Structure Determination of Different Casein Components

Abstract

Casein is the main protein component of milk. It is not a homogeneous polymer but consists of several types of casein, and forms large spherical micelles of remarkable stability. The reason for this stability is not yet fully understood. In co-operation with other laboratories, we have been able to obtain well-separated individual fractions which were studied with regard to their capability for micelle formation and the micelle structure. Studies have been carried out with the α, β and κ-caseins. Small-angle neutron scattering and combined static and dynamic light scattering have been applied, which gave information on the particle weight M_w, the radius of gyration R_g and the hydrodynamic radius R_H, and on the particle scattering factor P_z(q), where q = (4π/λ) sin θ/2.

The three caseins show strikingly different behaviour. For the β-casein, a star-like structure was detected which corresponds to the aggregation pattern of common soap micelles. The aggregation number is 38. The aggregates of κ-casein appear to be composed of star-like sub-micelles; each sub-micelle contains nine κ-casein molecules and the total aggregation number is 140. α-Casein forms long fibrils which resemble worm-like filaments. The contour length of the cylinders was found to be L ≃ 1600 nm, and the filaments appear to be composed of about 12 Kuhn segments of l_K = 133 ± 12 nm in length.