The Zn2+ salt of pamidronate: a role for water in the metal-cation binding properties of bisphosphonates.

Pamidronate (3-ammonium-1-hydroxypropylidene-1,1-bisphosphonate) is used clinically in the treatment of diseases affecting bone tissue. In the salt zinc pamidronate dihydrate, Zn(2+).2C(3)H(10)NO(7)P(2)(-).2H(2)O, pamidronate is a zwitterion with an overall charge of -1. The carbon chain adopts a trans conformation, separating maximally the positively charged N atom from the negative phosphonate groups. The Zn(2+) ion lies on an inversion center and is surrounded by a sixfold coordination sphere provided by two bidentate chelating zwitterions and two water molecules. The bidentate O.Zn.O bond angle is 92.70 (7) degrees, while the O.O bite distance is 3.018 (3) A.