Depsipeptides and cyclodepsipeptides are analogues of the corresponding peptides in which one or more amide groups are replaced by ester functions. Reports of crystal structures of linear depsipeptides are rare. The crystal structures and conformational analyses of four depsipeptides with an alternating sequence of an α,α-disubstituted α-amino acid and an α-hydroxy acid are reported. The molecules in the linear hexadepsipeptide amide in (S)-Pms-Acp-(S)-Pms-Acp-(S)-Pms-Acp-NMe2 acetonitrile solvate, C47H58N4O9·C2H3N, (3b), as well as in the related linear tetradepsipeptide amide (S)-Pms-Aib-(S)-Pms-Aib-NMe2, C28H37N3O6, (5a), the diastereoisomeric mixture (S,R)-Pms-Acp-(R,S)-Pms-Acp-NMe2/(R,S)-Pms-Acp-(R,S)-Pms-Acp-NMe2 (1:1), C32H41N3O6, (5b), and (R,S)-Mns-Acp-(S,R)-Mns-Acp-NMe2, C30H37N3O6, (5c) (Pms is phenyllactic acid, Acp is 1-aminocyclopentanecarboxylic acid and Mns is mandelic acid), generally adopt a β-turn conformation in the solid state, which is stabilized by intramolecular N-H...O hydrogen bonds. Whereas β-turns of type I (or I') are formed in the cases of (3b), (5a) and (5b), which contain phenyllactic acid, the torsion angles for (5c), which incorporates mandelic acid, indicate a β-turn in between type I and type III. Intermolecular N-H...O and O-H...O hydrogen bonds link the molecules of (3a) and (5b) into extended chains, and those of (5a) and (5c) into two-dimensional networks.
Keywords: crystal structure; depsipeptide; peptide conformation; β-turn.