Conformations of peptides containing a chiral cyclic α, α‐disubstituted α‐amino acid within the sequence of Aib residues

A single chiral cyclic α,α‐disubstituted amino acid, (3 S ,4 S )‐1‐amino‐(3,4‐dimethoxy)cyclopentanecarboxylic acid [( S , S )‐Ac 5 c dOM ], was placed at the N ‐terminal or C ‐terminal positions of achiral α‐aminoisobutyric acid (Aib) peptide segments. The IR and 1 H NMR spectra indicated that the dominant conformations of two peptides Cbz‐[( S , S )‐Ac 5 c dOM ]‐(Aib) 4 ‐OEt ( 1) and Cbz‐(Aib) 4 ‐[( S , S )‐Ac 5 c dOM ]‐OMe (2) in solution were helical structures. X‐ray crystallographic analysis of 1 and 2 revealed that a left‐handed ( M ) 3 10 ‐helical structure was present in 1 and that a right‐handed ( P ) 3 10 ‐helical structure was present in 2 in their crystalline states. Copyright © 2010 European Peptide Society and John Wiley & Sons, Ltd.