Conformational heterogeneity of tripeptides containing Boc–Leu–Aib as corner residues in the solid state

A critical analysis of single crystal X-ray diffraction studies on a series of terminally protected tripeptides containing a centrally positioned Aib (α-aminoisobutyric acid) residue has been reported. For the tripeptide series containing Boc–Ala–Aib as corner residues, all the reported peptides formed distorted type II β-turn structures. Moreover, a series of Phe substituted analogues (tripeptides with Boc–Phe–Aib) have also shown different β-turn conformations. However, the Leu-modified analogues (tripeptides with Boc–Leu–Aib) disrupt the concept of β-turn formation and adopt various conformations in the solid state. X-ray crystallography sheds some light on the conformational heterogeneity at atomic resolution.