Protected 1–3 segment of the peptaibol antibiotics alamethicin and hypelcin.

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Abstract
A study of the modes of folding and self‐association of Z‐Aib‐l‐Pro‐Aib‐OMe (the protected 1–3 segment of the peptaibol antibiotics alamethicin and hypelcin) in the solid state was performed using i.r. absorption and X‐ray diffraction. The stereochemically constrained tripeptide molecules adopt a 4 ± 1 intramolecularly H‐bonded form (β‐turn), where the single intramolecular H‐bond is found between the peptide N‐H group of the Aib3 residue and the urethane C = O group of the N‐blocking benzyloxycarbonyl moiety. This folded structure is stabilized by an intermolecular H‐bond between the urethane N‐H group of the Aib1 residue and the peptide C = O group of the Pro2 residue of a symmetry related molecule. According to the i.r. absorption data, in CH2Cl2 and TMP solutions the same intramolecularly H‐bonded form occurs as that found in solid state. Compared to the situation in the solid state, in CH2Cl2 and TMP solvation of the urethane N‐H group replaces self‐association (through the same N‐H group). The results are also discussed in relation to those obtained for other protected ‐Aib‐X‐Aib‐(X = Aib, l‐Ala, l‐Val) tripeptide segments of peptaibol antibiotics.