Structure of guanylyl‐3′,5′‐cytidine monophosphate. II. Description of the molecular and crystal structure of the calcium derivative in space group P2₁

The structural features of calcium guanosine‐3′,5′‐cytidine monophosphate (GpC) have been elucidated by X‐ray diffraction analysis. The molecule was crystallized in space group P2₁ with cell constants of a = 21.224 Å, b = 34.207 Å, c = 9.327 Å, and β = 90.527°, Z = 8. The hydration of the crystal is 21% by weight with 72 water molecules in the unit cell. The four GpC molecules in the asymmetric unit occur as two Watson‐Crick hydrogen‐bonded dimers related by a pseudo‐C face centering. Each dimer consists of two independent GpC molecules whose bases are hydrogen bonded to each other in the traditional Watson‐Crick fashion. Each dimer possesses a pseudo twofold axis broken by a calcium ion and associated solvent. The four molecules are conformationally similar to helical RNA, but are not identical to it or to each other. Instead, values of conformational angles reflect the intrinsic flexibility of the molecule within the range of basic helical conformations. All eight bases are anti, sugars are all C3′‐endo, and the C4′‐C5′ bond rotations are gauche‐gauche. The R factor is 12.6% for 2918 observed reflections at 1.2‐Å resolution.