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The regulation of water loss through the skin is a poorly understood but crucial process in maintaining terrestrial life-forms. In mammalian skin, the outermost layer, called the stratum corneum (SC), is rate-limiting to water loss. We have evaluated temperature-dependent changes in water vapor permeability and infrared spectra of porcine SC. In particular, we have analyzed the infrared absorption peaks due to the extracellular lipids of the SC. These results show a remarkable correlation between water permeability and the frequency of the C-H stretching vibrations over a broad range of temperature. Since the spectral changes reflect an increased number of alkyl gauche conformers, these results suggest that water permeability is dependent upon the hydrocarbon-chain disorder of SC lipids.While all life-forms depend on membrane-like structures for their existence, nowhere is that more dramatically demonstrated than in the skin. This is particularly true in higher mammals, where this complex membrane has evolved into a multifunctional organ. In terrestrial animals, one ofthe skin's most vital functions is to regulate the amount of water lost to the environment. This crucial regulatory function is achieved by a remarkable structure known as the stratum corneum (SC), the thin, outermost layer of the skin. The SC is composed of alternating layers of flat, protein-rich cells surrounded by an extracellular lipid matrix (1, 2) in an array similar to "bricks and mortar" (1). The extracellular lipids form an extended multilamellar domain and provide the only continuous phase from top to bottom of this tissue. Biophysical evidence from our laboratory suggests that SC lipid domains are the primary barrier to water loss (3) and to penetration of small compounds into the skin (4). In addition, removal of lipids from the SC by solvent extraction leads to a 1000-fold increase in water permeability (5). Thus, the role of SC lipids in regulating water loss is well established.The mechanism by which water passes through the SC, or any other lamellar lipid phase, is not well characterized. A number of investigators have suggested that water permeates through lipid lamellae via free-volume voids created due to random fluctuations in alkyl chain packing (6,7). In this investigation, we have compared temperature-induced changes in SC water vapor permeability with lipid conformational changes determined by Fourier-transform infrared (IR) spectroscopy. The findings show a strong correlation between water permeability and SC lipid alkyl-chain disorder and, thus, support the free-volume hypotheses.MATERIALS AND METHODS Permeability Measurements. Water vapor transport was measured by adapting the technique described by Blank et al. (8). These measurements were made by placing a sheet of isolated SC between two halves of a vapor permeation cell.Two milliliters of a saturated aqueous solution of NaCl was introduced into both sides of the permeation cell, and the entire apparatus was immersed in a water bath at the appropriate temperat...

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Oleic acid is known to be a penetration enhancer for polar to moderately polar molecules. A mechanism related to lipid phase separation has been previously proposed by this laboratory to explain the increases in skin transport. In the studies presented here, Fourier transform infrared spectroscopy (FT-IR) was utilized to investigate whether or not oleic acid exists in a separate phase within stratum corneum (SC) lipids. Per-deuterated oleic acid was employed allowing the conformational phase behavior of the exogenously added fatty acid and the endogenous SC lipids to be monitored independently of each other. The results indicated that oleic acid exerts a significant effect on the SC lipids, lowering the lipid transition temperature (Tm) in addition to increasing the conformational freedom or flexibility of the endogenous lipid alkyl chains above their Tm. At temperatures lower than Tm, however, oleic acid did not significantly change the chain disorder of the SC lipids. Similar results were obtained with lipids isolated from the SC by chloroform:methanol extraction. Oleic acid, itself, was almost fully disordered at temperatures both above and below the endogenous lipid Tm in the intact SC and extracted lipid samples. This finding suggested that oleic acid does exist as a liquid within the SC lipids. The coexistence of fluid oleic acid and ordered SC lipids, at physiological temperatures, is consistent with the previously proposed phase-separation transport mechanism for enhanced diffusion.(ABSTRACT TRUNCATED AT 250 WORDS)