Effects of hydrogen-bond deletion on peptide helices: Structural characterization of depsipeptides containing lactic acid

The insertion of α-hydroxy acids into peptide chains provides a convenient means for investigating the effects of hydrogen bond deletion on polypeptide secondary structures. The crystal structures of three oligopeptides containing L-lactic acid (Lac) residue have been determined. Peptide 1, Boc-Val-Ala-Leu-Aib - Val-Lac-Leu-Aib-Val-Ala-Leu-OMe (Boc: tert-butyloxycarbonyl; Aib:- aminoisobutyric acid; OMe: methyl ester), and peptide 2, Boc-Val-Ala-Leu-Aib - Val-Lac-Leu-Aib-Val-Leu-OMe, adopt completely helical conformations in the crystalline state with the Lac(6) residue comfortably accommodated in the center of a helix. The distance between the O atoms of Leu(3) CO group and the Lac(6) O (ester) in both the structures is 3.1-3.3 Å. The NMR and CD studies of peptide 1 and its all-amide analogue 4, Boc-Val-Ala-Leu-Aib - Val-Ala-Leu-Aib-Val-Ala-Leu-OMe, provide firm evidence for a continuous helical conformation in solution in both the cases. In a 14-residue peptide 3, Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-Val-Ala-Leu-Aib-Val-Lac-Leu-OMe, residues Val(1)-Leu(10) adopt a helical conformation. Aib(11) is the site of chiral reversal resulting in helix termination by formation of a Schellman motif. Residues 12-14 adopt nonhelical conformations. The loss of the hydrogen bond near the C-terminus appears to facilitate the chiral reversal at Aib(11)