Infrared
9,42–54 and fluorescence anisotropy
42 studies show that meibum lipid hydrocarbons align to maximize Van der Waal's interactions between chains. Therefore, the WE and CE hydrocarbon chains are not randomly orientated as they are in an oil phase, as many schematic pictures in literature show them to be. At lower temperatures, meibum is in a liquid crystalline phase. The term “liquid crystalline phase” is used because meibum is not a solid crystal (100%
trans) as the meibum hydrocarbon chains contain 72%
trans rotamers, allowing them to pack tightly together (
Fig. 4A).
54 Thus, the term “liquid crystalline phase” is used rather than crystalline phase. At higher temperatures, meibum is in the gel phase, and the conformation of the meibum lipid hydrocarbon chains are 18%
trans rotamers (
Fig. 4A).
54 Thus, meibum is not a liquid (0%
trans) but rather in the gel phase. X-ray crystallography of WE (
Fig. 4A) shows that WE packs together in lamellar layers with their hydrocarbons, with terminal methyl and ester carbonyl moieties adjacent to one another (
Fig. 4A).
55–57 X-ray crystallography and biophysical studies of CE (
Fig. 4B) show that at lower temperatures, in the smectic phase, they also pack together in lamellar layers with their hydrocarbons, steroid nuclei, with terminal methyl and ester carbonyl moieties adjacent to one another and their side chains interdigitated.
17,58 Adjacent terminal methyl moieties would be expected, especially with CE that contains branched terminal methyl groups. It is unknown how a mixture of WE and CE pack together, but it is reasonable that the minimal energy structures of CE and WE crystals alone are maintained in a mixture of the two. In the mixture, we speculate that the lipids would align to maximize hydrocarbon chain interactions just as they do for CE and WE alone (
Fig. 5). The arrangement of molecules in
Figure 5 allows for the interdigitation of CE side chains and maximizes the adjacent packing of the steroid nuclei and CE carbonyl moieties, just as they are in the crystalline state alone. The speculative model for the mixture of CE and WE (
Fig. 5) has phospholipids with their hydrophilic head group facing the tear aqueous layer. Phospholipids do not interact with CE.
11–13 It is speculative if the phospholipids in tears arise from debris
59 or form a monolayer alone with other amphipathic molecules, such as OAHFA. Note that the carboxylate groups of the diester of OAHFA and the CE of OAHFA extend to the interface region where they may interact with water. For a 100-nm thick TFL layer, the motif structure of hydrophobic region and side chain-to-side chain region above the phospholipid monolayer in
Figure 5 would stack about 26 times.