The impact of ceramides NP and AP on the nanostructure of stratum corneum lipid bilayer. Part I: neutron diffraction and 2H NMR studies on multilamellar models based on ceramides with symmetric alkyl chain length distribution

Abstract

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.