Long polypeptide 3 10 -helices at atomic resolution

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Abstract
The crystal-state preferred conformation of the terminally blocked homooctapeptide from the C α,α -dimethylated α-aminoisobutyric acid (Aib) residue, p BrBz-(Aib) 8 -OBu t , in which p BrBz is para -bromobenzoyl and OBu t is tert -butoxy, determined by x-ray diffraction analysis using direct methods, was found to be a 3 10 -helix stabilized by six consecutive intramolecular N—H....O=C hydrogen bonds of the C 10 -III (or III′) type. This is the first observation at atomic resolution of a regular 3 10 -helix longer than two complete turns. The solid-state structural analysis was extended to the terminally blocked, α-aminoisobutyric acid-rich octapeptide corresponding to the 2-9 sequence of the peptaibol antibiotics emerimicins III and IV, p BrBz-Aib 3 -L-Val-Gly-L-Leu-Aib 2 -OMe. Again, this peptide adopts a (right-handed) 3 10 -helical structure, although slightly distorted at the level of the L-leucine residue. The role of specific amino acid sequence and peptide main-chain length in stabilizing either the 3 10 - or the α-helical conformation and their possible implications on the nature of the channel formed by peptaibol antibiotics in the membrane are also briefly discussed.