Octacalcium phosphate

Both monocalcium phosphate and dicalcium phosphate dissolve incongmently in water, disproportionating to more basic calcium phosphate and phosphoric acid. The extent of these reactions varies with the temperature and the amount of water. If water is added gradually to anhydrous monocalcium phosphate, equiUbrium conditions first correspond to a mixture of the anhydrous salt and its monohydrate. After conversion to the monohydrate, further reaction affords dicalcium phosphate plus free phosphoric acid. Dicalcium phosphate decomposes in aqueous solution to the more basic hydroxyapatite and phosphoric acid via intermediate octacalcium phosphate. The compHcated stepwise conversion of the acidic mono- and dicalcium phosphates to hydroxyapatite is summarized in equations 6—9. The kinetics are quite complex. 

Sodium carboxymethyl chitin and phosphoryl chitin had most evident influences on the crystallization of calcium phosphate from supersaturated solutions. They potently inhibited the growth of hydroxyapatite and retarded the rate of spontaneous calcium phosphate precipitation. These chitin derivatives were incorporated into the precipitate and influenced both the phase and morphology of the calcium phosphate formed (flaky precipitate resembling octacalcium phosphate instead of spherical clusters in the absence of polysaccharide) [175]. 

Chitosan scaffolds were reinforced with beta-tricalciiun phosphate and calcium phosphate invert glass [177]. Along the same line, composites of Loligo beta-chitin with octacalcium phosphate or hydroxyapatite were prepared by precipitation of the mineral into a chitin scaffold by means of a double diffusion system. The octacalciiun phosphate crystals with the usual form of 001 blades grew inside chitin layers preferentially oriented with the 100 faces parallel to the surface of the squid pen and were more stable to hy-... 

Despite the importance of the precipitation of calcium phosphates, there is still considerable uncertainty as to the nature of the phases formed in the early stages of the precipitation reactions under differing conditions of supersaturation, pH, and temperature. Although thermodynamic considerations yield the driving force for the precipitation, the course of the reaction is frequently mediated by kinetic factors. Whether dicalcium phosphate dihydrate (CaHPO HoO, DCPD), octacalcium phosphate (Ca HfPO, 2.5 H20, OCP), hydroxyapatite (Cag (PO fOH), HAP), amorphous calcium phosphate (ACP), or a defect apatite form from aqueous solution depends both upon the driving force for the precipitation and upon the initiating surface phase. Thermodynamically, the relative supersaturation, o, is given by... 

Fluoridated apatite crystals can grow using the dual membrane system involving on the one hand a calcium acetate solution and on the other hand a phosphate solution at physiological temperature with a pH of 6.5. lijima et al. showed that the combination of fluoride ions, added to the phosphate solution, and amelogenin (a major protein in the enamel extracellular matrix), present in the reaction space between the two membranes, controlled the transformation of octacalcium phosphate (OCP) into fine rod-like fluoridated apatite crystals with habit, size... 

M.S. Tung, D. Skrtic, Interfacial properties of hydroxyapatite in fluorapatite and octa-calcium phosphate, in L.C. Chowm, E.D. Eanes (Eds.), Octacalcium Phosphate, Karger, Basel, 2001, pp. 112-129. 

Hydroxyapatite (with some carbonate inclusions) is the most stable of the possible calcium phosphate salts that can be formed under physiological conditions. However, it is not the most rapid one to form. Instead, octacalcium phosphate (OCP) will precipitate more readily than hydroxyapatite. This led Brown in 1987 to propose that, as the kinetically favoured compound, OCP precipitates first, and then undergoes irreversible hydrolysis to a transition product OCP hydrolyzate [68]. This hypothesis is consistent with the observation that enamel comprises hydroxyapatite crystals that have the long, plate-like morphology that is generally considered characteristic of OCP crystals [69]. Overall, it seems that enamel crystals, with their elongated form, result from early precipitation of OCP, which forms a template on which hydroxyapatite units grow epitaxially [70,71]. This leads to enamel mineralisation with the observed thin, ribbon-like structure of crystals. 

