Ab initio structure determination of anhydrous sodium alendronate from laboratory powder X-ray diffraction data

Sodium alendronate, a member of bisphosphonate class of compounds commonly used for treatment of generalized bone disorders, exists in various hydrated forms. Dehydration of sodium alendronate trihydrate has been studied using variable temperature X-ray powder diffraction technique. The crystal structure of anhydrous sodium alendronate, prepared by heating the trihydrate sodium alendronate at 150 degrees C, has been determined from X-ray powder data using direct space global optimization technique for structure solution, followed by the Rietveld refinement. The structure of the anhydrous form of sodium alendronate is compared with that of the trihydrate form, which was determined previously from single crystal X-ray diffraction data. Both anhydrous and trihydrate sodium alendronate crystallize in monoclinic system with space group P2(1)/n. The crystal structure of the anhydrous sodium alendronate is built by edge-sharing of NaO(6) octahedra into a two-dimensional molecular sheet in the (011) plane, whereas in the trihydrate compound, one-dimensional chain along the (010) direction is generated by corner sharing of NaO(6) octahedra.