Affinity of tin(II) and tin(II) diphosphonates for hydroxyapatite: An experimental and model study

The adsorption of tin(II) on hydroxyapatite (a model for the mineral phase of bone) was studied in vitro in the presence of either 1-hydroxy ethylidene-1,1-diphosphonate (HEDP) or methylenediphosphonate (MDP). Experimental results suggest the separate adsorption of tin and diphosphonate after dissociation of the tin diphosphonate complex. A detailed analysis of these results was performed using a set of equations (including acid dissociation, complex stability, and affinity constants of tin and diphosphonate) to describe the binding on hydroxyapatite of the various species in solution. By varying guessed values for the affinity constants for adsorption of tin diphosphonate and uncomplexed tin, the deviations between experimental and calculated adsorption results were minimized. Accurate fitting was obtained when attributing a value of 10(-3) L mmol(-1) to the affinity constant of the tin diphosphonate complex and a value of 3.5 x 10(11) L mmol(-1) to the affinity constant of uncomplexed tin(II). The latter appeared to be the primary adsorbing tin species. Thus, tin diphosphonate species dissociate near the hydroxyapatite surface, where tin is transferred from the diphosphonate environment in solution to the phosphate environment at the surface. Diphosphonates carry tin in solution, preventing untimely hydrolysis.