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The role of calcium, phosphate and citrate ions in the stabilization of casein micelles

SUMMARY To obtain greater insight into the interaction of Ca, citrate and phosphate ions with casein, 31P NMR measurements were performed on combinations of these ions with αs- and κ-caseins. It was found that addition of αs-casein to a Ca phosphate solution in D2O at 27 °C and pD 6·4 resulted in a downfield shift of the 31P singlet. An almost identical shift was observed with κ-casein, but no shift was found when only phosphate ions were present or when Ca2+ were added to phosphate ions in the absence of casein. Separate experiments with poly-l-lysine, mol. wt approx. 35000, resulted in similar downfield 31P chemical shifts of Ca phosphate as with both caseins, whereas no shift was observed when poly-glycine was added. From these results it can be concluded that Ca and phosphate ions associate with casein in a co-operative manner, probably in the way described by ter Horst (1963) as a complex with the NH3+-groups of lysine or arginine in a structure such as: (casein—NH3+)—P043-—Ca2+. The formation of this complex may be enhanced by citrate ions, since preliminary results have shown that addition of Na citrate to a solution of αs-casein with added Ca phosphate produces a broadening of the 31P signal as well as a chemical shift.