Crystal Structures and Conformational Calculations of Fragments of Alamethicin Containing Aminoisobutyric Acid

The crystal structures of four peptide fragments (Boc-Pro-Aib-Ala-Aib-OBzl, Boc-Leu-Aib-Pro-OBzl, Boc-Leu-Aib-Pro-OH, and Boc-Gly-Aib-OH) of alamethicin containing five aminoisobutyric acid (Aib) residues have been determined. In all cases, the conformation of the Aib residue is in excellent agreement with the predictions of previous theoretical calculations. Viewed together with other observations in the literature, these data clearly demonstrate that the crystallographically determined average value of ±(50, 42) for (ø, Ψ) for Aib is a low-energy form. Systematic calculations of the possible conformers for acetyl-Aib-Ala-Aib-methylamide show four minima with one corresponding closely to that observed in Boc-Pro-Aib-Ala-Aib-OBzl.