Nucleation hydroxyapatite

There are several reports on the coating of bone-like hydroxyapatite onto natural polymer substrates. Kawashita et at. [57] reported that carboxymethylated chitin and gellan gum gels, which have carboxyl groups, can form hydroxyapatite on their surfaces in SBF if they are treated with a saturated Ca(OH)2 solution in advance, while curdlan gel, which has no carboxyl group, does not form hydroxyapatite in SBF, even if it is treated with Ca(OH)2 solution. These results support the hypothesis that carboxyl groups induce hydroxyapatite nucleation. Kokubo et at. [58,59] reported that non-woven fabrics of carboxymethylated chitin and alginate fibers also form hydroxyapatite on their surfaces in SBF if they are treated with Ca(OH)2 solution. 

Harris NL, Rattray KR, Tye CE, Underhill TM, Somerman MJ, D Errico JA, Chambers AF, Hunter GK, Goldberg HA (2000) Functional analysis of bone sialoprotein Identification of the hydroxyapatite-nucleating and cell-binding domains by recombinant peptide expression and site-directed mutagenesis. Bone 27 795-802... 

Damen and Ten Cate [98] studied calcium phosphate precipitation with and without the presence of silicic acid and showed that polysilicic acid, not monomer, acted as a substrate for hydroxyapatite nucleation, caused a 60% reduction in the induction period in seeded reactions and overcame part of the inhibitory effect of phosprotein. In all cases hydroxyapatite grew in the presence of silica even when the system was seeded with a different polymorph of calcium phosphate. It was concluded that silica promotes dental calculus formation. 

Nudelman E, Pieterse K, George A, Bomans PHH, Friedrich H, Brylka LJ et al (2010) The role of collagen in bone apatite formation in the presence of hydroxyapatite nucleation inhibitors. Nat Mater 9 1004-1009... 

Huang, J., Wong, C., George, A., and Kaplan, D. L. (2007). The effect of genetically engineered spider silk-dentin matrix protein 1 chimeric protein on hydroxyapatite nucleation. Biomaterials 28, 2358-2367. 

Calcium phosphate precipitation may also be involved in the fixation of phosphate fertilizer in soils. Studies of the uptake of phosphate on calcium carbonate surfaces at low phosphate concentrations typical of those in soils, reveal that the threshold concentration for the precipitation of the calcium phosphate phases from solution is considerably increased in the pH range 8.5 -9.0 (3). It was concluded that the presence of carbonate ion from the calcite inhibits the nucleation of calcium phosphate phases under these conditions. A recent study of the seeded crystal growth of calcite from metastable supersaturated solutions of calcium carbonate, has shown that the presence of orthophosphate ion at a concentration as low as 10-6 mol L" and a pH of 8.5 has a remarkable inhibiting influence on the rate of crystallization (4). A seeded growth study of the influence of carbonate on hydroxyapatite crystallization has also shown an appreciable inhibiting influence of carbonate ion.(5). 

Lower et al. (1998b) can shed more light on this phenomenon. They again used AFM, SEM, TEM, SEM-EDS, electron diffraction, and XRD to study the reactions between 0.5 and 500 mg/L of Pb with hydroxyapatite at pH 6 and a reaction temperature of 22 °C. A commercial hydroxyapatite was used at sorbent concentrations of 0.5 g/L. Reactions were observed over a 2 h period. At high initial Pb concentrations, Pb solution concentrations dropped from 500 mg/L to <100 mg/L. At concentrations of 0.5-100 mg Pb/L, after reaction, Pb levels dropped to less than 15 pg/L. In both cases, hydroxyapatite dissolved and hydroxypyromorphite formed. The authors applied some nucleation and crystal growth theory developed... 

Valsami-Jones et al. (1998) conducted similar studies with a synthetic, microcrystalline hydroxyapatite in either a Pb or Cd buffered solution. Using AFM, hydroxypyromorphite was observed to grow epitaxially on the hydroxyapatite surface in a clear example of heterogeneous nucleation. Cd removal differed, with the likely formation of a Ca-Cd phosphate solid solution. 

Mahrapatra, P. P., Sarangi, D. S. Mishra, B. 1995. Kinetics of nucleation of lead hydroxylapatite and preparation of solid solutions of calcium-cadmium-lead hydroxyapatite an x-ray and IR study. Journal of Solid State Chemistry, 116, 8-14. 

Hydroxyapatite (calcium phosphate) is deposited in nucleation sites between the ends of tropocollagen molecules as the first step in bone formation. 

The spaces between the ends of the tropocollagen molecules in a collagen fiber (see Fig. 4) are the nucleation sites for the deposition of a form of calcium phosphate, hydroxyapatite, in bone formation. Further hydroxyapatite is added until the nucleation sites grow and join with one another to form the mature bone structure. 

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