pH-induced modifications to stratum corneum lipids investigated using thermal, spectroscopic, and chromatographic techniques.
The effects of nonphysiological pH on stratum corneum lipid content and structure have been studied. Human stratum corneum samples were soaked in solutions at pH 1, 2, 6, 11, or 12 for up to 24 h. After removal of the stratum corneum, the buffer solutions were analyzed for lipid composition using thin-layer chromatography analysis and the stratum corneum sheets were examined using differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy. The results demonstrate that only buffers of pH 11 or higher affect the stratum corneum lipids. No large difference in the contents of ceramides and cholesterol extracted by buffers of varying pH was observed. In contrast, free fatty acid extraction was pH dependent; amounts removed by 24-h treatment with pH 11 or 12 buffers were comparable, and were similar to amounts extracted with a methanol-chloroform mixture for 15 min. No appreciable changes in DSC and FTIR spectra were detected between untreated stratum corneum and stratum corneum samples treated with buffers at pHs in the range 1-6. For tissue treated with pH 11 and 12, the position of the endothermal melting peak T2 shifted from 72 to 74 degrees C on the DSC thermograms. Small changes in the broadness of spectral peaks at 2855 cm(-1) [attributable to upsilon(CH(2)) stretching of stratum corneum lipids and 1655 cm(-1) upsilon(C=O) stretching amide I band] can be seen in the FTIR spectra from the treated stratum corneum samples, although no shifts in peak positions were observed. Intensity changes in peaks from extraneous lipids [upsilon(C=O) stretching mode at 1735 cm(-1)] were observed after buffer treatments. The changes provoked by the alkaline buffers are not dramatic and it may be concluded that the stratum corneum appears remarkably resilient to extended exposure in both highly acidic (pH 1) and highly alkaline (pH 12) environments.