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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 NMRspectroscopic 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.
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 NMRspectroscopic 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.
Purpose: To visualize the behavior of perfluorohexyloctane (PFHO), an eye drop to treat dry eye disease (DED), on the surface of saline in vitro and on the human ocular surface using infrared emissivity. Methods: Emissivity videos were used to measure the spreading and disappearance rates of PFHO on saline (with and without mucin for spreading rate) and layered over a 125 nm film of meibum on the surface of saline using a TearView camera. Ocular surface emissivity was videoed in a volunteer without DED before and after instillation of 1 drop of PFHO. Videos were exported and converted to still photographs, and grayscale levels measured. Results: PFHO formed a layer over saline that spread at a mean (SD) rate of 0.89 (0.5) cm2/s and disappeared at 0.0760 (0.0055) μL/min, consistent with reported rates of evaporation for PFHO. Mucin in the subphase did not alter spreading rate (P > 0.2). In vitro, a single drop of PFHO spread over the top of a 125-nm thick film of meibum remaining for 3.3 hours. In the volunteer, an increase in emissivity was detected on the ocular surface for ≥5 hours. Conclusions: PFHO quickly spread to form a layer over the surface of saline or meibum in vitro and was detected on the ocular surface in vivo for ≥5 hours after topical administration. This supports findings that PFHO forms a long-lasting barrier to evaporation at the air–liquid interface of the tear film and thus reduces signs and symptoms of DED.
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