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Effect of methylene group insertions on the structural rigidity of Aib containing helices

Nonprotein amino acids are being extensively used in the design of synthetic peptides to create new structure mimics. In this study we report the effect of methylene group insertions in a heptapeptide Boc‐Ala1‐Leu2‐Aib3‐Xxx4‐Ala5‐Leu6‐Aib7‐OMe which nicely folds into a mixed 310‐/α‐helical structure when Xxx= Ala. Analogs of this peptide have been made and studied by replacing central Xxx4 residue with Glycine (α‐residue), β‐Alanine (β‐Αla), γ‐aminobutyric acid (Gaba), and ε‐aminocaproic acid (ε‐Aca). NMR and circular dichroism were used to study the solution structure of these peptides. Crystals of the peptides containing alanine, β‐Αla, and Gaba reveal that increasing the number of central methylene (‐CH2‐) groups introduces local perturbations even as the helical structure is retained. © 2015 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 104: 720–732, 2015.