Natural terpenes II. Concentration-dependent profile of effects on dynamic organization of biological and model...

Monoterpenes (MTs), the major constituents of plant essential oils, cover a broad spectrum of biological activities through their interaction with biomembranes. MTs are highly hydrophobic substances with a net electrical dipole, but are not clearly amphipathic. As a result, they aggregate at increasing concentrations in aqueous media, and in membrane environments their behavior changes from dynamics modulators to disruptors. In the present work we have tried to find the boundaries between these conditions. Using spin-label electron paramagnetic resonance (EPR) spectroscopy, we studied the effect of a wide range of concentrations of camphor, cineole, thymol, menthol and geraniol on the dynamics of phospholipid model membranes, with or without cholesterol, organized in different initial lyotropic phases, as well as on synaptosomal membranes from bovine brain cortex. EPR spectra of two fatty acid spin probes incorporated into the membranes (5-SASL and 12-SASL) provided information on molecular order and mobility at different bilayer depths. Qualitative changes in the spectra indicated transitions between monomeric and aggregated MTs, as well as membranes with different degrees of order and disrupted membranes. Analyzed in conjunction with the kinetics of [³H]flunitrazepam membrane binding, either non-specifically to the membrane or specifically to the GABA_A receptor, the present results allowed us to define 200 μM as the concentration limit to safely evaluate MT effects in non-disrupted biological and model membranes. These findings may be helpful in the interpretation and design of pharmacological assays and provide concentration references that allow correlating a variety of biophysical and pharmacological data in the literature.