Amorphous calcium phosphates

Stabilization of amorphous calcium phosphate by Mg and ATP Calcified Tissue Research 23 245-250. 

Growth of an amorphous calcium phosphate layer by incorporation of soluble calcium and phosphates from the solution. 

The estimation of the crystallinity index (Cl) of bone is based on one of the four vibrational modes associated with the apatite phosphate group. In amorphous calcium phosphate, the absorption band at 550-600 cm-1 appears as a single broad peak, whilst in hydroxyapatite it is split into bands of unequal intensity by the apatite crystal field (Sillen and Parkington 1996). Based on the splitting factor introduced by Termine and Posner (1966), Weiner and Bar-Yosef (1990) proposed the use of a crystallinity index to measure the crystallinity of bone mineral. As illustrated in Fig. 4.7, the Cl is estimated by drawing a base line from 750 to 495 cm 1 and measuring the heights of the absorption peaks at 603 cm-1 (measurement a), 565 cm 1 (measurement b) and the distance from the base line to the lowest point between the two peaks (c). Cl is calculated from the formula ... 

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... 

BIOACTIVE POLYMERIC COMPOSITES BASED ON HYBRID AMORPHOUS CALCIUM PHOSPHATES... 

Bioactive Polymeric Composites Based on Hybrid Amorphous Calcium Phosphates... 

Fluorhydroxyapatite solid solutions can be prepared by the sol-gel method [134,135]. Cheng et al. reported the control of fluoride content in fluorhydroxyapatite solid solutions by the amounts of triethanolamine (N(CH2CH20H)3) and trifluoroacetic acid (CF3COOH) in the mixed ethanol solutions of Ca(N03)2 and P0(CH2CH20H)x(0H)3 x with a Ca/P ratio of 1.67 [134]. After evaporation of the mixed ethanol solution at 150°C on a hot plate, the powder obtained, comprising a homogeneous mixture of calcium nitrate crystallites and amorphous calcium phosphates, is then heated at 500 or 900°C for 1 h to be transformed into the pure apatitic phase. 

Although the submicellar model of the casein micelle readily explains many of the principal features and physicochemical reactions undergone by the micelles and has been widely supported, it has never enjoyed unanimous support and two alternative models have been proposed recently. Visser (1992) proposed that the micelles are spherical conglomerates of individual casein molecules randomly aggregated and held together partly by salt bridges in the form of amorphous calcium phosphate and partly by other forces, e.g. hydrophobic bonds, with a surface layer of K-casein. Holt (1992, 1994) depicted the casein micelle as a tangled web of flexible casein... 

Eanes, E. D., Termine, J. D., Nylen, M. U. An electron microscope study of the formation of amorphous calcium phosphate and its transformation to crystalline apatite. Calc. Tiss. Res. 12, 144 (1973)... 

Boskey, A. L., Posner, A. S. Conversion of amorphous calcium phosphate to microcrystalline hydroxyapatite pH-dependent, solution-mediated, solid-solid conversion. J. Phys. 

Wuthier, R. E., Bisaz, S., Russell, R. G. G., Fleisch, H. Relationship between pyrophosphate, amorphous calcium phosphate and other factors in the sequence of calcification in vivo. Calc. Tiss. Res. 10, 198 (1972)... 

The insoluble Ca(II) salts of weak acids, such as calcium phosphate, carbonate, and oxalate, serve as the hard structural material in bone, dentine, enamel, shells, etc. About 99% of the calcium found in the human body appears in mineral form in the bones and teeth. Calcium accounts for approximately 2% of body weight (18,19). The mineral in bones and teeth is mosdy hydroxyapatite [1306-06-5] having unit cell composition Ca10(PO4)6(OH)2. The mineralization process in bone follows prior protein matrix formation. A calcium pumping mechanism raises the concentrations of Ca(II) and phosphate within bone cells to the level of supersaturation. Granules of amorphous calcium phosphate precipitate and are released to the outside of the bone cell. There the amorphous calcium phosphate, which may make up as much as 30—40% of the mineral in adult bone, is recrystallized to crystallites of hydroxyapatite preferentially at bone collagen sites. These small crystallites do not exceed 10 nm in diameter (20). 

These data may imply that cellular mechanisms for calcium transfer and homeostasis are intimately tied up with mitochondria and that biomineralization is an essential element in Ca2+-regulation processes481,482). For a review on the role of mitochondria in the deposition of amorphous calcium phosphate in the early steps of biological calcification, see473,474. ... 

Eanes, E. D. Thermochemical studies on amorphous calcium phosphate. Calc. Tiss. Res. 5, 133-145 (1970). 

Fig. 18. Average environment of calcium ions in amorphous calcium phosphates as determined by X-ray absorption spectroscopy near the K absorption edge of calcium. The diagram shows two bidentate and two monodentate phosphate tetrahedra and two water molecules contributing to the eight oxygens of the first coordination sphere of the calcium ions. A third shell, possibly comprising the two phosphorus atoms of the monodentate phosphate ions, can be seen in some of the preparations. The positions of the protons are not established (Holt et al., 1988, 1989b Holt and Hukins, 1991).

The approach adopted for the interpretation of the spectra of poorly crystalline calcium phosphates was to take the shell model of the crystalline phase having the greatest chemical similarity and progressively simplify and refine the model while maintaining a good fit to the observed spectrum. For the amorphous calcium phosphates, however, it was found that virtually identical shell models resulted from simplification and refinement of either the hydroxyapatite or brushite shell models to give the structure depicted in Fig. 18. 

Reynolds, E.C. 1998. Anticariogenic complexes of amorphous calcium phosphate stabilized by casein phosphopeptides a review. Spec. Care Dent. 18, 8—16. 

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