The features of the crystal structure of the layered series hydrates of uridine-5'-monophosphate salts (UMPNa x · y H 2 O).

Almost all reported salts of nucleotides crystallized from solutions are in the form of hydrate. Layered hydrates often occur in crystals with more than five water molecules per host molecule. In the present report, five single-crystal structures of uridine-5'-monophosphate (UMP) series hydrates of acid or salts (UMPNa x · y H 2 O, x = 0-2) were determined and analysed. It was found that all crystal hydrates were orthorhombic with a C 222 1 space group but with mere variation in the plane angle of adjacent bases and the distance between phosphate arms. The packing arrangements of UMPNa x · y H 2 O hydrates present typical layered sandwich structures and show that the UMP molecular layers alternate with water molecular layers parallel to the ac plane, linked by hydrogen bonds or coupled with coordinate bonds besides ionic electrostatic interaction. Metal ions were located in water molecular layers as a form of hydration. In addition, we tried to deduce and give insights into the formation of UMPNa x · y H 2 O hydrates. The effect of water molecules and metal ions on the crystal structure and stability was investigated. It was found that the coexistence of relatively rigid architectures constructed by host molecules and flexible interlayer regions was a key factor to the formation of these hydrates. Excessive loss of lattice water would give rise to the irreversible collapse of the host structure and loss of ability to recover to the initial state under humidity. Approximately seven crystal-water molecules were the balance point of sodium salt hydrates at room temperature under 43-76% RH conditions. The number of sodium ions in the crystal lattice is positively correlated with their thermal stability.