Interactions of oleic acid and model stratum corneum membranes as seen by 2H NMR

We have investigated the mechanism through which the penetration enhancer oleic acid acts on stratum corneum (SC) model membranes (bovine brain ceramide:cholesterol:palmitic acid, 1:1:1 molar ratio). We used solid state deuterium nuclear magnetic resonance to monitor such multilamellar SC dispersions containing either cholesterol-d6, palmitic acid-d31, or oleic acid-d2 as a function of both fatty acid concentration (2:2:1:1 and 1:1:1:1 bovine brain ceramide:cholesterol:palmitic acid:oleic acid) and temperature (18–75 °C). Our results show that below 40 °C, oleic acid (OA) is in an ‘isotropic’ phase, indicating that it has not incorporated into the lamellar membrane phase. At and above the SC model membrane's crystalline to liquid crystalline melting temperature, Tm = 40–42 °C, OA interacts with lamellar SC membranes with a slight dependence on OA concentration. Tm does not change upon the exposure of the SC model membrane to OA, nor do we see any significant change in membrane chain disorder as monitored by the labelled PA. However, the spectra of both the palmitic acid (PA) and cholesterol SC model membrane components contain an isotropic peak that grows with increasing temperature. Our results thus indicate that oleic acid extracts a fraction of the endogenous SC membrane components, promoting phase separation in the SC membrane system. Reducing the proportion of crystalline lipids and creating more permeable OA-rich domains is a plausible mechanism that explains how OA enhances transdermal penetration.