Conformations of helical Aib peptides containing a pair of l ‐amino acid and d ‐amino acid

A pair of l ‐leucine ( l ‐Leu) and d ‐leucine ( d ‐Leu) was incorporated into α‐aminoisobutyric acid (Aib) peptide segments. The dominant conformations of four hexapeptides, Boc‐ l ‐Leu‐Aib‐Aib‐Aib‐Aib‐ l ‐Leu‐OMe (1a), Boc‐ d ‐Leu‐Aib‐Aib‐Aib‐Aib‐ l ‐Leu‐OMe (1b), Boc‐Aib‐Aib‐ l ‐Leu‐ l ‐Leu‐Aib‐Aib‐OMe (2a), and Boc‐Aib‐Aib‐ d ‐Leu‐ l ‐Leu‐Aib‐Aib‐OMe (2b), were investigated by IR, 1 H NMR, CD spectra, and X‐ray crystallographic analysis. All peptides 1a,b and 2a,b formed 3 10 ‐helical structures in solution. X‐ray crystallographic analysis revealed that right‐handed ( P ) 3 10 ‐helices were present in 1a and 1b and a mixture of right‐handed ( P ) and left‐handed ( M ) 3 10 ‐helices was present in 2b in their crystalline states. Copyright © 2012 European Peptide Society and John Wiley & Sons, Ltd.