Interactions in human casein systems: Self-association of nonphosphorylated human β-casein

Since caseins were originally defined as phosphoproteins, nonphosphorylated β-casein, comprising nearly 5% of the total β-casein in the isoelectric precipitate from human milk, appears to be unique. Despite the relatively small amount present, its properties suggest that it may play an important role in micelle formation and structure. It has a partial specific volume, v, of 0.749 ± 0.008 and an absorbancy, E^1%_{1 cm,280 nm} of 6.2 ± 0.2. Sedimentation and viscosity data yield a solvation of 3 g H₂O/g protein and an axial ratio of about 5 for the monomer. This would be consistant with a prolate ellipsoid of 10 nm length and 2 nm width. Equilibrium in the system is attained quite slowly and the temperature-dependent polymerization was found to be reversible. With calcium, the solubility behavior reflects an increased hydrophobicity and lower electrostatic repulsion in the molecule. There is essentially no strong calcium binding to this protein but there is evidence which strongly suggests that calcium binds to nonphosphate groups at higher concentrations. Increasing the temperature from 4 to 37 °C causes an apparent conformational change and an increase in protein aggregation which is further increased by addition of NaCl at 37 °C until a limiting size is reached at about 0.1 m NaCl. This limiting size polymer contains about 75 monomers and is nearly spherical with a radius of about 12 nm and a solvation of 1.5 g H₂O/g protein. Laser light scattering measurements on the solution in 0.25 m NaCl revealed a relatively homogeneous particle size with a corrected diffusion coefficient, D_20,W, of 2.8 × 10^-7 cm²/s.