Temperature-dependent molecular motions of saturated acyl cholesteryl esters: A 13C NMR study

High resolution 13C NMR spectra were obtained vs temperature in the isotropic liquid and liquid-crystalline phases of four cholesteryl esters with saturated acyl chains [(6:0), (14:0), (16:0), and (18:0)] and a mixture rich in cholesteryl esters with unsaturated acyl chains [10% 16:0, 45% 18:1, 45% 18:2, w/w/w]. Except for peaks from carbons in the middle and/or distal part of the fatty acyl chain, the narrow peaks seen for the isotropic liquid broadened beyond detection below the isotropic→cholesteric phase transition, indicating a marked decrease in the rates and/or amplitudes of motions of most portions of the cholesteryl ester molecule. Rotational correlation times for the long molecular axis and the short nonunique axis (τrz and τrx, respectively), were calculated from the linewidth of the steroid ring methine C3 and C6 resonance for each ester system in the liquid phase. With decreasing temperature, τrx and τrz showed little or no increase for the saturated esters but a significant differential increase for the mixture. Because of stronger intermolecular interactions of neat saturated esters, molecular ordering near the liquid→liquid-crystalline phase transition may occur in such a narrow temperature range that the large pretransitional anisotropies observed for unsaturated esters are not observed for the saturated esters.