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D. Borchman, G. N. Foulks, M. C. Yappert, D. Tang, D. V. Ho; Temperature-Induced Conformational Changes in Human Tear Lipids Determined by Fourier Transform Infrared and Fluorescence Spectroscopies. Invest. Ophthalmol. Vis. Sci. 2007;48(13):430.
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As a first step in determining the relationships between tear lipid composition, structure and function, the molecular structure/conformation, dynamics and packing of human tear lipids were investigated.
Infrared and fluorescence spectroscopies were applied to human meibum and tear lipids.
Differences between the infrared CH stretching band region suggest that tear-fluid lipids contain less double bonds and less hydrocarbons compared to meibum lipids. Both meibum lipid and tear-fluid lipid were in an orthorhombic perpendicular packing geometry. The packing of meibum lipids was found to be tighter than in tear-fluid lipids. This tighter packing was also reflected in the higher cooperativity and enthalpy observed during the gel to liquid phase transition of meibum lipids, as compared to tear lipids. As the temperature increased from 25 to 40oC, hydrocarbon chains became disordered (20% to 62%) and the probe anisotropy diminished significantly. The C=O stretching band observed in the infrared spectra of tear lipids revealed that most C=O groups are not involved in strong hydrogen bonds, however, a fraction of the meibum lipid C=O are. Bands due to the polar moieties C=O and PO2- did not change significantly with increasing temperature, suggesting that they may not play an appreciable thermodynamic role in the lipid hydrocarbon chain phase transition. Components in tears bind to tear lipids and exclude water at the water-lipid boundry where the polar headgroups of phospholipids are located. If similar interactions occur in vivo at the tear film lipid-aqueous interface, they would reduce the rate of evaporation.
Temperature induced structural changes may contribute to the enhanced delivery of meibum observed during therapeutic warming of the eye lid. The results provide a foundation for future studies to assess possible differences with age and sex in tears from normal and dry eye subjects.
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