DYNAMIC STRUCTURE OF THE LOWER DENSITY LIPOPROTEINS .2. DEUTERIUM NMR-STUDIES OF THE MONOLAYER OF VERY LOW AND LOW-DENSITY LIPOPROTEINS

The order of phosphatidylcholine (PC) acyl chains in the surface monolayer of very low density lipoproteins (VLDL) and low density lipoproteins (LDL) has been determined from 2H nuclear magnetic resonance order parameters, S(CD), using selectively deuterated PC or palmitic acids. From the computer simulated line shapes, we find two distinct phospholipid domains within the amphiphilic monolayer of both VLDL and LDL. In the more ordered domain of LDL, S(CD) was ~ 0.3 for the ''plateau'' chain region. The S(CD) values of VLDL particles are similar to those of LDL for the 5,6- and 11,12-positions, hence we suggest the organization of the more ordered region of VLDL and LDL are similar. The domain of low order in LDL comprises < 10% of the phospholipid molecules (we do not distinguish between PC and sphingomyelin), having approximately the same order (S(CD) < 0.1) as egg PC - sphingomyelin unilamellar vesicles. In VLDL, the domain of low order comprises between ~ 10 and ~ 20% of the phospholipid molecules and the entire acyl chain is in an essentially isotropic environment (S(CD) < 0.02). We prepared VLDL-sized microemulsions composed of egg PC, deuterated PC, and triolein to test whether the apoproteins were responsible for creating the two differently organized domains in VLDL and LDL. Surprisingly, these protein-free particles also showed two domains of different order at two temperatures. The high order region, however, is less ordered than in VLDL and LDL. We explain two surface domains of PC in terms of lipid organization and the unique interactions of lipids in the various lipoprotein particles.