Dynamics and partitioning of spin-labeled stearates into the lipid domain of stratum corneum

The EPR spectra of the positional isomers n-doxyl stearic acids (n-DSA), with n = 5, 12 and 16, and 5-doxyl methyl stearate (5-DMS) structured in the lipid domain of intact stratum corneum (SC), are characterized by the thermodynamic equilibrium of two distinct spectral components provided by two different motional states of the spin-labeled chains. A two-component model used in the EPR spectra simulations provided the relative populations of the components, allowing for the calculation of the thermodynamic profile. Based on a detailed investigation, the more motionally restricted population of spin labels (component 1) is found to arise when the spin label is hydrogen-bonded to the polar surfaces of the membranes, while the less motionally restricted population (component 2) is generated by spin labels nonhydrogen-bonded and more deeply inserted in the hydrophobic core. The 5-DSA is bound tightly to the polar surfaces (ΔGo2 → 1 = − 1.75 kcal/mol and ΔHo2 → 1 = − 13.8 kcal/mol), whereas the more lipophilic 5-DMS has a major spin population stabilized in the hydrophobic core (ΔGo2 → 1 = − 0.57 kcal/mol and ΔHo2 → 1 = − 9.1 kcal/mol). Upon lipid-depleting SC increases the interactions of the probe with the polar surfaces, thereby decreasing its rotational diffusion. In contrast, the treatment of SC with oleic acid, a permeation enhancer, drastically increases the mobility of the spin labels, particularly that of component 1, and the thermodynamic equilibrium shifts towards the formation of component 2. A mechanism for water permeation in SC is also proposed.