Skin hydration plays an important role in the optimal physical properties and physiological functions of the skin. Despite the advancements in the last decade, dry skin remains the most common characteristic of human skin disorders. Thus, it is important to understand the effect of hydration on Stratum Corneum (SC) components. In this respect, our interest consists in correlating the variations of unbound and bound water content in the SC with structural and organizational changes in lipids and proteins using a non-invasive technique: Raman spectroscopy. Raman spectra were acquired on human SC at different relative humidity (RH) levels (4-75%). The content of different types of water, bound and free, was measured using the second derivative and curve fitting of the Raman bands in the range of 3100-3700 cm(-1). Changes in lipidic order were evaluated using νC-C and νC-H. To analyze the effect of RH on the protein structure, we examined in the Amide I region, the Fermi doublet of tyrosine, and the νasymCH3 vibration. The contributions of totally bound water were found not to vary with humidity, while partially bound water varied with three different rates. Unbound water increased greatly when all sites for bound water were saturated. Lipid organization as well as protein deployment was found to be optimal at intermediate RH values (around 60%), which correspond to the maximum of SC water binding capacity. This analysis highlights the relationship between bound water, the SC barrier state and the protein structure and elucidates the optimal conditions. Moreover, our results showed that increased content of unbound water in the SC induces disorder in the structures of lipids and proteins.

Proper hydration of the stratum corneum (SC) is important for maintaining skin's vital functions. Water loss causes development of drying stresses, which can be perceived as 'tightness', and plays an important role in dry skin damage processes. However, molecular structure modifications arising from water loss and the subsequent development of stress has not been established. We investigated the drying stress mechanism by studying, ex vivo, the behaviors of the SC components during water desorption from initially fully hydrated samples using Raman spectroscopy. Simultaneously, we measure the SC mechanical stress with a substrate curvature instrument. Very good correlations of water loss to the mechanical stress of the stratum corneum were obtained, and the latter was found to depend mainly on the unbound water fraction. In addition to that, the water loss is accompanied with an increase of lipids matrix compactness characterized by lower chain freedom, while protein structure showed an increase in amount of α-helices, a decline in α-sheets, and an increase in folding in the tertiary structure of keratin. The drying process of SC involves a complex interplay of water binding, molecular modifications, and mechanical stress. This article provides a better understanding of the molecular mechanism associated to SC mechanics.

Atopic dermatitis (AD) is the most common skin inflammatory disease, affecting up to 3% of adults and 20% of children. Skin barrier impairment is thought to be the primary factor in this disease. Currently, there is no method proposed to monitor non-invasively the different molecular disorders involved in the upper layer of AD skin. Raman microspectroscopy has proved to be a powerful tool to characterize some AD molecular descriptors such as lipid content, global hydration level, filaggrin and its derivatives. Our investigations aimed to extend the use of in vivo Raman microspectroscopy as a rapid and non-invasive diagnostic technique for lipid conformation and organization, protein secondary structure and bound water content analysis in atopic skin. Our approach was based on the analysis of Raman data collected on the stratum corneum (SC) of 11 healthy and 10 mild-to-moderate atopic patients. Atopic skin revealed a modification of lipid organization and conformation in addition to the decrease of the lipid-to-protein ratio. This study also highlighted a reduction of the bound water and an increase in protein organized secondary structure in atopic skin. All these descriptors worsen the barrier function, state and appearance of the skin in AD. This precise and relevant information will allow an in vivo follow-up of the pathology and a better evaluation of the pharmacological activity of therapeutic molecules for the treatment of AD.