Polypeptide Helices in Hybrid Peptide Sequences

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Abstract
A new class of polypeptide helices in hybrid sequences containing α-, β-, and γ-residues is described. The molecular conformations in crystals determined for the synthetic peptides Boc-Leu-Phe-Val-Aib-βPhe-Leu-Phe-Val-OMe 1 (βPhe: (S)-β3-homophenylalanine) and Boc-Aib-Gpn-Aib-Gpn-OMe 2 (Gpn: 1-(aminomethyl)cyclohexaneacetic acid) reveal expanded helical turns in the hybrid sequences (ααβ)n and (αγ)n. In 1, a repetitive helical structure composed of C14 hydrogen-bonded units is observed, whereas 2 provides an example of a repetitive C12 hydrogen-bonded structure. Using experimentally determined backbone torsion angles for the hydrogen-bonded units formed by hybrid sequences, we have generated energetically favorable hybrid helices. Conformational parameters are provided for C11, C12, C13, C14, and C15 helices in hybrid sequences.