Crystal structure and conformation of the cyclic dipeptide cyclo-L-prolyl-D-phenylalanyl

The structure of cyclo-L-prolyl-D-phenylalanyl has been rifined to a final R of 0.030 for 1439 observed reflexions. The space group is P2 t , with a=8.107, b=6.638, c=12.083 A, β=96.43°, Z=2. The molecular packing coefficient is about 0.72, with pairs of molecules held together by an N-H…O hydrogen bond between the peptide N and O (2.855 A). The molecule is in the folded conformation (γ½=75.0°) with tha aromatic side chain facing the diketopiperazine ring. The latter assumes a boat conformation with the α-carbon atoms deviating about 0.2 A from the plane through the two peptide bonds. The pyrrolidine ring assumes a half-chair conformation with approximate C 2 symmetry.