Effects of ethanol/l-menthol on the dynamics and partitioning of spin-labeled lipids in the stratum corneum

The interaction of ethanol as well as ethanol/l-menthol mixtures with the uppermost layer of epidermis, the stratum corneum, was investigated by electron paramagnetic resonance (EPR) spectroscopy utilizing spin-labeled analogs of androstanol (ASL), stearic acid (5-DSA) and methyl stearate (5-DMS). The EPR spectra of these spin probes structured in stratum corneum tissue of neonatal rat are characterized by the coexistence of two spectral components indicating the presence of two classes of spin labels with very different states of mobility. Probably, one class of spin labels is H-bonded to the polar surface of the membrane and another class corresponds to spin labels more deeply inserted in the hydrophobic core. EPR results showed that in the ethanol range 0-70% neither fluidity in stratum corneum membranes nor the relative fractions of these two components changes were observed. Instead, ethanol only caused a selective extraction of spin labels. The removal of the steroid ASL began at 30% ethanol, reaching extraction levels over 50% at ethanol concentrations of 60-70%, whereas the more hydrophobic 5-DMS was partially removed only with 70% ethanol. Addition of 5% l-menthol to the solvent containing 20% ethanol increases both the mobility and the fraction of those spin labels situated in the hydrophobic core (more mobile spectral component). Altogether, these findings suggest that the l-menthol stabilizes mainly in the central region of stratum corneum membranes attracting the membrane lipids and causing hydrogen bond ruptures in the polar membrane interface.