Molecular Packing and Fluidity of Lipids in Human Serum Low Density Lipoproteins

Abstract

The structural transition of lipoprotein B (LpB) from human serum low density lipoproteins in the temperature range between 15-30 C was studied by analyzing the X-ray small angle scattering curves measured at 4, 21 and 37.degree. C. The radius of gyration shifts from 13.8 nm at 4.degree. C to 17.0 nm at 37.degree. C, indicating a radial rearrangement of the internal electron density distribution. The spherically averaged radial electron density distribution calculated from the experimental data under the assumption of ideal spherical symmetry suggests a thermotropic transition of the molecular packing in the particle core, up to a radius of about 7.5 mm. The outer shell between radii of 7.5 and approximately 12.0 nm is essentially unaffected by temperature changes in the observed range. Together with the chemical composition of LpB, these results lead to a structural model in which the neutral lipid constituents, cholesteryl esters and triglycerides, form the particle core. The thermotropic changes relating to this region are consistent with a transition from a smectic-like to a more isotropic state. The dimensions and electron densities of the outer shell suggest a monolayer-type arrangement of the amphiphilic constituents, i.e., phospholipids, free cholesterol and protein, at the particle surface.