Effects of D-Leu residues on the helical secondary structures of L-Leu-based nonapeptides.

The influence of D-Leu residues on the helical structures of L-Leu-based-nonapeptides was investigated. Specifically, the preferred conformations of four diastereomeric nonapeptides, Boc-(L-Leu-L-Leu-Aib)3-OMe (1); Boc-(L-Leu-L-Leu-Aib)2-L-Leu-D-Leu-Aib-OMe (2), which contained one D-Leu residue; Boc-L-Leu-D-Leu-Aib-L-Leu-L-Leu-Aib-L-Leu-D-Leu-Aib-OMe (3), which contained two D-Leu residues; and Boc-(L-Leu-D-Leu-Aib)3-OMe (4), were analyzed in solution and in the crystalline state. Peptide 1 formed a right-handed (P) 310-helix in solution. Peptides 2 and 3 both formed (P) 310-helices in solution and (P) α-helices in the crystalline state. Peptide 4 formed a (P) α-helix both in solution and in the crystalline state.