acid residue satisfied the α-helix classification. These experimental results confirm previous theoretical studies in our laboratory which predicted that the cage moiety would be a strong/active β-turn inducer.