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 ethyliden e-1,1-diphosphonate (HEDP) or methylenediphosphonate (MDP). Experimental re sults suggest the separate adsorption of tin and diphosphonate after dissoc iation of the tin diphosphonate complex. A detailed analysis of these resul ts was performed using a set of equations (including acid dissociation, com plex stability, and affinity constants of tin and diphosphonate) to describ e the binding on hydroxyapatite of the various species in solution. By vary ing guessed values for the affinity constants for adsorption of tin diphosp honate and uncomplexed tin, the deviations between experimental and calcula ted adsorption results were minimized. Accurate fitting was obtained when a ttributing 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 affini ty constant of uncomplexed tin(II). The latter appeared to be the primary a dsorbing tin species. Thus, tin diphosphonate species dissociate near the h ydroxyapatite surface, where tin is transferred from the diphosphonate envi ronment in solution to the phosphate environment at the surface. Diphosphon ates carry tin in solution, preventing untimely hydrolysis.