Insights about α-tocopherol and Trolox interaction with phosphatidylcholine monolayers under peroxidation conditions through Brewster angle microscopy

Modulation of membrane physical properties by natural insecticidal ketones

The insecticidal activity of Mentha oil and its main components has been tested and established for various insects/pests. Several mint ketones have demonstrated to act on GABAA receptors (GABAA-R), a transmembrane channel target of several important insecticides whose activity can be modulated by surface-active compounds and by changes in the physical properties of the lipid membrane. In the present work, we analyze the capacity of monoterpenic ketones most commonly found in Mentha species, pulegone and menthone, to interact with DPPC membranes by molecular dynamics (MD) simulations and Langmuir monolayers. The experimental results indicate that the presence of menthone and pulegone in the subphase modify the interfacial characteristics of DPPC isotherms. The changes were reflected as expansion of the isotherms and disappearance or bringing forward of DPPC phase transition. MD simulation corroborate these results and indicate that both ketones are located at the region of the carbonyl group, at the interface with the acyl chains. Ketone intercalation between lipid molecules would induce an increasing intermolecular interaction, diminishing the film elasticity and causing an ordering effect. Our results suggest that the insecticidal activity of both ketones could involve their interaction with lipid molecules causing disturbance of the cell membrane as postulated for several larvicide compounds, or at least modulating the receptor surrounding.

Licofelone-DPPC Interactions: Putting Membrane Lipids on the Radar of Drug Development

(1) Background: Membrane lipids have been disregarded in drug development throughout the years. Recently, they gained attention in drug design as targets, but they are still disregarded in the latter stages. Thus, this study aims to highlight the relevance of considering membrane lipids in the preclinical phase of drug development. (2) Methods: The interactions of a drug candidate for clinical use (licofelone) with a membrane model system made of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) were evaluated by combining Langmuir isotherms, Brewster angle microscopy (BAM), polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and grazing-incidence X-ray diffraction (GIXD) measurements. (3) Results: Licofelone caused the expansion of the DPPC isotherm without changing the lipid phase transition profile. Moreover, licofelone induced the reduction of DPPC packing density, while increasing the local order of the DPPC acyl chains. (4) Conclusions: The licofelone-induced alterations in the structural organization of phosphatidylcholine monolayers may be related to its pharmacological actions. Thus, the combination of studying drug-membrane interactions with the pharmacological characterization that occurs in the preclinical stage may gather additional information about the mechanisms of action and toxicity of drug candidates. Ultimately, the addition of this innovative step shall improve the success rate of drug development.

Studies on the Behavior of Eucalyptol and Terpinen-4-ol-Natural Food Additives and Ecological Pesticides-in Model Lipid Membranes

Effective application of the essential oils requires detailed exploration of their mechanism of action and the origin of diverse activity of their components. In this work, the influence of eucalyptol and terpinen-4-ol on artificial membranes was studied to verify whether the differences in the activity of these compounds are related to their effect on membranes. The properties of monolayers formed from structurally different lipids in the presence of terpenes were examined based on the results of the surface pressure-area measurements, penetration studies, and Brewster angle microscopy experiments. Both compounds were able to incorporate into the membrane and alter lipid/lipid interactions, making the monolayer less stable and more fluid. These effects were determined by monolayer composition (but not by its condensation per se) and the resulting rheological properties and were stronger in the presence of terpinen-4-ol. These findings confirm the hypothesis that differences in the antimicrobial potency of these terpenes are membrane-related, and membrane composition may determine their selectivity.

Miscibility of dl-α-tocopherol β-glucoside in DPPC monolayer at air/water and air/solid interfaces

The role of newly synthesized tocopherol glycosidic derivative in modifying molecular organization and phase transitions of phospholipid monolayer at the air/water interface has been investigated. Two-component Langmuir films of dl-α-tocopheryl β-D-glucopyranoside (BG) mixed with dipalmitoyl phosphatidylcholine (DPPC) in the whole range of mole fractions were formed at the water surface. An analysis of surface pressure versus mean molecular area (π-A) isotherms and Brewster angle microscope images showed that the presence of BG molecules changes the structure and packing of the DPPC monolayer in a BG concentration dependent manner. BG molecules incorporated into DPPC monolayer inhibit its liquid expanded to liquid condensed phase transition proportionally to the BG concentration. The monolayers were also transferred onto solid substrates and visualized using an atomic force microscope. The results obtained indicate almost complete miscibility of BG and DPPC in the monolayers at surface pressures present in the biological cell membrane (30-35·10(-3) N·m(-1)) for a BG mole fraction as high as 0.3. This makes the monolayer less packed and more disordered, leading to an increased permeability. The results support our previous molecular dynamics simulation data.

