_exchange Mineralization Fluoride离子交换成矿作用氟课件.ppt

Section 12: Mineralized Tissues 3. Ion exchangeMineralizationFluoride Hydroxyapatite (HA): ion exchange in biological HA, ions are continuously in motion, dissociating, associating, etc. (dynamic equilibrium) even when theres no net precipitation or dissolution,exchange of ions is occurring ion exchange rates depend on location: HA: ions substituted because bulk solutions in contact with HA vary in ion content, biological HA contains other ions: HA ions substitute ions Ca 2+ Mg 2+, Sr 2+, Pb 2+, Fe 2+ PO43– CO32– HPO42– citrate 3– OH – Cl –, F – , HCO3– HA: ions substituted because bulk solutions in contact with HA vary in ion content, biological HA contains other ions: HA ions substitute ions Ca 2+ Mg 2+, Sr 2+, Pb 2+, Fe 2+ PO43– CO32– HPO42– citrate 3– OH – Cl –, F – , HCO3– most substitutions increase HA solubility e.g., enamel solubility higher due to presence of 2– 4% CO32– (1–2 CO32–/20 PO43– ) the term carbonated apatite sometimes used F – exception: substitution with F – lowers solubility Ion exchange in HA: CO3= for PO43– Mineralization if a solution of calcium Pi with ion product, e.g.,1.5 mM 2 (supersaturated relative to HA) is prepared,no HA forms if crystal of HA added,it will grow until solutions ion product = Ksp these experiments illustrate: Ca-Pi solutions tend to stay supersaturated precipitation on existing crystals (seed or nucleator) is much faster stoichiometry of HA formation: 10 Ca2+ + 6HPO42– + 8 OH – ? Ca10(PO4)6(OH)2 + 6H2O Mineralization in vitro if above solutions ion product increased to 3 mM2, a different CaP salt forms: calcium hydrogen phosphate: CaHPO4.2H2O Ksp = 2.6mM2 aka brushite, amorphous calcium phosphate (ACP) this illustrates:though more soluble, other simpler salts form faster than HA stoichiometry of formation dissolution of ACP: Ca2+ + HPO42– + 2 H2O ? CaHPO4.2H2O How do the first crystals form in mineralizing tissues? i.e., how is mineralization initiated? Initiation (nucleation) heterogeneous n