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 PO43-) in the hydration shell of the crystals. Exchangeable PO43- and Ca2+ in the hydration shell were calculated from the 45Ca2+ and 32PO43- 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.
ASJC Scopus subject areas
- Molecular Biology