Solid-state structure of cyclic dipeptides : an X-ray and computational study of cis- and trans-diketo-piperazines of N-methyl-phenylalanine with the thia-pipecolic acids and thia-prolines

Ten new crystal structures of cis and trans bicyclic diketopiperazines (DKPs) of thia-pipecolic acid (with sulfur in the beta, gamma or delta position) or thia-proline (with sulfur in the beta or gamma position) and N-methyl phenylalanine [(NMe) Phe]: cyclo[(beta-S) Pip-(NMe) Phe], cyclo[(gamma-S) Pip-(NMe) Phe], cyclo[(delta-S) Pip-(NMe) Phe], cyclo[(beta-S) Pro-(NMe) Phe] and cyclo[(beta-S) Pro-(NMe) Phe] were determined with X-ray crystallography. Density functional theory calculations of these molecules in the gas phase succeed in reproducing the observed molecular conformations in the crystal remarkably well. This illustrates the weak to moderate impact of intermolecular packing forces in the absence of classical N-H center dot center dot center dot center dot O hydrogen bonds. The effect of sulfur on the geometry of the DKP ring and details of amide bond non-planarity are discussed. Molecular flexibility of the DKP ring, as estimated from the calculated deformation energies of its endocyclic ring torsion angles, is not in general the decisive factor for the occurrence of multiple symmetry independent molecules in the unit cell (Z' > 1), though in some cases a correlation is observed