Solid-state transformation of 4,2 '-anhydro-5-(beta-D-arabinofuranosyl)uracil dihydrate to 4,2 '-anhydro-5-(beta-D-arabinofuranosyl)uracil monohydrate

Crystals of 4,2' -anhydro-5-(beta-d-arabinofuranosyl)uracil, (I), obtained from an aqueous solution, were characterized as the dihydrate, C9H10N2O5. 2 H(2)O, (Ia). In air, these crystals slowly transform to the monohydrate, C9 H10N2O5. H2O, (Ib), but remain crystalline. The solid-state transformation proceeds with the loss of one water molecule and a rearrangement of hydroge n-bonded layers of molecules. The furanose ring in (I) has an approximate C 4'-exo,O4'-endo twist conformation. The central five-membered ring is sligh tly puckered. The uracil group is planar within experimental uncertainty.