ABILITY OF A BETA-CASEIN PHOSPHOPEPTIDE TO MODULATE THE PRECIPITATIONOF CALCIUM-PHOSPHATE BY FORMING AMORPHOUS DICALCIUM PHOSPHATE NANOCLUSTERS

The ability of casein in the form of colloidal-sized casein micelles t o modulate the phase separation of calcium phosphate during milk secre tion is adapted to produce nanometre-sized particles of calcium phosph ate stabilized by a casein phosphopeptide (nanoclusters). The nanoclus ters were prepared from an undersaturated solution of salts and the pe ptide by raising the pH homogeneously from about 5.5 to 6.7 with urea plus urease. Chemical analysis and IR spectroscopy showed that they co mprise an amorphous dicalcium phosphate bound to the phosphopeptide. M ultinuclear NMR spectroscopy of the cluster solutions showed that the small ions and free peptide in the solution were in a state of dynamic exchange with the nanoclusters. The peptide is linked to the calcium phosphate through its sequence of phosphorylated residues, but, in a p roportion of adsorbed conformational states, the termini retain the co nformational freedom of the unbound peptide. The ability of casein to form nanoclusters in milk suggests a more general mechanism for avoidi ng pathological calcification and regulating calcium flow in tissues a nd biological fluids exposed to or containing high concentrations of c alcium.