Oligopeptides with Equal Amounts of l- and d‑Amino Acids May Prefer a Helix Screw Sense

We investigated the preferred conformations
of two nonapeptides,
Boc-(l-Leu-d-Leu-Aib)3-OMe (2) and its enantiomer Boc-(d-Leu-l-Leu-Aib)3-OMe (ent-2), four dodecapeptides,
Boc-(l-Leu-d-Leu-Aib)4-OMe (3), Boc-(l-Leu-Aib-d-Leu)4-OMe (4), Boc-(Aib-l-Leu-d-Leu)4-OMe
(5), and Boc-(l-Leu-Aib-d-Leu-Aib)3-OMe (6), and a decapeptide, Boc-l-Leu-(d-Leu-l-Leu-Aib)3-OMe (7),
in solution and in the crystalline state. The nonapeptide 2 formed a right-handed (P) α-helix, and its
enantiomer ent-2 formed a left-handed
(M) α-helix. The dodecapeptides 3 and 5 were folded into (P) helices,
and 4 formed an (M) helical structure.
As for 6, roughly equivalent amounts of (P) and (M) helices were observed in solution, and
two (M) α-helices were detected in the crystalline
state. Furthermore, the decapeptide 7, which possesses
four l-Leu residues and three d-Leu residues, was
folded into an (M) α-helix.