Structural Study on Hen-egg Yolk High Density Lipoprotein (HDL) Granules

Hen egg yolk's main fractions, granules and plasma contribute to the formation and the stability of disperse systems like emulsions by constituting an interfacial film. However, yolk is still used empirically and physical properties of yolk fractions are not entirely understood. Therefore, one of the main approaches is to fractionate egg yolk in its single components and to close the lack of understanding of the molecular and structural characteristics of each fraction. As our sedimentation experiments show, separation efficiency of granules from hen egg yolk varies with changing solution pH. Under isoelectric conditions at pH 4.0, granules bind more low density lipoprotein (LDL; plasma-fraction), as atomic force microscopy reveals. We show that besides reduced electrostatic repulsion forces also structural changes of the granules support a higher incorporation of LDL-vesicle at pH 4.0. Varying amounts of lipid with pH make a structural analysis difficult by imaging techniques alone. We report on X-ray scattering experiments in reflection geometry performed on films of hen-egg granules. The observed scattering patterns provide prominent lateral distances in the film and a repeating distance from the phospholipids of the LDL vesicles. At pH 4.0 the repeating distance is larger and less diffuse compared to pH 6.5. We extracted two prominent lateral particle sizes in the HDL-granule film from two significant shoulders in the scattering functions. At pH 4.0 the obtained radii of gyration are Rg(1) = 83 nm and Rg(2) = 2.4 nm. At pH 6.5 we observed a significant expansion of both sizes. This indicates rougher contact areas and a stronger steric hindering for the incorporation of the LDL vesicle in the HDL-granules under native milieu conditions.