α-Amino acid residues with a ϕ,ψ constrained conformation are known to significantly bias the peptide backbone 3D structure. An intriguing member of this class of compounds is (αMe)Aze, characterized by an N α -alkylated four-membered ring and C α -methylation. We have already reported that (S)-(αMe)Aze, when followed by (S)-Ala in the homochiral dipeptide sequential motif -(S)-(αMe)Aze-(S)-Ala-, tends to generate the unprecedented γ-bend ribbon conformation, as formation of a regular, fully intramolecularly H-bonded γ-helix is precluded, due to the occurrence of a tertiary amide bond every two residues. In this work, we have expanded this study to the preparation and 3D structural analysis of the heterochiral (S)-Ala/(R)-(αMe) Aze sequential peptides from dimer to hexamer. Our conformational results show that members of this series may fold in type-II β-turns or in γ-turns depending on the experimental conditions. KEYWORDS azetidines, heterochiral sequences, peptide conformation, peptide synthesis, X-ray diffraction, β/γturns 1 | INTRODUCTION Main-chain reversals (turns) in naturally occurring and synthetic polypeptide molecules are ubiquitous. 1,2 Among them (α-, 3 β-, 4-7 γ-, 8-12 δ-, 13 ε-, 14 and π-15 turns), β-turns are by far the most extensively investigated both experimentally and computationally. All other types of turns, although drastically less frequently reported, are currently the focus of an increasing number of publications, in particular γ-turns. 12Except for the case of αand β-turns (a repetition of one type of αturn generates the classical α-helix 16 while that of some β-turns produces either the 3 10 -helix, 17-20 also well known, particularly in the peptide structural biochemistry literature, or β-turn ribbons 21-23 ), incipient helices or ribbon-like stretches 23,24 arising from all other types of turns have attracted much less interest from chemists.Recently, with the aim at reducing this gap, we focused our attention on γ-turns. 12,24 We wished in particular to discover and investigate in detail noncoded α-amino acids with a high propensity to fold into single turns, and possibly offering a good chance to create new, longer ordered secondary structures. To this end, we especially considered two C α -tetrasubstituted α-amino acids as potentially very promising building blocks, the chiral (αMe)Aze 24-31 (Figure 1) and the achiral 2-aminoadamantane-2-carboxylic acid. 32 In particular, by examining a series of repeating, homochiral, sequential -(S)-(αMe)Aze-(S)-Ala-dipeptides, we found a novel stable peptide conformation, the γ-bend ribbon structure, 24 characterized by only half of the C¼O … H-N intramolecular H-bonds typical of the γ-helix 12 because the strong γ-bend inducer (αMe)Aze, which forms Xxx-(αMe)Aze tertiary amides, lacks the N-H donor function.