Decipher 'Em All: A Profiling Study on the Effects of Acyl Groups in O-Acyl-ω-hydroxy Fatty Acids.

The O-acyl-ω-hydroxy fatty acids (OAHFAs) are an intriguing class of surface-active lipids which can be found in the human tear film lipid layer (TFLL). Recent studies have suggested that OAHFAs exist in the polar lipid layer and play a central role in TFLL function. Surprisingly, biophysical profiling studies have only shed light on the properties of OAHFAs bearing an oleate acyl group and insights on species with other acyl groups are scarce. Herein, we seek to address this issue through (1) focusing on the synthesis and characterization of a representative library of OAHFA analogues bearing a palmitate, palmitoleate, stearate, and linoleate acyl group, and (2) performing an in-depth mapping of their biophysical properties. Our results indicate that NMR-spectroscopic techniques can be utilized for rough estimation of the amounts of distinct acyl groups in a sample and more importantly, how the subtle variations in both parent chains and acyl groups influence the core properties of the OAHFAs. We reach the conclusion that the correlation between melting points and film properties is not as clear-cut as previously thought. Nevertheless, grouping of OAHFA species into three separate categories which display distinct behavior seems to be possible utilizing the melting points as a guiding parameter. Altogether, our study suggests that the properties of OAHFAs need to be assessed from a viewpoint which combines both the parent chain and acyl group instead of independent analysis based on either fragment alone.

Another Brick in the Wall of Tear Film Insights Added Through the Total Synthesis and Biophysical Profiling of anteiso-Branched Wax and Cholesteryl Esters.

The tear film lipid layer (TFLL) plays a vital part in maintenance of ocular health and represents a unique biological barrier comprising unusual and specialized lipid classes and species. The wax and cholesteryl esters (WEs and CEs) constitute roughly 80-90% of the TFLL. The majority of species in these lipid classes are branched and it is therefore surprising that the synthesis and properties of the second largest category of species, i.e., the anteiso-branched species, remain poorly characterized. In this study, we have developed a total synthesis route and completed a detailed NMR spectroscopic characterization of two common anteiso-branched species, namely: (22S)-22-methyltetracosanyl oleate and cholesteryl (22'S)-22'-methyltetracosanoate. In addition, we have studied their structural properties in the bulk state by wide-angle and small-angle X-ray scattering and their behavior at the aqueous interface using Langmuir monolayer techniques. A comparison to the properties displayed by iso-branched and straight-chain analogues indicate that branching patterns lead to distinct properties in the CE and WE lipid classes. Overall, this study complements the previous work in the field and adds another important brick in the tear film insights wall.

New Insights into the Molecular Structure of Tear Film Lipids Revealed by Surface X-ray Scattering.

The tear film lipid layer (TFLL) is a unique biological membrane that serves a pivotal role in the maintenance of ocular surface health. Reaching an overarching understanding of the functional principle of the TFLL has been hampered by a lack of insights into the structural and functional roles played by individual lipid classes. To bridge this knowledge gap, we herein focus on studying films formed by principal lipid classes by surface scattering methods. Through grazing incidence X-ray diffraction and X-ray reflectivity studies, we reveal quantitative data about the lattice distances, molecular tilt angles, and mono/multilayer thickness and density profiles for central TFLL lipid classes under close to simulated physiological conditions. In addition, we discuss the correlation of the results to those obtained previously with the natural lipid composition of meibum.