Teeth carbonate ions

The hydroxyapatite crystals in bone and teeth are imperfect due to other anions and cations, especially magnesium, chloride, carbonate, and fluoride ions. Carbonate (C032-) is the most important. At low carbonate contents (<4% by weight), a carbonate ion replaces a phosphate ion in the crystal ( A site substitution), but at higher contents (>4% by weight) it replaces a hydroxide ion ( B site substitution). Either substitution slightly shortens and fattens the crystal ( c or a axes increase) and increases solubility. In contrast, if hydroxide ions are present, they can be replaced by fluoride, which decreases apatite solubility (Sect. 16.2.1). Crystallographic analyses indicate that, in bone and dentin, phosphate is often replaced by carbonate, whereas in enamel it is more often replaced with chloride (Cl1-). Carbonated hydroxyapatite is critical for enamel development (see Sect. 9.5.3). 

A = Ba, Ca, Ce, K, Na, Pb, Sr, Y, X = As, P, Si, V and Z = F, Cl, O, OH, H2O. Solid-solution between end-members is extensive, but complete only in certain cases. Carbonate ion may partially replace the XO4 group, with appropriate charge compensation [2,3]. Fluorapatite (Ca5(P04)3F) is the most common member of the group and the major constituent of phosphorites, which are the main raw materials for the manufacture of phosphoric acid derivatives including fertilizers, foods, pharmaceuticals and other chemicals. Fluorapatite in phosphorites is usually partially carbonated and hydroxylated. Hydroxyapatite (Ca5(P04)30H) is the primary mineral constituent of bones and teeth and hosts a variety of chemical substituents in its structure. These materials are stable for billions of years, even during tectonic events, and over a wide range of solution pH and geological conditions, as can be inferred from the existence of ancient sedimentary phosphorites [4]. 

Up to this point, we have focused on aqueous equilibria involving proton transfer. Now we apply the same principles to the equilibrium that exists between a solid salt and its dissolved ions in a saturated solution. We can use the equilibrium constant for the dissolution of a substance to predict the solubility of a salt and to control precipitate formation. These methods are used in the laboratory to separate and analyze mixtures of salts. They also have important practical applications in municipal wastewater treatment, the extraction of minerals from seawater, the formation and loss of bones and teeth, and the global carbon cycle. 

Solid solutions between carbonated HA and fluorapatite occur naturally in the body, in bone and teeth. In particular, the presence of fluoride ions offers the low solubility and good acid resistance needed for protecting teeth [57]. Note, however, that at very high pH levels, the HA end-member becomes less soluble than fluorapatite [58]. That is why it is often difficult to prepare stoichiometric fluorapatite by precipitation methods involving generally alkaline pH levels. 

Fluoride ions are absorbed from both the stomach and the small intestine. The soluble salts are efficiently absorbed, and the peak increase of fluoride in blood plasma is within 1 hour of ingestion. Ions are rapidly cleared from plasma into tissue in exchange with anions, such as hydroxyl, citrate, and carbonate. At least 95% of the 2.6 g of total body fluoride is located in bones and teeth. Almost 90% of excess fluoride is excreted in urine. 

The resonance contributors of the enolate ion show that it has two electron-rich sites the a-carbon and the oxygen. The enolate ion is an example of an ambident nucleophile (ambi is Latin for both dent is Latin for teeth ). An ambident nucleophile is a nucleophile with two nucleophilic sites ( two teeth ). 

Phosphorus is absorbed by animals from food (and by plants from the soil (Chapter 12.2)) in the form of phosphate ions HPO and H2PO4. In animals some of the element is found in this form in blood, urine and tissue fluids, but mostly as inorganic calcium salts in bones and teeth. The remaining phosphorus is present as organic phosphate , which is almost all in the form of numerous mono-and di-esters in which fully oxidised P is almost always linked indirectly to carbon through P-O-C bonds. In a few compounds P-NH-P linkages are formed. 

Once absorbed, metal ions and compounds enter the blood, mostly bound to blood cells and/or plasma proteins, which can be very specific (transferrins, ceruloplasmin). By the bloodstream metals are usually distributed throughout the body. Metallothioneins play an important role in distribution, function, detoxification, and maybe also toxicity of heavy metals [8]. There is a blood-brain barrier which can only be crossed by lipid-soluble molecules. Liver and kidney have a high capacity to bind metals. Bones and other mineralized tissues such as teeth can serve as storage organs for metals such as Ba, Be, Tl, Pb, Sr, La, Y. A number of metals have been shown to cross the placenta and to enter the fetal blood circulation. Biotransformation includes changes in the oxidation state, methylation processes, and cleavage of metal-carbon bonds. Gastrointestinal... 

Tanase et al. [1347] isolated four tetracycline antibiotics (tetracycline, demeclo-cycline, minocycline, oxytetracycline) from powdered teeth. Separation was achieved on a C,g column (A = 354 tun) using a 74/26 water (50 mM NaH2P04 at pH 1.75)/acetonitrile with 10 mM sodium pentanesulfonate mobile phase. Plots of k vs. ion pair carbon chain length and ion-pair reagent eoncentration were presented. Peak shapes and resolution were excellent. Elution was complete in 10 min. Linear ranges of lOng/mL to 7.5pg/mL were reported. 

A variation of calcium phosphate is the major component of bones and teeth in all vertebrates including humans. These calcium phosphates are usually referred to collectively as biological apatites, which are nonstoichiometric compounds based on pure apatites, Ca5(P04)3X, where X can be fluorine (F), chlorine (Cl), or hydroxyl (OH). (These are called fluoro-, chloro-, and hydroxyapatite, respectively.) In biological apatites the calcium cations can be replaced with varying amounts of strontium, magnesium, sodium, and potassium ions, and the phosphate anions can be replaced with hydrogen phosphates and carbonates. 

---