CRYSTAL AND MOLECULAR STRUCTURE OF TRANS-3,6-DIMETHYL-2,5-PIPERAZINEDIONE (L-ALANYL-D-ALANYL-D-ALANYL-2,5-DIKETOPIPERAZINE)

The crystal structure of trans-3,6-dimethyl-2,5-piperazinedione (meso cyclic dimer of alanylalanine) has been determined. The unit cell is monoclinic with a = 6.349, b = 6.214, c = 9.040 Å; β = 95.8°. The space group is P21/n with two molecules per unit cell. The structure has been solved by the application of close-packing criteria and has been refined to an R value of 5.6%, for 521 nonzero independent reflections (Cu Kα radiation). The piperazinedione ring is found to be essentially planar. A comparison of our results with the data for bond distances and bond angles of the cyclic dimer of glycine and with the suggested value for the cis-peptide unit is made. The crystal structure is held together by formation of hydrogen-bonded layers of molecules approximately in the (101) plane.