M. lijima, H. Tohda, Y. Moriwaki, Growth and structure of lamellar mixed crystals of octacalcium phosphate and apatite in model system of enamel formation, J. Cryst. Growth 116 (1992) 319-326. 

M.J. Mura-Galelli, FI. Narusawa, T. Simada, M. lijima, T. Aoba, Effect of fluoride on precipitation and hydrolysis of octacalcium phosphate in an experimental model simulating enamel mineralization during amelogenesis. Cells Mater. 2 (1992) 221-230. 

Brushite has a very wide distribution field, from about pH 5 to pH 13, with minimum Ca2+ solubilities at around 1 x 10 3 m between pH 7 and 11. At the solute concentrations modelled, only the CaOH+ complex influences the solubility of the solid phase. Monetite also has a very wide distribution field, from about pH 5 to pH 13, with minimum Ca2+ solubilities at around 1 x 10-3m between pH 7 and 11. At the solute concentrations modelled, only the CaOH+ complex influences the solubility of the solid phase. Octacalcium phosphate has a very wide distribution field, from about pH 5 to pH 14, with minimum Ca2+ solubilities at around 1 x 10 5 m around pH 12. At the solute concentrations modelled, only the CaOH+ complex influences the solubility of the solid phase. Low whitlockite has a very wide distribution field, from about pH 4 to pH 14, with minimum Ca2+ solubilities at around 1 x 10-8 M around pH 12. At the solute concentrations modelled, only the CaOH+ complex influences the solubility of the solid phase. Hydroxyapatite has a very wide distribution field, from about pH 4 to pH 14, with minimum solubilities at around 5 x 10 9 m around pH 12. At the solute concentrations modelled, only the CaOH+ and CaCO ligands influence the solubility of the solid phase. 

Hydroxyapatite vertebrates, mammals, fish Octacalcium phosphate vertebrates Amorphous SiO limpets, chitons Magnetite chitons Goethite fimpets Phosphoferrite chitons... 

M. lizima and Y. Moriwaki, In vitro study of the formation mechanism of tooth enamel apatite crystals - Effects of organic matrices on crystal growth of octacalcium phosphate(OCP),/.Japan. Assoc. Crystal Growth, 26,1999,175-83 (in Japanese with English abstract)... 

However, methane-diphosphonate could not prevent the growth of apatite crystals in vitro on prepared sinews of rats tail out of a metastable solution with calcium and phosphate ions. On the contrary, the precipitated crystalline particles were bigger and better crystallized than those from control solutions. This is in surprising contrast to most of the information from the literature. No other calcium phosphate minerals besides apatite have been found by X-ray diffraction, whereas under comparable conditions brushite and octacalcium phosphate grow on collagenous sinews549. ... 

The present studies on calcium phosphate in bacteria are mostly of dental interest. Through the mineralization of the dental plaque dental calculus forms the soft, adherent and predominant coating which forms on the surface of teeth567. X-ray diffraction studies have shown the dental calculus to be composed of four principal minerals hydroxyapatite, octacalcium phosphate, brushite and whitelockite568. The mechanism by which mineralization of oral calculi is initiated is not fully understood567. Two types of mineralization centres can be distinguished which... 

Whereas octacalcium phosphate has been found as a minor constituent in young bones, the claim has been made that the main crystalline phase in bones is octa-. calcium phosphate carbonate373. This suggestion is certainly not correct, yet, one aspect of the study is quite significant in that attention is drawn to the possibility... 

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. 

Newesely, H. Conditions for the existence of octacalcium phosphate, withlockit and carbonate apatite. A contribution to the crystal chemistry of biological hard substances. Dtsch. zahnarztl. Ztschr. 20, 754-766 (1965). 

In a subsequent study mineralization in cultured fetal murine calvarial sections was followed for up to 72h [7]. Transient octacalcium phosphate (OCP) or OCP-like intermediates were observed and converted to CA over periods of a few hours. Increased levels of OCP were found in sutures undergoing fgf2-induced accelerated mineralization. These results supported the transient precursor mineralization mechanism, which had been hypothesized but not previously observed [41, 42]. 

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