Molecular weight and size distribution of bovine milk casein micelles

Abstract

We have characterized the distribution of hydrodynamic radii, sedimentation coefficients, and molecular weights of native bovine casein micelles, using samples fractionated by rate-zone centrifugation in sucrose density gradients. Diffusion coefficients were measured by inelastic laser light scattering, and were combined with sedimentation coefficients to yield molecular weights. The distribution of molecular weights is quite broad, with a ratio of weight-average to number average of about 3. The weight-average molecular weight is about 2.5·108, corresponding to about 104 casein monomers per micelle. The micelle radii obtained by these solution measurements are somewhat larger than those obtained in most earlier electron microscopic studies, but agree quite closely with those found by Carroll and coworkers. The mean radius on a weight basis is about 800 Å. The relation between sedimentation coefficient and molecular weight is s20,w = 1.79·10−3M0,665, indicating that casein micelles behave like highly hydrated spheres.