Antagonistic effects of α-tocopherol and ursolic acid on model bacterial membranes

α-tocopherol (Toc), the most active component of vitamin E can exert antagonistic effects disabling the therapy of cancers and bacterial infections. Such antagonisms were observed also between Toc and bioactive pentacyclic triterpenes (PT) exhibiting anticancer and antibacterial properties. Both Toc and PT are water-insoluble membrane active substances. Thus, our idea was to emulate their interactions with model Escherichia coli membranes. E. coli inner membranes were selected for the experiments because their lipid composition is quite simple and well characterized and the two main components are phosphatidylethanolamine and phosphatidylglycerol. As a model of E. coli membranes we applied Langmuir monolayers formed by the E. coli total extract of polar lipids (Etotal) as well as by the main lipid components: phosphatidylethanolamine (POPE) and phosphatidylglycerol (ECPG). The antagonistic effects of ursolic acid (Urs) and Toc were investigated with the application of ternary Langmuir monolayers formed by Urs, Toc and one of the phospholipids POPE or ECPG. Our studies indicated that the affinities of Urs and Toc towards the POPE molecule are comparable; whereas there are profound differences in the interactions of Urs and Toc with ECPG. Thus, the model experiments prove that in the case of E. coli membrane, the differences in the interactions between Urs and Toc with the anionic bacterial phosphatidylglycerol can be the key factor responsible for the antagonistic effects observed between PT and Toc in vivo.

Physicochemical aspects of reaction of ozone with galactolipid and galactolipid-tocopherol layers

The impact of reaction of galactolipids with ozone on the physicochemical properties of their monolayers was examined. In Megli and Russo (Biochim Biophys Acta, 1778:143–152, 2008), Cwiklik and Jungwirth (Chem Phys Lett, 486:99–103, 2010), Jurkiewicz et al. (Biochim Biophys Acta, 1818:2388–2402, 2012), Khabiri et al. (Chem Phys Lett, 519:93–99, 2012), and Conte et al. (Biochim Biophys Acta, 1828:510–517, 2013), the properties of layers formed from model mixtures composed of chosen lipids and selected oxidation products were studied, whereas in this work, question was raised as to how the oxidation reactions taking place in situ affect the physical properties of the galactolipid layers. So, set experiment should take into account the effect of all reaction products. The mechanical characteristics of monolayers of monogalactosyldiacyl-glycerol (MGDG) and digalactosyldiacylglycerol (DGDG) were determined by Langmuir trough technique, and the electrical properties of liposomes formed from these lipids by measuring their electrophoretic mobility. Considerable loss of galactolipid molecules forming monolayers was found at ozone concentrations (in aqueous medium) higher than 0.1 ppm with a stronger effect measured for MGDG. That goes along with the greater amounts of MDA found in the extracts of oxidized MGDG films compared with DGDG. Based on this, it was concluded that an additional galactose group present in DGDG molecules acts protectively under oxidative conditions. The surface tension of the solutions (of small volume) contacting the oxidized galactolipids films was significantly reduced, indicating the presence of soluble in polar media, surface active reaction products. The presence of α-tocopherol in mixtures with tested galactolipids at a molar ratio of lipid to tocopherol equal to 1.7:1 caused some inhibition of lipid oxidation, reducing the decrease of amount of lipid particles forming the monolayer. Here, also protective effect of α-tocopherol was greater for the MGDG compared to DGDG.

Lipid Peroxidation in Membranes: The Peroxyl Radical Does Not "Float"

Lipid peroxidation is a fundamental phenomenon in biology and medicine involved in a wide range of diseases. Some key microscopic aspects of this reaction in cell membranes are still poorly studied. In particular, it is commonly accepted that the propagation of the radical reaction in lipid bilayers is hampered by the rapid diffusion of peroxyl intermediates toward the water interface, that is, out of the reaction region. We investigated the validity of this "floating peroxyl radical" hypothesis by means of molecular modeling. Combining quantum calculations of model systems and atomistic simulations of lipid bilayers containing lipid oxidation products, we show that the peroxyl radical does not "float" at the surface of the membrane. Instead, it remains located quite deep inside the bilayer. In light of our findings, several critical aspects of biological membranes' peroxidation, such as their protection mechanisms, need to be revisited. Our data moreover help in the design of more efficient antioxidants, localized within reach of the reaction propagating radical.