The effect of pH and salt on the molecular structure and dynamics of the skin

The 'acid mantle' of the human skin is considered crucial for several protective functions of the skin. It has been associated with numerous biological processes including enzymatic activities and barrier function for diffusional transport. The outer thin layer of the skin- the stratum corneum (SC) - is primarily responsible for the skin barrier function. In this paper, we investigate how the structure and dynamics of SC lipid and protein components are influenced by variations in pH, ionic strength and salt composition. We characterize molecular mobility and organization in pieces of SC exposed to buffer solutions with pH ranging from acidic to neutral to basic conditions. Using solid state NMR we obtain details on molecular dynamics in intact SC at close to atomistic resolution, providing detailed information on the molecular responses. The highest mobility in both lipid and protein components is observed at the lowest pH 4.0. We also study the effects of adding salt, either with monovalent Na⁺ or divalent Mg²⁺ ions at the different pH conditions. Our results provide new understanding how SC molecular properties respond to solution conditions that can be utilized in the development of topical drug delivery and cosmetic formulations.