Studies of Casein. III. The Molecular Size of α-, β-, and γ-Casein

α-, β-, and x-casein are aggregated at neutral pH and room temperature in salt solution. They may be disaggregated at high pH or at neutral pH in concentrated urea solution. At neutral pH in salt solution, α-casein forms aggregates centring around a preferred size. As the pH is increased the size decreases until at pH 11 (I 0.20) it is disaggregated completely. Measurements of the molecular weight by sedimentation and diffusion at pH 11 give a value of 24,800�1000. Approach to sedimentation-equilibrium measurements at pH 12, using the Archibald method, gives a value of 25,500�1000. In 6M urea solution at pH 7.3 sedimentation and diffusion give a value of 27,600�1000. At room temperature and neutral pH, β-casein is present as single molecules in equilibrium with an aggregate of very high molecular weight. At pH 11, it is disaggregated with a molecular weight of 17,300�800 (by sedimentation-diffusion). At pH 7 in 631 urea a value of 19,800�1000 is obtained for the molecular weight. Preliminary measurements by the Archibald method at pH 12 give a value of 26,000�3000 for the molecular weight of x-casein. These results are discussed in relation to those of other workers.