Pseudopeptides Designed to Form Supramolecular Helixes: The Role of the Stereogenic Centers

The two epimers Boc-l-Phe-d-Oxd-(S)-β3-hPhg-OBn (1) and Boc-l-Phe-d-Oxd-(R)-β3-hPhg-OBn (2) have been prepared by standard methods in solution, and their conformation was analyzed both in solution and in the solid state. While in solution 1 shows a random coil structure, 2 tends to assume a γ-turn conformation that is nearly retained in the solid state. On the other hand, in the solid state molecules of 1 associate generating a helix that involves the formation of elongated crystals with hexagonal cross-section. This effect is not observed in the crystals formed by Boc-l-Phe-d-Oxd-(R)-β3-hPhg-OBn 2.