Ionic interation with bone mineral. II. The control of Ca²⁺ and PO₄^3-exchange by univalent cation-Ca²⁺ substitution at the hydroxyapatite crystal surface

Evidence is presented for one-to-one substitution of Ca²⁺ by Li⁺, Na⁺, K⁺, and Cs⁺ at the surface of synthetic hydroxyapatite crystals. Such cation substitution alters the surface charge asymmetry, which is then compensated by an altered balance in the opposite sign of ions (Ca²⁺ and PO₄^3-) in the hydration shell of the crystals. Exchangeable PO₄^3- and Ca²⁺ in the hydration shell were calculated from the ⁴⁵Ca²⁺ and ³²PO₄^3- studies. Li⁺, Na⁺, K⁺ and Cs⁺ increased the exchangeable Ca²⁺ without altering exchangeable phosphate. This suggests that Ca²⁺ accumulate in the hydration shell to compensate for the reduction in the net positive charge at the crystal surface which is brought about the substitution of divalent Ca²⁺ by univalent cations. Conversely, Ca²⁺ in the solution reduced Ca²⁺ exchange, probably by producing a net increase in the positive charge at the crystal surface. By these criteria, it could be shown that tetramethylammonium (TMA⁺) and tris(hydroxymethyl)-aminomethane (Tris⁺) do not participate in cation substitution. Additional proof for cation substituttion was obtained from ²²Na⁺ and ⁴²K⁺ wash-out studies indicating that Na⁺ and K⁺ exchange with the crystal surface, and from the estimation of net Ca²⁺ loss from the crystal, demonstrating depletion of surface Ca²⁺ with cation substitution.