Hydroxyapatite occurs naturally as a mineral in phosphate rock and also constitutes the mineral portion of bone. It may also be used to fractionate protein by chromatography.
Hydroxyapatite is prepared by mixing a solution of sodium phosphate (Na2HPO4) with calcium chloride (CaCl2). A white precipitate known as brushite is formed. Brushite is then converted to hydroxyapatite by heating to 100°C in the presence of ammonia:
Ca2HPO4 2H2O 100°C/NH3 ) Ca10(PO4)6(OH)2
The underlying mechanism by which this substance binds and fractionates proteins is poorly understood. Protein adsorption is believed to involve interaction with both calcium and phosphate moieties of the hydroxyapatite matrix. Elution of bound species from such columns is normally achieved by irrigation with a potassium phosphate gradient.
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