We investigated the lamellar structure of ternary stratum corneum (SC) lipid model systems based on either the phytosphingosine type ceramide (CER) [NP] or CER[AP], supplemented by cholesterol and stearic acid as representative free fatty acid species. For the CER[NP] based membrane, neutron diffraction measurements revealed the coexistence of two lamellar phases, which markedly differ in their hydration properties. CER[NP] forms an extremely rigid and stable bilayer backbone and is at least partly sequestered in a separate phase which coexists with a second lamellar phase. At increased temperature, a structural re-organization of the lipids was observed. One of the lamellar phases disappeared, while the remaining phase increased its repeat distance by about 1 Å. Such a behaviour has not been described for SC lipid model membranes based on CER[AP] so far. Further, 2H NMR spectroscopic measurements on two SC lipid model systems based on either CER[NP] or CER[AP] in addition to cholesterol and perdeuterated stearic acid revealed a state of high lamellar order present in both samples, emphasizing the importance of the phytosphingosine-type ceramides for the proper formation of stable SC bilayer structures. However, the CER[NP] based ternary model showed a state of higher lamellar order than the CER[AP] based system. Our results demonstrate that slight changes in the ceramides' head groups (CER[NP] with 3 hydroxyl groups vs. CER[AP] with 4 hydroxyl groups) have a dramatic influence on the morphology of the lipid structures formed by these lipids.
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