Core Polymers of Casein Micelles

The cores of casein micelles are considered to be composed of insoluble salts of αs- and β-caseins. Core structure has been investigated by examining, for αs-casein, β-casein and a unit weight ratio mixture of the two, at pH 6.6, the relationships between degree of monomer association and ionic strength (NaCl), divalent cation concentration (Ca(II)) or temperature. Data obtained were sedimentation coefficient, S20,w, and reduced viscosity, [η]. For β-casein and the mixture at 37°, with calcium absent, as NaCl increases, S20,w increases and [η] decreases. Both plateau at ionic strengths above 1.5 until precipitation occurs; near saturation for β-casein and at about 3.5 M NaCl for the mixture. The degree of association also increases as the temperature is increased. Plateau values for S20,w and [η] have been observed just prior to precipitation over a temperature range near 23° for β-casein at 0.048 M NaCl and 0.015 M CaCl₂ and for the mixture at 0.048 M NaCl and 0.009 M CaCl₂. αs-Casein behaves differently. For example, it precipitates near ionic strength 0.6 (NaCl) in the absence of calcium and plateaus in S20,w and [η] are not observed. Plateaus in both S20,w and [η] just before precipitation suggest that polymers have arrived at limiting average values of size, shape, and solvation. Application of the equations of Simha and Perrin indicates that limiting polymers are spherical, have a maximum degree of association near 30, and have solvations ranging from 2.2 to 4.4 g of water per g of protein. The degrees of association calculated using S20,w and [η] are in agreement with those obtained independently by Archibald analysis of polymer weights. The limiting (core) polymer is considered to contain radially arranged anisometric monomers. It is proposed that monomers consist of elongated bodies of low net charge having characteristic solvent accessible acidic peptides attached near their ends. Electrostatic free energy is minimized by placing the acidic peptides near the polymer surface. When the acidic peptides are sufficiently discharged by ion shielding or binding, interpolymer interactions allow precipitation (core formation) to occur. A comparison of the behavior of individual components and the mixture shows that αs- and β-caseins intermingle at the molecular level in the mixture and that polymers of αs-casein have a larger interpolymer attractive energy than those of β-casein.