Studies on bovine casein.

A model for explaining the differences in physical chemical properties of the caseins was proposed, and the results of the experiments supporting this model are presented. κ, αsl , β and whole-casein were shown to exhibit circular dichroic measurements typical of random coiled or denatured proteins, all being similar. Modification of the proteins with Na-tetrathionate, S-mercapto-succinic anhydride, reduction and esterification or changes in the solvent system with urea or polyethylenimine effected the spectra only slightly. Significant dichroic changes were observed when pH adjustments were made. The interaction of polyethylenimine or κ-casein with αsl produced interaction products which were detectable with a nuclear magnetic resonance spectrometer. The interaction of polyethylenimine and as αsl -casein were detected by electrophoresis, ultracentrifugation, light scattering, solubility and nuclear magnetic resonance. This interaction was dependent upon pH, concentration and amino nitrogen content. αsl and κ-caseins were observed to be heat labile while their interaction product was more stable than each individual casein. The results suggest that the interaction of αsl and κ-caseins is through carboxyl and amino electrostatic bonds, and that their association is purely random due to lack of stoichiometry in the molecules. The role of urea in the dissociation of caseins is the disruption of both ionic and hydrogen bonds.