Molecular dynamics of lipids in human plasma high density lipoproteins. A high field 13C NMR study.

Natural abundance Fourier transform 13C NMR spectra were obtained at 63.4 kG for human plasma high density lipoproteins (HDL) at several temperatures between 21°C and 36.5”C. At a given temperature, spectra of HDL, HDL2, and HDL3 are similar; linewidths for corresponding resonances are not distinguishable within experimental error. Improved resolution in HDL spectra at high field yields a number of new resonances assigned to specific carbon atoms of lipids, including single-carbon resonances for each of the principal lipid classes of HDL, and permits linewidth determinations for several peaks incompletely resolved in low field (14.2 kG) 13C NMR spectra. The cholesterol ring C-6 methine resonance is resolved into two components: a narrow resonance at 122.3 ppm from the ester form and a weaker resonance at 120.7 ppm from the unesterified form. The cholesterol ring methine carbon atoms C-3 and C-9, which have parallel C-H vectors orthogonal to the C-6-H vector, show well resolved resonances assigned to the ester form. Since these resonances have a linewidth which is about 50% greater than that for the cholesteryl ester C-6 resonance, the reorientation of the steroid ring of HDL cholesteryl esters is not isotropic. Both the C-6 and C-9 (or C-3) linewidths in HDL are nearly identical with linewidths for the corresponding resonances in spectra obtained for a neat cholesteryl ester/triglyceride mixture (58% cholesteryl linoleate, 20% cholesteryl oleate, 22% triolein) which does not exhibit liquid-crystalline phase transitions, suggesting that the rate of rotation and the degree of anisotropic rotation is similar in both systems. Cholesterol ring carbon resonqnces of HDL cholesteryl esters remain narrow at 21°C. The unesterified cholesterol C6 linewidth for HDL is similar to the corresponding linewidth in a spectrum obtained for the above lipid mixture containing solubilized cholesterol (54% cholesteryl linoleate, 19% cholesteryl oleate, 6.5% cholesterol, 20.5% triolein). Resonances for glyceryl backbone carbon atoms of triglycerides and phospholipids are observed in a region which is poorly resolved at low field. The phospholipid C-l’ and C-3’ glyceryl carbon atoms appear as a doublet of unequal intensity: the C-l’ resonance is apparently broader than the C-3’, possibly as a result of greater motional freedom at the C-3’ carbon (adjacent to the phosphate group). Several specific fea-