Mechanism of Action of Thymol on Cell Membranes Investigated through Lipid Langmuir Monolayers at the Air-Water Interface and Molecular Simulation.  A major challenge in the design of biocidal drugs is to identify compounds with potential action on microorganisms and to understand at the molecular level their mechanism of action. In this study, thymol, a monoterpenoid found in the oil of leaves of Lippia sidoides with possible action in biological surfaces, was incorporated in lipid monolayers at the air-water interface that represented cell membrane models. The interaction of thymol with dipalmitoylphosphatidylcholine (DPPC) at the air-water interface was investigated by means of surface pressure-area isotherms, Brewster angle microscopy (BAM), polarization-modulation reflection-absorption spectroscopy (PM-IRRAS), and molecular dynamics simulation. Thymol expands DPPC monolayers, decreases their surface elasticity, and changes the morphology of the lipid monolayer, which evidence the incorporation of this compound in the lipid Langmuir film. Such incorporation could be corroborated by PM-IRRAS since some specific bands for DPPC were changed upon thymol incorporation. Furthermore, potential of mean force obtained by molecular dynamics simulations indicates that the most stable position of the drug along the lipid film is near the hydrophobic regions of DPPC. These results may be useful to understand the interaction between thymol and cell membranes during biochemical phenomena, which may be associated with its pharmaceutical properties at the molecular level.