Rotational and Segmental Motions in the Lipids of Human Plasma Lipoproteins

Abstract Natural abundance 13C NMR spectra of high density, low density, and very low density lipoproteins isolated from human plasma have been recorded and assigned, and the spin lattice relaxation times (T1) of some well resolved resonances have been determined. The 13C NMR spectra reflect differences in protein-lipid ratios and in lipid composition between the three classes of lipoproteins. All three lipoproteins yield numerous narrow 13C resonances assignable to the lipid component. The high density lipoprotein also yields several narrow peaks that arise from the protein moiety. All of the above narrow resonances must originate from liquid-like lipid and protein components. T1 values of lipid carbon resonances of lipoproteins and some model systems yielded semiquantitative information about rotational and segmental mobilities of the lipid components of lipoproteins. The T1 values of C6 of the cholesteryl moiety of high and low density lipoproteins revealed that the fused ring systems occupy regions of much higher microviscosity than that of the aqueous environment. T1 values of carbons of fatty acyl chains yielded information about segmental motions of these chains in the environment of the lipoprotein complex. Segmental mobilities of fatty acyl chains are identical, within experimental error, in high density and low density lipoproteins, despite marked compositional (and probably structural) differences. T1 values of fatty acyl chains of very low density lipoproteins are slightly longer than the corresponding values of the denser lipoproteins, a result that suggests somewhat greater segmental mobilities in very low density lipoproteins. Our 13C T1 values indicate that all or most of the lipid molecules of plasma lipoproteins, although in a liquid-like state, are significantly restricted in their rotational and segmental mobilities relative to lipids in organic solvents.