Solid-state conformation of diastereomeric -Pro-Pro-(Aib)4 sequences

The crystal structures of two diastereomeric -Pro-Pro-(Aib)4- sequences, Cbz-l-Pro-l-Pro-(Aib)4-OMe (1) and Cbz-d-Pro-l-Pro-(Aib)4-OMe (2), have been determined by X-ray crystallographic analysis. The crystals of the two compounds were characterized by the following parameters: (1) monoclinic, P21, a=10.543 Å, b=8.103 Å, c=22.642 Å, β=97.679, Z=2, R1=0.104, and Rw=0.327; (2) orthorhombic, P212121, a=10.470 Å, b=10.953 Å, c=32.405 Å, Z=4, R1=0.040, and Rw=0.046. In the asymmetric unit of 1, the homochiral l-Pro1-l-Pro2 adopts a polyproline II structure, which induces a left-handed (M) 310-helical structure in the following -(Aib)4- sequence. The preferred conformation of diastereomeric 2, which contains heterochiral d-Pro1-l-Pro2 segments, was similar to that of 1 with differences at the N-terminal d-Pro residue.