Temperature-dependent molecular motions of cholesterol esters: a carbon-13 nuclear magnetic resonance study

Carbon-13 NMR spectroscopy at 50.3 MHz has been used to study four long-chain cholesterol esters with a double bond in the omega-9 position: cholesteryl oleate, C18:1, omega-9; cholesteryl linoleate, C18:2, omega-6,9; cholesteryl erucate, C22:1, omega-9; cholesteryl nervonate, C24:1, omega-9. The linoleate and oleate esters exhibit two metastable liquid-crystalline phases (cholesteric and smectic), whereas the longer chain esters form a stable smectic phase but no cholesteric phase [Ginsburg, G. S., & Small, D. M. (1981) Biochim. Biophys. Acta 664, 98-107]. Line widths (nu 1/2), spin--lattice relaxation times (T1), and nuclear Overhauser enhancements (NOE) were measured for all well-resolved resonances from ring and fatty acyl (FA) carbons at different temperatures in the isotropic liquid of each ester. T1 and NOE values of FA resonances were constant between the FA-2 carbon and olefinic region of each acyl chain and increased markedly for carbons near the chain terminus. FA carbon motions are thus restricted and/or highly correlated in the region between the ring and the olefinic carbons, suggesting that strong interactions occur between cholesterol ester molecules in this region of the FA chain. These results also suggest that the FA chains are approximately extended in the isotropic liquid. Steroid ring methine C-6 and C-3 nu 1/2's increased differentially on cooling to the liquid leads to liquid crystal transition temperature (Tm) of each ester, indicative of increasingly anisotropic ring rotations. The rotational anisotropy was quantitated by using a prolate ellipsoid model for the cholesterol ester molecule for which two correlation times (corresponding to rotations about the long and short molecular axes) were calculated from the C-3 and C-6 nu 1/2 values. The C-3/C-6 nu 1/2 ratio was directly proportional to the anisotropy of the ring motions as measured by the ratio of the two correlation times. At any given temperature relative to Tm, the C-3 and C-6 nu 1/2's and the C-3/C-6 nu 1/2 ratios were larger for cholesterol esters which have a cholesteric phase than for esters which have no cholesteric phase, showing that steroid ring motions were more restricted and more anisotropic prior to the formation of a cholesteric phase. Cholesteryl erucate and cholesteryl nervonate have longer regions of FA chain interactions which result in greater chain cooperativity, apparently preventing the preordering of steroid rings to the degree necessary for formation of a cholesteric phase. Thus, these esters form the smectic phase directly from the isotropic liquid. These results are applied to the cholesterol ester transition in plasma low-density lipoproteins.