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Section 12: Mineralized Tissues

Section 12: Mineralized Tissues. 2. Ionic solids; hydroxyapatite. 2/28/06. Solubility of ionic solids. the solubility of a salt or ionic solid is a resultant of 2 tendencies:

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Section 12: Mineralized Tissues

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  1. Section 12: Mineralized Tissues 2. Ionic solids; hydroxyapatite 2/28/06

  2. Solubility of ionic solids the solubility of a salt or ionic solid is a resultant of 2 tendencies: • aggregation or formation of the solid state (crystallization, precipitation)favored by the strength of attraction between ions (ion-ion interaction) • dispersion or formation of the dissolved state (dissolution)favored by attraction between ions and solvent (ion-dipole interaction) 1

  3. Types of ionic solids • slightly soluble salts • stable in the solid state • ions: • have high charge density (e.g., Ca2+, Al3+, PO43–) • pack so as to: maximize separation of same-charge ions minimize separation of opposite-charge ions • highly soluble salts • stable in the dissolved state • ions: • have low charge density (Na+, NH4+) • pack relatively poorly • are attracted to H2O 2

  4. [M(H2O)6]+ : ball & stick space-filling Solid–solution transition • solid formation & dissolving described as a chemicalequation: dissolution [M(H2O)6]+ + [X(H2O)6]- M+X- + 12 H2O precipitation • the ions on the left side of the above equation are shown hydrated (aquo ions) with the common coordination # of 6 3

  5. Solid–solution transition • solid formation & dissolving described as a chemicalequation: dissolution [M(H2O)6]+ + [X(H2O)6]- M+X- + 12 H2O precipitation • the ions on the left side of the above equation are shown hydrated (aquo ions) with the common coordination # of 6 • usually ion'shydration statenot shown:M+ + X- M+X– MX 3 [M(H2O)6]+ : ball & stick space-filling

  6. Solubility & rates ofprecipitation & dissolution • the rates of these processes depend on the surface area of the solid, Asolid:ratedsln = kdsln(Asolid) ratepptn = kpptn(Asolid)[M+][X–] • [M+][X–] is often called the ion productkdsln & kpptnare rate constants • at equilibrium,ratedsln = ratepptn kdsln(Asolid) = kpptn(Asolid) [M+]eq[X–]eq 4

  7. Solubility: Ksp • dividing by kpptn(Asolid), • kdsln/kpptn = [M+]eq[X–]eq • this ratio of rate constants is an equilibrium constant • Ksp = [M+]eq[X–]eq • this last equation states that at equilibrium the product of the component ions is equal to a constant, termed the solubility product constant, Ksp • the solution is said to be saturated • there is no net dissolution or precipitation,but in general the equilibrium is dynamic • Ksp: a measure of solubility larger values indicate greater solubility 5

  8. Deviation from equilibrium often for salts in vivo, applicable ion product ≠ Ksp , so under-standing these nonequilibrium conditions is also important: solution tendency DG'dsln DG'pptn in vivo process condition favored saturated: no net change 0 0 ion product (equilibrium) = Ksp supersaturated: precipitation + – mineralization: ion product M+ + X–® MX formation of > Ksp bone, enamel & calculus undersaturated: dissolution – + demineralization: ion product MX ®M+ + X– caries, bone < Ksp resorption 6

  9. Hydroxyapatite (HA): composition, Ksp • calcium & phosphate form a variety of salts • at pH  7, the calcium phosphate solidwith the lowest solubility is hydroxyapatite • empirical formula: Ca10(PO4)6(OH)2 • despite its evident complexity, at constant pHa reasonable approximation of its solubility is obtained by the simple equation: K'sp(HA)= [Ca2+][Pi] where [Pi] = [H2PO4–] + [HPO4=] + [PO43–] • in vitro, with only Ca2+, Pi in H2O, pH = 7, K'sp 0.01 mM2 • in solutions containing physiological concentrations of other ions, its solubility is higher:K'sp = 0.7 mM2 7

