Crystal structure and conformation of the cyclic dipeptide cyclo-(L-histidyl-L-aspartyl) trihydrate

The crystal structure of cyclo-(L-histidyl-L-aspartyl) trihydrate has been determined by X-ray diffraction techniques are refined to a final R index of 0.056 for 1601 reflections. The molecule is in a folded conformation, with the imidazole ring facing the diketopiperazine ring. However, since the diketopoperazine ring is essential planar, the intraction berween the two rings is not as intimate as in those cyclic dipeptides in which the diketopiperazine ring is in a boat conformation with the side chain occupying ab axial, or flagpole, site. Planarity of the diketopiperazine ring may be dictated by sreric interactions between the imidazole ring and the aspartyl side chain. The molecule is a zwitterion, a proton having been transferred from the corboxyl group of the aspartyl side chain to the imidazole ring