Determination of the quaternary structural states of bovine casein by small-angle X-ray scattering: Submicellar and micellar forms

Abstract

Whole casein occurs in milk as a spherical colloidal complex of protein and salts called the casein micelle, with approximate average radii of 650 Å as determined by electron microscopy. Removal of Ca 2+ is thought to result in dissociation into smaller noncolloidal protein complexes called submicelles. Hydrodynamic and light scattering studies on whole casein submicelles suggest that they are predominantly spherical particles with a hydrophobic core. To investigate whether the integrity of a hydrophobically stabilized submicellar structure is preserved in the electrostatically stabilized colloidal micellar structure, small-angle X-ray scattering (SAXS) experiments were undertaken on whole casein from bovine milk under submicellar (without Ca 2+) and micellar (with added 10 m m CaCl 2) conditions. All SAXS results showed multiple Gaussian character and could be analyzed best by nonlinear regression in place of the customary Guinier plot. Analysis of the SAXS data for submicellar casein showed two Gaussian components which could be interpreted in terms of a particle with two concentric regions of different electron density, designated as a “compact” (subscript C) core and a “loose” (subscript L) shell, respectively. The submicelle was found to have an average molecular weight of 285,000 ± 14,600 and a mass fraction of higher electron density core, k, of 0.212 ± 0.028. The radius of gyration of the core, R c, was 37.98 ± 0.01 Å with an electron density difference, Δ ρ C, of 0.0148 ± 0.0014 e −/Å 3, while the loose region had values of R L = 88.2 ± 0.8 A ̊ with Δρ L = 0.0091 ± 0.0003 e −/ A ̊ 3 . Calculated distance distribution functions and normalized scattering curves also were consistent with an overall spherical particle with a concentric spherical inner core of higher electron density. These results, and in particular the remarkably low electron densities of the shells, can be interpreted in terms of a loosely packed spherical aggregate stabilized by a hydrophobic inner core and surrounded by an even more loosely packed hydrophilic region, in agreement with the results of other studies. The SAXS data for the colloidal micellar casein, which yield only cross-sectional information related to a window of scattered intensity, were analyzed by a sum of three Gaussians with no residual function. The two Gaussians with the lower values of the radius of gyration were interpreted again as an indication of an inhomogeneous spherical particle of two electron densities with the same centroid. The third Gaussian was shown to reflect the packing number of these particles, which was 3:1 for this system. The molecular weight determined from the two Gaussians of lower radii of gyration was in agreement with the value obtained from SAXS of submicellar casein, as were the k and Δ ρ C values. However, a lower value of Δ ρ L = 0.0065 ± 0.0003 was observed under these micellar conditions. These results are an indication of the existence of submicellar inhomogeneous particles containing a hydrophobically stabilized inner core within the colloidal micellar structure.