Lipid membranes play a crucial role in regulating the body’s water balance by adjusting their properties in response to hydration. The intercellular lipid matrix of the stratum corneum (SC), the outermost skin layer, serves as the body’s primary defense against environmental factors. Osmolytes, including urocanic acid (UCA) and glycerol, are key components of the natural moisturizing factor that help the SC resist osmotic stress from dry environments. This study examines the effects of UCA and glycerol (each at 5 mol %) on isolated human SC lipids. For this, different techniques were employed, offering complementary information of the system’s multiscale characteristics, including humidity-scanning quartz crystal microbalance with dissipation monitoring, infrared spectroscopy, x-ray diffraction, electrical impedance spectroscopy, and studies of water loss and permeability. Our results show that UCA increases water sorption and makes lipid films more liquid-like at high relative humidity, without significantly altering the lipid lamellar structure, chain order, or orthorhombic chain packing. Lipid films containing UCA exhibited higher water loss and significantly higher model drug permeability compared to lipid films without UCA. Further, incorporation of UCA resulted in kinetically faster changes in electrical properties upon contact with aqueous solution compared with control lipids. These observations suggest that UCA reduces lipid cohesion in regions other than the acyl chain-rich leaflets, which may impact SC desquamation. In contrast, glycerol did not influence the hydration or permeability of the SC lipid matrix. However, it increased the proportion of orthorhombic domains at high humidities and slowed the kinetics of the hydration process, as evidenced by slower changes in the dielectric properties of the lipid film. These findings suggest that glycerol enhances lipid cohesion rather than increasing water uptake, which is typically the expected function of humectants. Consequently, UCA and glycerol appear to have distinct roles in maintaining epidermal homeostasis.

中文翻译：

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皮肤屏障脂质的结构和功能：体外补水和天然保湿剂的作用


脂质膜通过调整其特性以响应水合作用，在调节身体的水分平衡方面起着至关重要的作用。角质层 （SC） 的细胞间脂质基质是最外层的皮肤层，是身体抵御环境因素的主要防御系统。渗透剂，包括尿酸 （UCA） 和甘油，是天然保湿因子的关键成分，可帮助 SC 抵抗干燥环境的渗透压。本研究检查了 UCA 和甘油（各为 5 mol%）对分离的人 SC 脂质的影响。为此，采用了不同的技术，提供了系统多尺度特性的互补信息，包括具有耗散监测功能的湿度扫描石英晶体微天平、红外光谱、X 射线衍射、电阻抗光谱以及水损失和渗透性的研究。我们的结果表明，UCA 增加了水吸附，使脂质膜在高相对湿度下更像液体，而不会显着改变脂质层状结构、链序或正交链堆积。与不含 UCA 的脂质膜相比，含有 UCA 的脂质膜表现出更高的水分损失和显著更高的模型药物渗透性。此外，与对照脂质相比，UCA 的掺入导致与水溶液接触时电性能的动力学变化更快。这些观察结果表明，UCA 会降低富含酰基链的小叶以外的区域的脂质凝聚力，这可能会影响 SC 脱屑。相比之下，甘油不影响 SC 脂质基质的水合作用或通透性。 然而，它在高湿度下增加了斜方晶域的比例并减慢了水合过程的动力学，脂质膜介电特性的较慢变化证明了这一点。这些发现表明，甘油增强了脂质凝聚力，而不是增加水分吸收，这通常是保湿剂的预期功能。因此，UCA 和甘油似乎在维持表皮稳态方面具有不同的作用。