Crystal structure and conformation of cyclo-L-cystine

X-ray crystallographic investigations on cyclo-L-cystine show that the diketopiperazine ring is in a twisted boat form and the C-S-S-C bridge across the C alpha atoms has a negative chirality with chi 3 angle of - 94 degrees, The two peptides are significantly nonplanar with the omega values of - 10 degrees and - 16 degrees. The crystals are orthorhombic, space group P212121 with a = 11.216(4), b = 12.874(4), c = 5.978(3) and Z = 4. The structure was solved by heavy atom method and refined to an R index of 0.065.