Alendronate zwitterions bind to calcium cations arranged in columns

Alendronate is used clinically in the treatment of skeletal disorders, the mode of action depending on the adsorption to calcium hydroxyapatite crystals (bone). In the title compound, calcium 4-ammonium-1-hydroxybutylidene-1,1-bisphosphonate, Ca(2+).2C(4)H(12)NO(7)P(2)(-), alendronate is a zwitterion, possessing one negative charge on each PO(3) group and a protonated N atom. The zwitterion is disposed with its negative end facing the Ca(2+) ion, while its positive end is stretched in the opposite direction. The geometry of the carbon chain is all-trans, while the hydroxy group is approximately gauche. The Ca(2+) ion lies on a twofold axis parallel to b. The coordination sphere around the metal cation is octahedral and is determined by monodentate- and bidentate-coordinated alendronate zwitterions. The O.O bite distance is 3.080 (2) A. Coordinated Ca(2+) metal cations are arranged at the centre of a column running along c.