Temperature-dependent molecular motions and phase behavior of cholesteryl ester analogues

The phase behavior and temperature-dependent molecular motions of three cholesteryl ethers (caproyl, myristyl, oleyl) and a cholesteryl carbonate (oleyl) were characterized. The properties of each ether were qualitatively similar to, but quantitatively different from, those of the corresponding cholesteryl ester. For example, cholesteryl oleyl ether exhibited the same phase transitions as cholesteryl oleate, but at much lower temperatures (e.g., the ether isotropic liquid to cholesteric transition is at 29 degrees C). 13C NMR spectra of ethers in the isotropic liquid and liquid crystalline phases were similar to those of the ester analogue. However, near the liquid to liquid crystalline transition, the steroid ring C3 and C6 linewidths, the C3/C6 linewidth ratio, and the steroid ring rotational correlation times tau rx and tau rz calculated from the linewidths were larger for the ether than the ester analogue. The oleyl carbonate had qualitatively different properties from its analogues (e.g., stable vs. metastable cholesteric and smectic phases). Quantitative results (e.g., relatively long tau rx and tau rz in the isotropic liquid phase) for the carbonate were also distinct from those of both the ester and ether analogues. A comparison of analogues in which the polar linkage is the only structural variable yielded insights into the intermolecular interactions which influence phase behavior.