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Abstract
The stratum corneum (SC) is the outermost layer of skin and provides a barrier to the outside environment. Critical components of the SC are lipid molecules that prevent water permeation while sometimes binding with water molecules to sequester them in the SC to hydrate the skin. The exact composition of lipids in the SC varies with evolutionary history and environment. Recently, lipids in the SC of the American Alligator (Alligator mississippiensis) were found to share many similarities in composition with those of birds, reflecting their shared ancestry as Archosaurs. Among the most notable similarities was the presence of a lipid called a cerebroside, a polar lipid that forms strong hydrogen bonds with water molecules in birds. However, it is not known whether cerebrosides interact with water similarly in alligator SC. In this study, we investigate the effects of hydration on lipids in alligator SC by exposing pieces of SC (n=9) to 0%, 75%, and 100% relative humidity (RH) at 33oC for 24 hours. We then used infrared spectroscopy to analyze the relative strength of hydrogen bonding and the prevalence of gauche defects in lipid chains. We found that the strength of hydrogen bonding in the SC decreased only after exposure to 100% RH, indicating that cerebrosides in alligator skin form strong hydrogen bonds with water even at moderate levels of hydration, whereas birds exhibit weaker hydrogen bonding at moderate levels. Additionally, the number of gauche defects in lipid chains did not change in response to hydration. Taken together, our results indicate that cerebrosides sequester water molecules outside lipid layers in a manner similar to their role in bird SC, and the greater abundance of cerebrosides we observe in alligator SC compared with many birds may enhance the role these lipids play in hydrating the skin.