Self-assembly and molecular packing in cholesteryl esters at interfaces

On the importance of chain branching in tear film lipid layer wax and cholesteryl esters

The tear film lipid layer (TFLL) is important to the maintenance of ocular surface health. Surprisingly, information on the individual roles of the myriad of unique lipids found therein is limited. The most abundant lipid species are the wax esters (WE) and cholesteryl esters (CE), and, especially their branched analogs. The isolation of these lipid species from the TFLL has proved to be tedious, and as a result, insights on their biophysical profiles and role in the TFLL is currently lacking. Herein, we circumvent these issues by a total synthesis of the most abundant iso-methyl branched WEs and CEs found in the TFLL. Through a detailed characterization of the biophysical properties, by the use of Langmuir monolayer and wide-angle X-ray scattering techniques, we demonstrate that chain branching alters the behavior of these lipid species on multiple levels. Taken together, our results fill an important knowledge gap concerning the structure and function of the TFLL on the whole.

The Role of Molecular Packing in Dictating the Miscibility of Some Cholesteryl n-Alkanoates at Interfaces

Cholesteryl n-alkanoates of saturated fatty acids and their mixtures are widely studied in different physical states and also due to their significance in biology. Here, we address the miscibility of some homologues of cholesteryl n-alkanoates at interfaces, which are known to exhibit different (cholesteryl octanoate, ChC8, and cholesteryl stearate, ChC18) or the same (cholesteryl nonanoate, ChC9, and cholesteryl laurate, ChC12) molecular packing in bulk. Surface manometry and Brewster angle microscopy studies on ChC8 (cholesteryl-cholesteryl interaction, referred to as m-i packing)/ChC9 (cholesteryl-chain interaction, referred to as m-ii packing) and also on ChC18 (chain-chain interactions, referred to as the crystalline bilayer)/ChC9 mixtures reveal phase separation at the air-water (A-W) interface plausibly due to the difference in the molecular packing. In contrast, ChC12/ChC9 (both m-ii packing) mixtures form a homogeneous phase and exhibit a higher collapse pressure (almost twice) than that of ChC9 indicating higher stability. At the air-solid (A-S) interface, the height profiles extracted from the surface topography images using an atomic force microscope yielded thicknesses of 3.6 ± 0.1 and 5.6 ± 0.1 nm for ChC18/ChC9 mixtures (at 0.66 and 0.5 mole fractions (MF)) corresponding to individual assembly, whereas a uniform thickness of 3.5 ± 0.2 nm is obtained for the case of ChC12/ChC9 mixtures (at 0.2, 0.5, and 0.8 MF) corresponding to m-ii packing. Ellipsometry studies reveal that the desorption temperature increases with the mole fraction of ChC9 and attains a maximum at 406.8 ± 4.8 K for 0.4 MF of ChC9, beyond which it decreases. Raman spectroscopy studies are carried out for ChC12/ChC9 mixtures in the homogeneous phase and in the collapsed state. Here, the dependency of peak positions on different physical states was assessed. Our studies offer new insights into the compatibility of molecular packing influencing the phase behavior and may be of relevance to tear film studies and on the formation of crystals in atherosclerosis.