Hydroxylapatite, also frequently called hydroxyapatite, is a mineral. It is a naturally occurring form of calcium apatite with the formula Ca5(PO4)3(OH), but is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two molecules. Hydroxylapatite is the hydroxyl endmember of the complex apatite group. The OH- ion can be replaced by fluoride, chloride or carbonate. It crystallizes in the hexagonal crystal system. It has a specific gravity of 3.08 and is 5 on the Mohs hardness scale. Pure hydroxylapatite powder is white. Naturally occurring apatites can however also have brown, yellow or green colorations. Compare to the discolorations of dental fluorosis.
Hydroxylapatite is the main mineral component of bone. Carbonated-calcium deficient hydroxyapatite is the main mineral of which dental enamel and dentin are comprised.
Hydroxylapatite can be used as a filler to replace amputated bone or as a coating to promote bone ingrowth into prosthetic implants. Although many other phases exist with similar or even identical chemical makeup, the body responds much differently to them. Coral skeletons can be transformed into hydroxylapatite by high temperatures; their porous structure allows relatively rapid ingrowth at the expense of initial mechanical strength. The high temperature also burns away any organic molecules such as proteins, preventing host vs. graft disease.
Some modern dental implants are coated with hydroxylapatite. It has been suggested that this may promote osseointegration, but there is not yet conclusive clinical proof of this.
Hydroxyapatite uses in chromatography
The mechanism of Hydroxyapatite (HAP) chromatography is complicated and has been described as "mixed-mode" ion exchange. It involves nonspecific interactions between positively charged calcium ions and negatively charged phosphate ions on the stationary phase HAP resin with protein negatively charged carboxyl groups and positively charged amino groups. It may be difficult to predict the effectiveness of HAP chromatography based on physical and chemical properties of the desired protein to be purified. For elution, a buffer with increasing phosphate concentration is typically used.