Structural Dimorphism of Achiral α,γ‐Hybrid Peptide Foldamers: Coexistence of 12‐ and 15/17‐Helices

Here, novel 12‐helices in α,γ‐hybrid peptides composed of achiral α‐aminoisobutyric acid (Aib) and 4‐aminoisocaproic acid (Aic, doubly homologated Aib) monomers in 1:1 alternation are reported. The 12‐helices were indicated by solution and crystal structural analyses of tetra‐ and heptapeptides. Surprisingly, single crystals of the longer nonapeptide displayed two different helix types: the novel 12‐helix and an unprecedented 15/17‐helix. Quantum chemical calculations on both helix types in a series of continuously lengthened Aib/Aic‐hybrid peptides confirm that the 12‐helix is more stable than the 15/17‐helix in shorter peptides, whereas the 15/17‐helix is more stable in longer sequences. Thus, the coexistence of both helix types can be expected within a definite range of sequence lengths. The novel 15/17‐ and 12‐helices in α,γ‐hybrid peptides with 5→1 and 4→1 hydrogen‐bonding patterns, respectively, can be viewed as backbone‐expanded analogues of native α‐ and 3 10 ‐helices.