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Micellar calcium phosphate has the chemical composition and physicochemical properties that are consistent with it being a complex of the phosphate centres of casein with an acidic amorphous calcium phosphate. Similar acidic amorphous calcium phosphates have been prepared in the laboratory and for these, as well as for micellar calcium phosphate, the most appropriate crystalline model compound from which the short-range structure may be derived is brushite, CaHPO 4 .2H 2 O. The predicted secondary structures around sites of phosphorylation in the Ca 2+ -sensitive caseins often comprise an a-hclix-loop-a-helix motif with the sites of phosphorylation in the loop region. This motif may be important in linking the colloidal calcium phosphate with casein in native casein micelles.In this paper, the term colloidal calcium phosphate (CCP) refers to the salt formed from small ions (mainly Ca 2+ , Pj, Mg 2+ and citrate) which is thought to exist in the native bovine casein micelle (Holt, 1985). Colloidal Ca or P 4 in milk can be clearly defined operationally by a dialysis equilibrium experiment as the difference between total and diffusible Ca or Pjj however, some colloidal Ca may not be associated completely with the calcium phosphate (the so-called caseinate Ca) making it impossible to determine the precise composition of the CCP. The term micellar calcium phosphate (MCP) is used here to refer to the complex of CCP with casein. It reflects a view of the complex in which the phosphate moieties of phosphoseryl residues and possibly other functional groups from the protein are incorporated in the CCP structure. Nevertheless, the definition of MCP is no more satisfactory than that of CCP since the boundary of the complex is a matter of choice. A practical definition of MCP is: that complex of casein phosphopeptides and small ions which is recovered following non-specific proteolytic digestion of casein micelles, carried out under conditions where the CCP does not dissolve 1986).Two different views of the nature of MCP have been put forward. In one (McGann et al. 1983;Holt, 1982) the CCP is regarded as a basic tricalcium phosphate (Ca/P = 1-5) and the interaction with phosphopeptides ignored. The hypothesis of Schmidt (1982) that the CCP comprises Ca 9 (PO 4 ) 6 ion clusters linked through Ca bridges to

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Amorphous calcium-magnesium phosphates were prepared by precipitation from moderately supersaturated aqueous solutions at pH 7. Chemical analysis of the samples by ion chromatography showed that up to about 50% of the phosphate ions were protonated, the proportion increasing with the magnesium to calcium ion activity ratio in the solution. When left it contact with the supernatant, the amorphous precipitates matured to form the crystalline calcium phosphate brushite (CaHPO 4.2H20). The amorphous phases were characterized by X-ray absorption spectroscopy and by Fourier transform infrared spectroscopy and their properties compared with those of a basic amorphous tricalcium phosphate precipitated at pH 10. The X-ray absorption spectra near the K edge of calcium were very similar for all samples but there were differences in the infrared spectra between the basic and the more acidic salts. In the phosphate stretching region, the main band of the more acidic materials occurred at higher wavenumber and was broader. Also there was a broad band of medium intensity at about 890 cm 1 whereas there was virtually no absorption band in this region in the spectrum of the amorphous tricalcium phosphate. The acidic amorphous calcium phosphates may be useful as model compounds in describing some complex biological calcium phosphates that form near neutral pH.