Molecular Insights into the Penetration Enhancement Mechanism of Terpenes to Skin.

Terpenes are widely used in cosmetic formulations as chemical penetration enhancers (CPEs). However, the molecular mechanisms underlying the skin-penetration-enhancing effect have not been completely understood. In this work, molecular dynamics (MD) simulations were used to explore the influence of terpineol (TER), 1,8-cineole (CIN), and limonene (LIM) on the stratum corneum (SC) model. The results indicated that terpenes affected lipid membranes in a concentration-dependent manner and promoted skin permeation by disorganizing the cholesterol (CHOL) portion. The penetration of CPEs across the skin membrane also differed, with diffusion rates of limonene > 1,8-cineole > terpineol. The limonene molecules could penetrate the bilayer's center, forming a "triple layer" membrane structure. Furthermore, constrained simulations showed that the most favorable penetration pathway is via areas rich in CHOL. Terpineol could lower the energy barrier of the hydrophilic molecule (caffeic acid) across the cholesterol region. For the lipophilic molecule (osthole), limonene and 1,8-cineole could reduce the energy barrier across the cholesterol region. Each of the results provides novel insights into the mechanism of penetration of CPEs in the skin.