Neutron small angle scattering on selectively deuterated human plasma low density lipoproteins. The location of polar phospholipid headgroups.

Human plasma low density lipoproteins (LDL) were deuterated in the phospholipid headgroup region by exchange with phosphatidylcholine-N(CD3)3-apolipoprotein A complexes. The exchange was associated with a net transfer of phosphatidylcholine to LDL leading to an increase in total phospholipid content by 27%. Practically all of the endogenous phosphatidylcholine including lysophosphatidylcholine, and about one-third of the sphingomyelin pool was found to be exchangeable. Immunochemically, deuterated LDL was identical with native LDL. The hydrodynamic and ultrastructural properties were closely similar for the two particle species apart from a slight increase in overall particle size by about 2%. Both native and deuterated LDL were investigated by neutron small angle scattering at several representative contrasts in H2O/D2O buffers. Subtraction of the scattering amplitudes of native from deuterated LDL resulted in a radius of gyration of 103 +/- 5 A for the N(CD3)3 groups, and in a structure factor resembling that of a thin, spherical shell. Evaluation of the contrast variation experiments in combination with previous data from x-ray small angle scattering (Laggner, P., and Mueller, K. (1978) Q. Rev. Biophys. 11, 371-425) indicates that the phospholipids form a spherical monolayer shell in the radial range between 75 A and 103 A around the core of cholesteryl esters and triglycerides. For the protein moiety, a radius of gyration of 110 A was calculated, indicating that it is located, on average, 5 to 10 A from the polar phospholipid headgroups toward the aqueous environment.