title: Conformational analysis of cyclo (L-cystine).
abstract: Conformational analysis of cyclo(L-cystine) shows that the diketopiperazine ring has to exist only in the boat form. With this geometry, the molecule can adopt two distinct forms differing mainly in the chirality of the disulphide bridge. In both the P- and M-models, corresponding to dihedral angles of nearly +90 degrees and -90 degrees respectively about the S-S- bond, the molecule displays an approximate two-fold symmetry. According to our semi-empirical energy calculations, the minimum energy of the M-model is --9.2 kcal/mol, only 0.3 kcal/mol lower than that of the P-model. Because the difference between the two minima is so small, neither form is clearly superior to the other. However, the number of low energy conformations of the M-model in the allowed conformational space is significantly larger than that of the P-model by a ratio of 3 to 1, and therefore the former is likely to be thermodynamically favoured.
mesh_terms: D003553:Cystine; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008968:Molecular Conformation; D010456:Peptides, Cyclic / Q000032:analysis*; D010879:Piperazines / Q000032:analysis
chemical_list: D010456:Peptides, Cyclic; D010879:Piperazines; D003553:Cystine