Fourier transform infrared absorption (FTIR) on dry stratum corneum, corneocyte-lipid interfaces: experimental and vibrational spectroscopy calculations.

Many questions concerning the biophysical and physiological properties of skin are still open. Skin aging, permeability, dermal absorption, hydration, and drug transdermal delivery, are few examples of processes with unveiled underlying mechanisms. In this work, it is presented a comparison between Fourier transform infrared absorption (FTIR) of dry stratum corneum and stratum corneum under lipase action supported by first-principles density functional vibrational calculations. The molecular structure of stratum corneum was modeled by an archetype of its hygroscopic proteic portion inside the corneocytes, the natural moisturizing factor, coupled to glycerol molecules which represent the lipid fraction of stratum corneum. Vibrational spectra were calculated and compared to experimental data obtained on the animal model of stratum corneum. The experimental results indicated prominent spectral differences between dry and lipase-treated stratum corneum. Principal components analysis and hyerarchical clustering indicated that 1200, 1650, and 1695 cm-1 bands are the most influential on the discrimination. It is noticed that bands in the fingerprint region (800-1800 cm-1) were correctly assigned. Moreover, the calculations revealed the existence of two coupled vibration between the hydroxyl group of lipid and methylene (1120 and 1160 cm-1), which are of special interest since they probe the lipid-amino acid coupling. The model was also able to predict the shear modulus of dry stratum corneum in excellent agreement with the reported values from the literature. Other physical/chemical properties could be calculated exploring the chemical accuracy and molecular resolution of this model. Research in dermatology, cosmetology, and biomedical engineering in the specific topics of drug delivery and/or mechanical properties of skin are examples of fields that would potentially take advantage of this approach.