Molecular organization and motions of cholesteryl esters in crystalline and liquid crystalline phases: a 13C and 1H magic angle spinning NMR study.

Cholesteryl esters are a major lipid constituent of plasma lipoproteins and atherosclerotic lesions. Crystalline and liquid crystalline phases of several cholesteryl esters [oleate (C18:1, omega-9), erucate (C22:1, omega-9), hexanoate (C6:0), decanoate (C10:0), undecanoate (C11:0), myristate (C14:0), palmitate (C16:0), and stearate (C18:0)] have been studied by natural abundance 13C NMR with magic angle spinning (MASNMR) at 75 MHz (7.05 T). Spectra obtained with magic angle spinning, high-power proton decoupling, and cross-polarization transfer were highly resolved for crystalline cholesteryl esters. Acyl chain carbons had narrower lines than protonated steroid ring carbons, reflecting differential motions in the crystal (specifically, more rapid motions in the acyl chain than in the steroid ring). Esters which crystallize into the monolayer type II structure, in which all molecules are equivalent, gave rise to a single resonance for each carbon; esters of the monolayer type I and bilayer structures, in which there are two types of nonequivalent molecules in the unit cell, had two resonances (equal intensity and line width) for several carbons, such as the carbonyl and the steroid ring C5 and C6. Spectra for liquid crystalline phases did not show inequivalence of signals for the same carbon and were not enhanced by cross-polarization transfer. These changes are a result of increased molecular motions in the liquid crystals, which average the nonequivalent environments and reduce the dipolar interactions. Cholesteric and smectic liquid crystalline phases were distinguished by the broader C = O, C5, and C6 signals for the cholesteric compared with the smectic phase. In the smectic phase, chemical shifts of corresponding carbons of all cholesteryl esters are similar and are close to those for crystalline esters with a monolayer II structure, which suggests that the smectic phase has structural features resembling the monolayer II crystal structure. 13C MASNMR is thus a powerful approach for examining structure and motions of crystalline and liquid-crystalline cholesteryl esters. 1H MASNMR spectra did not give as detailed information on the molecular level but were unique for each phase and provided a simple and quick method for distinguishing the solid, smectic, cholesteric, and isotropic phases.