13C and 1H NMR ester region resonance assignments and the composition of human infant and child meibum

Recent NMR studies suggest that unsaturation may contribute to tear film instability in adults and loss of cholesteryl esters and squalene could reduce tear film stability in adults with meibomian gland dysfunction. The proton resonances were tentatively assigned in those studies. In this current investigation, meibum from seven infants and children, one adult and a pool of adult meibum have been analyzed using an NMR spectrometer with greater sensitivity and spectral resolution. The goals of this work are to confirm/correct the previous assignments and to determine possible age-related changes in composition. The initial resonance assignments were confirmed using heteronuclear single quantum correlation spectroscopy. Because there were no significant interferences in the spectral region corresponding to the resonances for cholesteryl and wax esters, the areas of these resonances were used to calculate their molar ratios. We calculated a wax ester: cholesteryl ester molar ratio of 1:0.57 +/- 0.05 for all our meibum samples and there were no age-related differences. At lower film thicknesses, the rate of evaporation measured in vitro was lower for wax esters mixed with a long chain cholesteryl ester compared to wax esters alone. However, the film thicknesses tested were non-physiological. Longer chain cholesteryl esters increase the interactions between hydrocarbon chains. Hydrocarbon chains were more saturated in meibum from infants and children compared to adults. Unsaturation may contribute to tear film instability in adults. Loss of cholesteryl ester and squalene could destabilize tear film in adults with meibomian gland dysfunction. (C) 2013 Elsevier Ltd. All rights reserved.