A β-turn-like conformation in “reduced” peptides Crystal structures of two dipeptide analogs with Pro-GlyΨ[CH2-NH] sequence

The conformational properties of the “reduced” dipeptides tBuCO-Pro-GlyΨ[CH2-NH]NRR′(R = H, R = Et; R = R′= Me) greatly depend upon the neutral or protonated state of the “reduced” amide bond. Due to protonation, the quite flexible neutral molecule turns into a very stable conformation resembling a β-turn in both the solid and solute states. The existence of a strong N +—H…O=C interaction closing a 10-membered cycle illustrates the possible specific properties induced by a chemical modification in pseudopeptide analogs.