The calcium-binding properties of pamidronate, a bone-resorption inhibitor.

The title compound, calcium bis(3-ammonio-1-hydroxypropylidene-1,1-bisphosphonate) dihydrate, Ca(2+).2C(3)H(10)NO(7)P(2)(-).2H(2)O, consists of calcium octahedra arranged in columns along the c axis and coordinated by hydrogen-bonded molecular anions. The Ca(2+) cation lies on a twofold axis. Pamidronate adopts a twisted conformation of the hydroxyalkylamine backbone that enables the formation of an intramolecular N-H...O hydrogen bond. The molecular anion is chelating monodentate as well as bidentate, with an O...O bite distance of 3.0647 (15) A.