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Douglas Borchman, Georgi Georgiev, Marta Yappert, Norihiko Yokoi; Confirmation of Squalene in Tears and Sebum and It’s Potential Function. Invest. Ophthalmol. Vis. Sci. 2013;54(15):927.
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Squalene (SQ) is found on the skin in sebum and in meibum on the surface of the eye lid and tears. A 1H NMR resonance at 5.2 ppm has been tentatively used to quantify SQ in human sebum and meibum. When the relative intensity of the resonance in the NMR spectra of human meibum is low as in dry eye associated with meibomian gland dysfunction, the tear film is unstable and patients have signs and symptoms of dry eye. When the intensity of the resonance is restored with azithromycin or doxycycline treatment, tear film stability is restored and patients no longer are afflicted by symptoms of dry eye. In this study, we confirmed that the 5.2 ppm resonance associated with human sebum and eye lid meibum lipid (ELML) is due to SQ.
Infrared spectroscopy was used to measure SQ-wax interactions. 1H and 13C NMR resonance assignments of SQ were confirmed in human meibum, human sebum, and in human eye lid lipid using heteronuclear single quantum correlation spectroscopy. Langmuir trough and Brewster angle microscopy were used to study the surfactant properties of SQ and SQ mixed with human meibum. Evaporataion rates were measured in vitro by gravimetric analysis.
The content of SQ in human sebum was found to be 28 mole %. We confirmed the presence of SQ in ELML (4 mole %) but not meibum. SQ was 87 % disordered and did not undergo a phase transition between 20 and 80o C. SQ at 20% by weight had no significant effect on the phase transition temperature, minimum or maximum frequency or cooperativity of wax but a mixture of wax and SQ decreased the rate of evaporation of buffer with increasing thickness. SQ did not possess surfactant properties and when mixed with human meibum did not contribute to the surface pressure of films at physiological surface pressure.
Due to the lack of SQ-SQ and SQ-wax interactions, SQ is likely to spread over the surface of skin or the tear film. It is possible that the thin layer of SQ could reduce the rate of evaporation on the surface of the skin and offer antioxidant, antibacterial, and anti-inflammatory protection to the skin and tears. At low surface pressures, SQ filled thinner regions of meibum films. It is this property of SQ that could potentially stabilize the tear film during break up by migrating to the areas without a tear film lipid layer offering protection to the cornea.
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