Peptaibol antibiotics: Conformational preferences of synthetic emerimicin fragments

The ir absorption and CD conformational analyses of solutions of the protected 2–9 fragment of the peptaibol antibiotics emerimicins III and IV documentclass{article}pagestyle{empty}begin{document}$ rlap{--} (Aib_3 rlap{--} )L - Val - Gly - L - Leurlap{--} (Aib_2 rlap{--} ) $end{document} and related short sequences are consistent with the presence of a right-handed α-helix for the octapeptide, while the tri-, tetra-, and pentapeptides adopt a 3₁₀-helix, either right- or left-handed, depending on the amino acid sequences. The structural preferences of solid-state documentclass{article}pagestyle{empty}begin{document}$ Zrlap{--} (Aib_3 rlap{--} )L - Val - OMe $end{document} and documentclass{article}pagestyle{empty}begin{document}$ Zrlap{--} (Aib_3 rlap{--} )L - Val - Gly - OMe $end{document} have been determined by x-ray diffraction. In accord with the solution data, incipient 3₁₀-helices, formed by two and three β-turns, have been found for the tetra- and pentapeptides, respectively. The tetrapeptide helix has the left-handed screw sense, while that of the pentapetide is right-handed, thus confirming the conclusions of the CD analysis of the solution.