Peptaibol antibiotics: a study on the helical structure of the 2-9 sequence of emerimicins III and IV.

Solution conformations of the protected 2-9 segment of the peptaibol antibiotics emerimicins III and IV [alpha-aminoisobutyric acid (Aib)]3-L-Val-Gly-L-Leu-(Aib)2 and the related short sequences benzyloxy-(Aib)3-L-Val-OMe and benzyloxy-(Aib)3-L-Val-Gly-OMe have been investigated by circular dichroism studies. For the latter two compounds the structural preferences in the solid state have been assayed by x-ray diffraction analyses. The experimental data described here, along with those previously reported, support the view that the shortest Aib-containing segments (from tri- through pentapeptides) adopt the 3(10)-helical structure both in solution and in the solid state. In contrast, the octapeptide appears to adopt the alpha-helical structure in solution. The role of peptide chain length and specific amino acid sequences in stabilizing either of the two helical structures and hence their possible implications on the nature of the channel formed by peptaibol antibiotics in the membrane are also briefly outlined.