The crystal-state preferred conformation of the terminally blocked homooctapeptide from the Ca'adimethylated a-aminoisobutyric acid (Aib) residue, pBrBz-(Aib)8-OBu', in which pBrBz is para-bromobenzoyl and OBu' is tert-butoxy, determined by x-ray diffraction analysis using direct methods, was found to be a 310-helix stabilized by six consecutive intramolecular N-H O=C hydrogen bonds of the C10-llI (or E') type. This is the first observation at atomic resolution of a regular 310-helix longer than two complete turns.The solid-state structural analysis was extended to the terminally blocked, a-aminoisobutyric acid-rich octapeptide corresponding to the 2-9 sequence of the peptaibol antibiotics emerimicins Im and IV, pBrBz-Aib3-L-Val-Gly-L-Leu-Aib2-OMe. Again, this peptide adopts a (right-handed) 310-helical structure, although slightly distorted at the level of the Lleucine residue. The role of specific amino acid sequence and peptide main-chain length in stabilizing either the 310-or the a-helical conformation and their possible implications on the nature of the channel formed by peptaibol antibiotics in the membrane are also briefly discussed.The polypeptide 310-helix, first proposed by Donohue in the early 1950s (1), has a three-residue repeat and a hydrogen bond between the C=O group of residue i and the N-H group of residue i + 3 [type III (III') C10 form or 3-bend (2, 3)]. Its 4,qi torsion angles are approximately ±60°,±300, within the same energy minimum in the conformational map as the a-(3.613)helix (4, 5). However, the H-bonding schemes are different in the two types of helices, being of the i +-i + 4 type [C13 form or a-bend (3)] in the a-helix.For a long periodic structure formed by Ca-monoalkylated a-amino acid residues with the same chirality at the a-carbon, the 310-helix is energetically considerably less favorable than the a-helix (4, 5). Therefore, it is not surprising that only short pieces of approximately 310-helix (particularly at the C end of an a-helix) have been found in protein crystal analyses (4-6).More recently, by theoretical (7-11) as well as experimental (12-27) investigations, it has been shown that the 4,q, angles ofthe achiral a-aminoisobutyric acid residue (Aib), the prototype of Caa-dialkylated a-amino acids, are restricted to values near those associated either with right-or left-handed a-or 310-helices, unless it is part of a strained cyclic compound (28). The x-ray diffraction structures of aaminoisobutyric acid homopeptides to the pentamer have provided examples of short (less than two complete turns) 310-helical conformations in the solid state (14,15,22,23).Our recent solution conformational analysis of monodispersed, terminally blocked (Aib), homooligopeptides to the dodecamer is strongly in favor of the formation of fully developed, stable 310-helices in chloroform, starting from the octamer (26).In this paper we present the results of a crystal structure analysis of pBrBz-Aib8-OBu' (pBrBz, para-bromobenzoyl; OBu', tert-butoxy) by x-ray diffraction using direct...