On the size of monomers and polymers of beta-casein

The size and number average molecular weight have been determined for beta-casein monomers and polymers from electron micrographs using the freeze-etching procedure with spray-frozen specimens. For the spherical beta-casein monomers we found a mol. wt of 22600 and a diam. about 10 nm, which compared quite well with data obtained from ultracentrifugation, light scattering and viscosity measurements. Polymer sizes were in agreement with molecular weight determinations from ultracentrifugation and light scattering, assuming that the volume and weight of the particles are proportional.