Comparative thermodynamic and spectroscopic properties of water interaction with human stratum corneum

Background/purpose: The water content of skin has a significant impact on skin properties; sufficient hydration is necessary to keep the skin supple, flexible, and smooth. To understand more completely the water retention properties of the human skin barrier, physical macroscopic properties must be related to the structural organization of the stratum corneum (SC). Water, lipids, and natural moisturizing factor (NMF) influence the molecular structures that affect the properties of SC, including water sorption and binding enthalpy. In the research reported here, isothermal microcalorimetry was used to study the interaction of water vapor with isolated human SC in intact, delipidized, and water‐washed delipidized forms to identify the influences of the principal components of SC on water sorption. The calorimetric data are interpreted in conjunction with spectroscopic results to identify the conformational changes in keratins induced by lipid and NMF removal and to assess the influence of these changes on water binding in SC. Methods: Isothermal calorimetry was used to measure the integral heat of water vapor sorption on intact, delipidized, and water‐washed delipidized human SC at 32 °C as a function of relative humidity using back and thigh skin from three donors. Calorimetric measurements were combined with water vapor sorption measurements to determine the differential thermodynamic properties of these systems. Attenuated total reflection–Fourier transform infrared spectroscopy was used to investigate effects of extraction on protein secondary structure. Results: The magnitudes of the differential enthalpy, entropy, and free energy were greatest for intact SC and least for water‐washed delipidized SC. Water sorption followed a similar trend. Delipidization led to a significantly reduced binding enthalpy at low water content; water washing the delipidized SC had only a small additional effect on binding enthalpy. Delipidization converts a fraction of keratin α‐helixes to turns and random coils, while water sorption converts a fraction of keratin α‐helixes to β‐sheets, turns, and random coils. Conclusions: The results of this study are consistent with a water sorption model in which keratin–keratin hydrogen bonds are replaced by keratin–water hydrogen bonds. Delipidization reduces the fraction of dry keratin that is in the α‐helix conformation, suggesting that lipids hold the keratins in a conformation conducive to optimal hydration.