Abstract
The secondary and tertiary structure of keratin and natural moisturizing factor (NMF) are of great importance regarding the water regulating functions in the stratum corneum (SC). In this in vivo study, the depth-dependent keratin conformation and its relationship to the hydrogen bonding states of water and its content in the SC, are investigated using confocal Raman microscopy. Based on the obtained depth-profiles for the β-sheet/α-helix ratio, the stability of disulphide bonds, the amount of cysteine forming disulphide bonds, the buried/exposed tyrosine and the folding/unfolding states of keratin, a “three layer model” of the SC, regarding the keratin-water-NMF interaction is proposed. At the uppermost layers (30–0% SC depth), the keratin filaments are highly folded, entailing limited water binding sites, and NMF is mostly responsible for binding water. At the intermediate layers (70–30% SC depth), the keratin filaments are unfolded, have the most water binding sites and are prone to swelling. At the bottom layers (100–80% SC depth), the water binding sites are already occupied with water and cannot swell substantially. The hydrogen bonding states of water molecules can only be explained by considering both, the molecular structure of keratin and the contribution of NMF as a holistic system.
Highlights
Keratin is one of the abundant proteins found in the mammalian epidermis
Since most of the water is present in the corneocytes, rather than in the lamellas of ICL21, and the corneocytes largely consist of keratin filaments and hygroscopic natural moisturizing factor (NMF) (Fig. 1a), the secondary and tertiary structures of keratin are important for the water regulation in the stratum corneum (SC)
Depth-dependent changes of the secondary structure of keratin determined by the β-sheet/αhelix ratio
Summary
Corneocytes of the stratum corneum (SC), the horny cells of human epidermis, are continuously proliferating from the stratum granulosum (SG) towards the skin surface[1,2], contain a lot of the fibrous keratin[3] which is embedded in a water-lipid matrix[4], and is almost homogenously distributed throughout the SC of healthy skin[5]. The distribution of bound/unbound water in the SC and the trans-epidermal water loss have been important issues in dermatology and cosmetology[15,17,18,19,20] Proteins such as keratin are described on different levels of biomolecular structure. Since most of the water is present in the corneocytes, rather than in the lamellas of ICL21, and the corneocytes largely consist of keratin filaments and hygroscopic natural moisturizing factor (NMF) (Fig. 1a), the secondary and tertiary structures of keratin are important for the water regulation in the SC. Confocal Raman microscopy (CRM) has been introduced for non-invasive determination of the chemicophysical properties of the SC in vivo and ex vivo, with sufficient spatial resolution[25,26]
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