Functional hydroxyapatite

Dicalcium Phosphate Dihydrate (DPD). Dicalcium phosphate cHhydrate is completely nonreactive at room temperature. At 65—71°C and in the presence of water, it dehydrates and decomposes into hydroxyapatite and acidic monocalcium phosphate, or a free phosphoric acid (18). It is used to some extent in cake mixes in combination with faster acting acid. Its primary function is to provide acidity late in the baking cycle and thus produce a neutral and palatable product. DPD has an NV of 33. It provides sufficient acidity only in products requiring long baking times. 

Bone is a porous tissue composite material containing a fluid phase, a calcified bone mineral, hydroxyapatite (HA), and organic components (mainly, collagen type). The variety of cellular and noncellular components consist of approximately 69% organic and 22% inorganic material and 9% water. The principal constiments of bone tissue are calcium (Ca ), phosphate (PO ), and hydroxyl (OH ) ions and calcium carbonate. There are smaller quantities of sodium, magnesium, and fluoride. The major compound, HA, has the formula Caio(P04)g(OH)2 in its unit cell. The porosity of bone includes membrane-lined capillary blood vessels, which function to transport nutrients and ions in bone, canaliculi, and the lacunae occupied in vivo by bone cells (osteoblasts), and the micropores present in the matrix. 

Once fluoride ions react with bone, they are not easily dissolved out or exchanged by other elements. If bone is buried for long periods of time, the relative amount of fluorine in the bone gradually increases as a function of time the "fluoridation" process continues until the maximum amount of fluorine (necessary to convert all the hydroxyapatite to fluorapatite) is reached. The total concentration of fluor in carbonated fluorapatite can reach levels as high as above 3%. There is ample room, therefore, for an increase in the relative amount of fluorine in buried bone. Determining the relative amount of fluorine in buried bone may thus serve as a tool for dating bone. 

Bone and teeth in mammals and bony fishes all rely on calcium phosphates in the form of hydroxyapatite [Ca5(P04)30H]2, usually associated with around 5% carbonate (and referred to as carbonated apatite). The bones of the endoskeleton and the dentin and enamel of teeth have a high mineral content of carbonated apatite, and represent an extraordinary variety of structures with physical and mechanical properties exquisitely adapted to their particular function in the tissue where they are produced. We begin by discussing the formation of bone and then examine the biomineralization process leading to the hardest mineralized tissue known, the enamel of mammalian teeth. 

The mechanism of interaction of amino acids at solid/ aqueous solution interfaces has been investigated through adsorption and electrokinetic measurements. Isotherms for the adsorption of glutamic acid, proline and lysine from aqueous solutions at the surface of rutile are quite different from those on hydroxyapatite. To delineate the role of the electrical double layer in adsorption behavior, electrophoretic mobilities were measured as a function of pH and amino acid concentrations. Mechanisms for interaction of these surfactants with rutile and hydroxyapatite are proposed, taking into consideration the structure of the amino acid ions, solution chemistry and the electrical aspects of adsorption. 

It is the divalent metal cation Ca + that is absolutely critical in human physiology. It is important both structurally and functionally. I noted above that hydroxyapatite, a phosphate salt of calcium, is an integral component of bone. As a component of bone, calcium is quantitatively one of the most abundant elements in higher organisms, such as humans. 

Phosphorus is a critically important element in every cell of the body and also in the form of hydroxyapatite in bone and in all other functions as phosphate. The concentration of phosphate in blood is 1.0 to 1.5 mmol/L existing as H2P0( and HPOl" the equilibrium between the two acts as a proton buffer... 

Bisphosphonates. Bisphosphonates are synthetic compounds designed to function as mimics of pyrophosphate, in which the oxygen atom in P-O-P is replaced with a carbon atom, creating a non-hydrolyzable backbone structure. The bisphosphonates selectively bind to the hydroxyapatite portion of the bone, decreasing the number of sites along the bone surface at which osteoclast-mediated bone resorption can occur. This permits the osteoblasts to lay down well-mineralized new bone without competition from osteoclasts. Clinically employed bisphosphonates include etidronate (8.109), tilu-dronate (8.110), risedronate (8.111), alendronate (8.112), and pamidronate (8.113). 

Apatite surface properties have been characterized with respect to their role as sorptive surfaces (Wu et al. 1991 Somasundaran Wang 1984 Chander Fuerstenau 1984 Leyva et al. 2001). The point of zero charge (PZC), as measured by titration, electrophoresis, or streaming potential varies from pH values of 7 to 10 for hydroxyapatite and from 4 to 12 for fluoroapatite, and is a function of (1) the presence of C02, (2) ionic strength, and (3) time/aging of the mineral. 

Laperche Traina (1998) studied Pb uptake on hydroxyapatite at low initial solution concentration of Pb (103 mg/L). For this, EXAFS was used to characterize the local coordination environment of Pb on the apatite. The baseline corrected, Fourier-transformed EXAFS spectra revealed fc-values at >3 A, suggesting that Pb was not randomly sorbed. Radial structure functions (RSF) showed three intense peaks, characteristic of pyromorphite. 

As summarized in Table 14.1, teeth, bones, shells, etc. are indispensable components, consisting of inorganic mineral crystals and protein film, with sizes, morphologies, and textures suitable to fulfil the function of the particular organs involved. In this section we will look at hydroxyapatite, aragonite and calcite (two polymorphs of CaCO ), and magnetite in greater detail. 

Figure 3. Saturation of lake water (winter only) with respect to hydroxyapatite as a function of temperature...

Phosphate Equilibria. Figure 3 is a plot of the ion product, (Ca2+)10-(P043 )G(0H")2, as a function of temperature. Unfortunately only data are available for low temperature (winter) waters. Samples shown by X are from water extracted from sediment in winter and are just at saturation with respect to hydroxyapatite. 

Figure 7. Ion product for hydroxyapatite as a function of temperature and pressure. Area outlined by dashed line bounds winter lake data, and double line represents solubility constant...

The different functions of the gla proteins and phosphoproteins must be related to the packing in hydroxyapatite and apatite [Ca2(OH)(P04)J,but probably depend too on the relative disposition of the oxygen donors in the gla and phosphoryl groups. 

The use of hydroxyapatites as a catalyst support has the following advantages (1) well-defined monomeric active species can be immobilized on the surface based on multiple functionalities, for example, cation exchange ability, adsorption capacity and nonstoichiometry (2) their hydrophilic character allows smooth reactions under aqueous conditions and (3) due to their robust structure, no leaching of metals occurs. 

Ban, S., Hasegawa, J., Maruno, S., (1999), Fabrication and properties of functionally graded bioactive composites comprising hydroxyapatite containing glass coated titanium , Mat. Sci. Forum., 308-311, 350-355. 

A hydroxyapatite-bound La complex (LaHAP), prepared by using a cation-exchange method, has been reported to function as an efficient heterogeneous catalyst for the Michael addition of 1,3-dicarbonyls to enones under aqueous or solvent-free conditions. Further application to an asymmetric version by a fluoroapatite-bound La complex catalyst modified with (R,R)-tartaric acid has also been described.171... 

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