Cyclodipeptides: Structure and conformation of cyclo(tyrosyl-prolyl)

The structure and conformation of the cyclodipeptide, cyclo(Tyrosyl-Prolyl), cyclo(Tyr-Pro) have been investigated with X-ray crystallographic and spectroscopic methods. Two conformations of cyclo(Tyr-Pro) crystallized in the space groupP2₁2₁2 with cell dimensionsa=11.890(3),b=12.057(1),c=18.528(4). Both these conformations are uncommon for cyclodipeptides containing a proline residue. The tyrosyl side chains of these conformers are folded towards the diketopiperazine (DKP) ring. The DKP ring adopts a flattened chair conformation in contrast to the typical boat conformation generally observed for DKP rings. The conformation of the pyrrolidine ring can be described as a pseudo C₂ symmetrical twist. One intermolecular hydrogen bond was observed for each of the two conformations of cyclo(Tyr-Pro). The hydrogen of the hydroxyl group of the tyrosyl residue is hydrogen bonded to the oxygen of the carbonyl group of the diketopiperazine ring, i.e., the carbonyl group originating from the tyrosyl residue. NMR spectroscopic studies indicated a different conformation for cyclo(Tyr-Pro) in solution similar to the generally observed conformation of cyclodipeptides, i.e., extended aromatic side chain and boat conformation for the DKP ring.