A serum low-density lipoprotein isolated from Rhesus monkeys that had been fed a coconut oil/cholesterol diet was studied by solution X-ray scattering experiments at variable contrast, at two temperatures, respectively, below (21 °C) and above (41 °C) the transition temperature. The experiments were carried out on an absolute scale, in the range (900 A ̊-1) < s < (20 A ̊-1) in water containing variable amounts of sodium bromide. Previous experiments (Aggerbeck et al., 1978) had shown that in this system the solution can be considered to be ideal and mono-disperse and that a volume can be ascribed to each macromolecule in solution inside of which the electron density distribution is independent of the density of the solvent. The structure analysis proceeds stepwise. First the morphological parameters are determined, namely those parameters (radius of gyration, volume, etc.) which are defined by some properties of the intensity and autocorrelation functions at their origin-value, slope or curvature; at the same time the three characteristic functions are determined. Next the autocorrelation function is plotted and analysed in the two-dimensional space (distance to the origin; solvent density). These steps lead to the following conclusions, which apply to both the low and the high-temperature forms: the particle is fairly isometric in shape; the outer surface is deeply convoluted; the outer region is sparsely occupied by a hydrated protein; the particle lacks a coarse centre of symmetry; the peripheral protein region is condensed in a small number of globules. It is also concluded that the temperature-induced structural transition involves both the external protein region and the core. All these conclusions are based upon a straightforward analysis of the X-ray scattering data. Pursuing the analysis beyond this stage requires making some assumption about the structure of the particle. These assumptions are focused on symmetry: it is argued, on the basis of formal considerations, of an analysis of the X-ray scattering data, and of the electron microscope observations (see Gulick-Krzywicki et al. (1979), accompanying paper), that the particle is likely to display a tetrahedral symmetry. Assuming that the symmetry belongs to point group 23, precise models are worked out for the shape of the predominantly lipid core and for the structure of the protein region. Moreover, at 21 °C the organization of the cholesteryl esters and the triglycerides inside the core is analyzed in terms of the organization of steroid-rich and hydrocarbon-rich regions of the cholesteryl esters, and it is shown to be more complicated than a concentric spherical multilayer. The models proposed are in excellent agreement with the X-ray scattering curves as well as with the chemical and physical properties of the particle; they also agree with the electron microscope observations. The most conspicuous features of the models and of the temperature-induced structural transitions are the following: the core of the particle, of low average electron density, is of almost spherical shape; this core contains the lipids and some protein; the amount of protein in the core is higher at 21 °C than at 41 °C; the core is surrounded by four protein globules, located at tetrahedral positions; each of these globules is compact at 21 °C and spreads out at 41 °C; at 21 °C the core displays a highly organized structure (tentatively described as a cubic organization of spherical elements); at 41 °C the core is less precisely organized although its structure is still more ordered than a liquid. The models are not intended to provide more than a coarse description of the structure; problems like the molecular weight of the apolipoprotein, the precise structure of the lipid in the core, the location of the protein which is not incorporated in the globules, are left open. Several formal aspects of the data processing adopted in this work are discussed; the information content of the data is shown to be redundant with respect to the models. © 1979.
