Apatite Ca2+

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. 

In systems where all components are initially soluble, it is simple to form pyromorphites (Nriagu 1974, 1984 Ma 1996) or ternary metal apatites where Pb21, Cd2+, Cu2+, and Zn2+ iso-morphically substitute for Ca2+ and form solid... 

Deep groundwaters from igneous rocks are usually saturated with respect to CaC03, which limits the maximum Ca2 -concentration and thereby also indirectly determines the F"- and H2P04"-concen-trations due to the limited solubility of CaF2 and Ca-phosphates. Both F" and H2PO4" are usually found in the groundwater due to the presence of accessory minerals like fluorite and apatite in the rock. 

Scientists agree that the main components of bone are calcium (Ca2+), phosphate (P04 ) and hydroxyl (OH ) ions. For that reason the mineral phase is usually denoted as hyroxyl apatite (HA) with the formula Ca10[(PO4)6/(OH)2], However, some literature sources claim that most HA is actually dahlite 3 Ca3[P04]2.2Ca[C03].H20 or francolite Ca5[F/(P04,C03,0H)3]. 

It was stated that hydrated calcium monohydrogen phosphate in amorphous or cryptocrystalline form is a potential precursor in the formation of hydroxyapatite because the structural position of Ca2+ on (010) and (110) crystal planes of both minerals essentially correspond to one another492. These planes of calcium ions could easily serve as transition boundaries with little distortion of crystal structure the same holds true for octacalcium phosphate or defect apatites. Thus apatite may form from amorphous or microcrystalline calcium monohydrogen phosphate possible via octacalcium phosphate or defect apatites. This process may already start inside the matrix vesicles and continue during extravesicular activities. 

The charge formation on the surface of tri-ionic crystals (apatite) is still more complex. Samani et al.56) postulate Ca2+ and HPO4- as PDI of fluoroapatite pH controls the concentration of these ions in the solution and the hydrolysis rate of appropriate surface species. In this way H+ and OH- indirectly become PDI. DobiaS et al.57) came to the same conclusion, but they also include F- as PDI. Somasundaran58 takes H+, OH-, and phosphate ions for major PDI Ca2+ and F- have a major influence on the potential of fluoroapatite. 

The phenomenon of bone growth under these conditions has as yet no accepted explanation. Ca2+and PO migrate in a field that has a frequency of 3 kHz. Perhaps the alternating nature of the current causes the formation of apatite [Ca5(P04)3 X where X is OH or a halogen] a constituent of bone, on both ends of the surfaces to be grown together. 

Pyrite formation (FeS2) is related to biologically mediated reduction of sulfate to sulfide and of Fe3+ to Fe2+ in anoxic zones. In the case of phosphate, this can be removed through precipitation reactions with Ca2+ and Fe3+ (by formation of apatite or iron phosphate, respectively), by co-precipitation, or by formation of surface complexes with Fe or Mn oxides or hydroxides. 

For mineralization, the normal, metastable state is adjusted by nucleation, measured by the seed and solubility tests. The seed test measures amount of solid apatite required to precipitate Ca2+ and HP042- ion concentrations exceeding their solubility product. The solubility test measures the minimal concentrations of Ca2+ and HP042- necessary to induce precipitation. Type I collagen fibers nucleate bone formation as the concentrations of Ca2+... 

It has been demonstrated that the release of citric acid from PHEMA hydrogels hinders the formation of calcium phosphates, especially hydroxyapatites. Because of this inhibitory effect, the calcium phosphate phases formed during in vitro calcification were mainly present as non-apatite phases, possibly MCPM and DCPD. The porous morphology of the outer surface of the spherical calcium phosphate deposits could be due to the dissolution of precipitates in the presence of citric acid. The results obtained after subcutaneous implantation ofPHEMA and PHEMA containing citric acid in rats confirmed the resistance of PHEMA-citric acid to calcification. The calcium phosphate deposits which formed in vivo consisted mainly of Ca2+ and OH deficient hydroxyapatites. However, it is not yet known whether or not the differences between the calcium phosphate phases found in vivo and in vitro arise from the presence of proteins/peptides in the in vivo calcifying medium. 

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