No abstract

Ceramides (Cer) are essential components of the skin permeability barrier. To probe the role of Cer polar head groups involved in the interfacial hydrogen bonding, the N-lignoceroyl sphingosine polar head was modified by removing the hydroxyls in C-1 (1-deoxy-Cer) or C-3 positions (3-deoxy-Cer) and by Nmethylation of amide group (N-Me-Cer). Multilamellar skin lipid models were prepared as equimolar mixtures of Cer, lignoceric acid and cholesterol, with 5 wt% cholesteryl sulfate. In the 1-deoxy-Cerbased models, the lipid species were separated into highly ordered domains (as found by X-ray diffraction and infrared spectroscopy) resulting in similar water loss but 4-5-fold higher permeability to model substances compared to control with natural Cer. In contrast, 3-deoxy-Cer did not change lipid chain order but promoted the formation of a well-organized structure with a 10.8 nm repeat period. Yet both lipid models comprising deoxy-Cer had similar permeabilities to all markers. N-Methylation of Cer decreased lipid chain order, led to phase separation, and improved cholesterol miscibility in the lipid membranes, resulting in 3-fold increased water loss and 10-fold increased permeability to model compounds compared to control. Thus, the C-1 and C-3 hydroxyls and amide group, which are common to all Cer subclasses, considerably affect lipid miscibility and chain order, formation of periodical nanostructures, and permeability of the skin barrier lipid models. Mammalian skin protects the body from external threats, such as chemicals and ultraviolet light, and prevents excessive water loss. The major skin permeability barrier is located in the outermost layer of the epidermis, the stratum corneum (SC) 1-3. The SC consists of corneocytes, epidermal cells in a terminal phase of differentiation, and extracellular lipid matrix, which is a multilamellar assembly of ceramides (Cer), free fatty acids and cholesterol (Chol) in an approximately 1:1:1 molar ratio, and minor components, such as cholesteryl sulfate (CholS) 1-3. Cer, i.e., N-acyl sphingosines, are simple sphingolipids that have from two to four hydroxyl groups and monosubstituted amide group, which behave as hydrogen bond donors and acceptors and affect Cer interactions with proteins and other lipids. For example, 1-deoxy-Cer do not mix well with other lipids 4 , removal of C-3 hydroxyl in Cer prevents the formation of water-extended cooperative H-bond network of Cer 5 , and Cer amide group appears to be fundamental in creating the signal-transducing membrane platforms 6. In the skin barrier, additional hydroxyls in Cer at C-4 (phytosphingosine Cer) or C-6 positions (6-hydroxyCer) modulate the lipid miscibility, lamellar arrangement and permeability of model SC lipid mixtures 7-10 , along with the correct acyl chain length 11,12 or sphingosine chain length of Cer 13,14. Topical supplementation of skin lipids in diseases such as atopic dermatitis or ichthyoses is an established therapeutic approach. Such Cer-dominant lipids are safe, can actually prevent inflammation in...