  10. K'sp of HA compared to [Ca2+] [Pi] ion product • by comparison, for a salt like NaCl, Ksp = 3×107 mM2 • knowing the K'sp and [ions], one can calculate the tendency of a fluid to dissolve mineral or form it • note that it is the product of the ions' concentrations that matters for example: [Ca2+] × [Pi] = ion product mM mM mM2 0.84 0.84 0.7 0.7 1.0 0.7 0.07 10 0.7 • despite differing ion concentrations, these 3 cases are at equilibrium under conditions where K'sp = 0.7 mM2 8

  11. Calcium & Pi in vivo Concentrations & ion products of calcium & Pi in some biological fluids [Ca2+]×[Pi] = ion product mM mM mM2 ECF: adult 1.2 1.3 1.6 child 1.2 1.9 2.3 Saliva: low flow rate 1 5 5 high " 2 2 4 • in all cases, ion product > K'sp, so the fluids are supersaturated with respect to HA (as long as the pH is close to 7) 9

  12. Conditions where ion product < K'sp • as the pH of saliva drops toward 5, however, the K'sp of HA increases & becomes larger than the ion product, i.e., saliva becomes undersaturated with respect to HA • HA thus has a tendency or potential to dissolve • this is a necessary but not sufficient condition for caries formation; a number of additional factors determine the actual rate and extent of dissolution Variation of K'sp with [H+] K'sp [H+] 10Ca2+ + 6PO43– + 2OH–↔Ca10(PO4)6(OH)2 10

  13. Effect of pH on solubility • for most simple salts (e.g., NaCl), solubility does not vary with pH • because the salt's ions don't react with H+ or OH– in the physiological pH range • because OH– is not part of crystal structure • for many other salts (e.g., phosphate salts), solubility is pH-dependent • because Pi forms are acids/bases with each [form] being pH-dependent due to these pKa values: H2PO4–H+ + HPO4= pKa = 7HPO4= H+ + PO43– pKa = 12 • because OH– is a component ion of the crystal Which ions account for thepH dependence of HA solubility? 11

  14. Crystal structure: general • unit cell: repeating unit of a crystal smallest sample of a crystal that includes an example of all of the interionic distances & angles which occur in the entire crystal • relatively simple example: NaCl • unit cell has4 Na+ & 4 Cl–ions (lighterspheres) naclb.jpg 12

  15. 300x HA crystals in enamel rods • hexagonal prisms that are small & variable in size • small size means large surface area/weight 4 rod fig. is enamrdyl.gif hydroxyapatite crystallite200 x 500 x 1000 Å packing of rods in enamel 13

  16. 60º HA crystal structure hydroxyapatiteunit cell9.4 × 9.4 × 6.9 Å • composed of unit cells ~105 unit cells/crystallite • composed of ions • polar surface • ion exchange with fluids at interface • adhesion of substances due to polar interactions 50× hydroxyapatitecrystallite200 × 500 × 1000 Å 14

  17. HA unit cellstructure HAuncel.gif • complex • 18 ions/unit cell: Ca10(PO4)6(OH)2 • formation difficult • supersaturation likely • unit cells pack to maximize attractive interactions • ions shared betweenunit cells • interlocking cells Ca HA.gif PO4 OH 15

  18. HA unit cell structure • 2 unit cells stacked together • ions at unit cellsurface interdigitate with complementary spaces on adjacent unit cells (knobs into grooves)[molecular LEGOs] • ion sharingCa2+ & Pi ionson faces are shared with adjacent unit cell 16

  19. HA unit cell structure: ion sharing this view shows how 2 OH– ions (arrows) are shared between 2 adjacent unit cells 17

  20. HA unit cell structure: ion sharing OH– ions shared with 3 adjacent unit cells (total=4)8 OH– shown ÷ 4 = 2/unit cell 18

  21. Next: 3. Mineralization & fluoride Open next slide-set

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