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The morphology and structural organisation of the coniplexes formed from the apoprotein of porcine high-density lipoprotein and dimyristoyl phosphatidylcholine (lecithin) have been studied using the technique of small-angle X-ray scattering. Scattering measurements made in solvents of varying electron density were interpreted in terms of a scattering-equivalent model for the structure of the complex. This model is described by an oblate ellipsoidal morphology with dimensions at 20 ' C : major axis 11.0 nm, minor axis 5.5 nm. Within this overall shape the lipid hydrocarbon chains are organised in an apolar core whilst the lipid polar head groups and protein are located in a outer shell 0.85 nm in thickness. The oblate morphology demonstrates that the structure of the complex is directed by the fundamental bilayer organisation of the lecithin. The dimension of the minor axis (5.5 nm) indicates that phospholipid hydrocarbon chains are orientated perpendicular to the interface.The plasma high-density lipoproteins represent a macromolecular assembly whereby both polar phospholipids and neutral lipids (cholesterol esters, triglycerides) are solubilised by interaction with a specific protein. Utilising the technique of smallangle X-ray scattering, we have recently presented models for the structural organisation of the high density lipoproteins [I, 21 (see also [3,4]). This model gives a more detailed description, in terms of dimensions and organisation, of the spherical "oil droplet" concept of high-density lipoprotein structure. The molecular organisation of an apolar core containing the lipid hydrocarbon chains and cholesterol esters surrounded by an outer shell containing the lipid polar groups and protein has now been shown to be consistent with detailed nuclear magnetic resonance studies [5].Following the demonstration [6] that the delipidated water-soluble apoprotein from plasma high density lipoproteins can re-combine with lipids, such systems have been widely studied [7 -91. Recombination of the apoprotein with the original high density lipoprotein lipids or with pure well characterised phospholipids and cholesterol esters, to give particles in which the conformation and environments of lipids and proteins are similar to the native assembly is well documented [lo].Visualisation by electron microscopy [l 1 -131 of the complexes formed by interaction of the apoprotein with phospholipids alone, however, has suggested that these complexes differ significantly in morphology to the native lipoprotein. Thus the observation of discoidal structures [ 11 -131 and the transformation to a more symmetrical shape following incorporation of cholesterol ester [l 1,131 has been used to infer that the cholesterol ester moiety has an important structural role in the natural complex. The lipoproteins of patients suffering from a deficiency of the lecithin-cholesterol acyl-transferase enzyme also have this discoidal appearance [14,15].In this and the accompanying communication we are concerned with the relative packing of the phos...

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