The insecticide fipronil affects the physical properties of model membranes: A combined experimental and molecular dynamics simulations study in Langmuir monolayers

Study and voltammetric determination of fipronil in bovine lactose-free milk by differential pulse voltammetry using a carbon paste electrode

A novel voltammetric screening method has been developed for the rapid determination of fipronil (FPN) residues in lactose-free milk samples with the use of a carbon-paste electrode (CPE) by differential-pulse voltammetry (DPV). Cyclic voltammetry indicated the occurrence of an irreversible anodic process at approximately +0.700 V (vs. Ag|AgCl, 3.0 mol L^-1 KCl) in a 0.100 mol L^-1 NaOH supporting electrolyte prepared as a 30% (v/v) ethanol-water solution. Quantification of FPN was carried out by DPV and analytical curves were constructed. In the absence of a matrix, the limits of detection (LOD) and quantification (LOQ) were 0.568 mg L^-1 and 1.89 mg L^-1, respectively. In the presence of a lactose-free skim milk matrix, the values of LOD and LOQ were 0.331 mg L^-1 and 1.10 mg L^-1. The recovery percentages for three different concentrations of FPN in lactose-free skim milk samples ranged between 95.3% and 109%. All assays could be conducted with milk samples without any prior extraction steps or pre-concentration of FPN, making this novel method rapid, simple, and relatively cheap.

Regulation of calcium ions on the interaction between amphotericin B and cholesterol-rich phospholipid monolayer in LE phase and LC phase

Amphotericin B, as a "gold standard", is used to treat invasive fungal infections. The AmB molecule can bind easily to cholesterol and damage cell membranes, so it produces the toxicity on cell membrane, which limits its clinical dose. However, the interaction between AmB and cholesterol-rich membrane is unclear now. The phase state of the membrane and the metal cation outside cell membrane may affect the interaction between AmB and the membrane. In this work, the effects of amphotericin B on the mean molecular area, elastic modulus and stability of mammalian cell membrane rich in cholesterol in the presence of Ca²⁺ ions were studied using DPPC/Chol mixed Langmuir monolayer as a model. The Langmuir-Blodgett method and AFM test were used to study the effects of this drug on the morphology and height of cholesterol-rich phospholipid membrane in the presence of Ca²⁺ ions. The influence of calcium ions on the mean molecular area and the limiting molecular area was similar in LE phase and in LC phase. The calcium ions made the monolayer more condensed. However, calcium ions can weaken the shortening effect of AmB on the relaxation time of the DPPC/Chol mixed monolayer in LE phase but enhance it in LC phase. Interestingly, calcium ions caused a LE-LC coexistence phase to occur in the DPPC/Chol/AmB mixed monolayers at 35mN/m, which was confirmed by atomic force microscopy. The results can help to understand the interaction between amphotericin B and cell membrane rich in cholesterol in the calcium ions environment.

Theoretical and Experimental Study of Molecular Interactions of Fluralaner with Lipid Membranes

Fluralaner is a relatively new insecticide belonging to the isoxazoline group, whose action mechanism involves the blocking of GABA_A-receptors in the insect nervous system. Because of its high hydrophobicity, fluralaner could bioaccumulate and reach toxic local concentrations. Since there are no data available about the penetration and persistence of isoxazolines in biological membranes, we intend to evaluate fluralaner permanence as a pollutant by using model membranes. We used experimental and in silico models to characterize the incorporation of fluralaner into the lipid phase at different packing states. We determined its impact in the membrane structure and organization. Our results confirm that fluralaner is capable of penetrating, holding, and accumulating in the lipid membrane and provide details on its precise location and orientation. These properties would allow fluralaner to reach high local concentrations in different membranes and organs, which could be dangerous for vertebrate organisms if its handling is not properly controlled.

A review of physiological resistance to insecticide stress in Nilaparvata lugens

Insecticides are widely used in agriculture as effective means to control pests. However, pests have not been completely mitigated with the increased use of insecticides. Instead, many side effects have arisen, especially the '3Rs' (resistance, resurgence, and residue). The brown planthopper, Nilaparvata lugens, is one of the most threatening rice pests. The main insecticides for controlling N. lugens belong to organochlorine, organophosphorus, carbamate, neonicotinoid and pyrethroid groups. However, metabolic enzymes, including cytochrome P450s, esterases, glutathione-S-transferases, and ATP-binding cassette transporters, effectively promote the detoxification of insecticides. Besides, mutations of neurological target sites, such as acetylcholinesterase, nicotinic acetylcholine, γ-aminobutyric acid receptor, and ryanodine receptor, result in insensitivity to insecticides. Here, we review the physiological metabolic resistance in N. lugens under insecticide stress to provide a theoretical basis for identifying and developing more effective and harmless insecticides.

The Past and the Future of Langmuir and Langmuir-Blodgett Films

The Langmuir-Blodgett (LB) technique, through which monolayers are transferred from the air/water interface onto a solid substrate, was the first method to allow for the controlled assembly of organic molecules. With its almost 100 year history, it has been the inspiration for most methods to functionalize surfaces and produce nanocoatings, in addition to serving to explore concepts in molecular electronics and nanoarchitectonics. This paper provides an overview of the history of Langmuir monolayers and LB films, including the potential use in devices and a discussion on why LB films are seldom considered for practical applications today. Emphasis is then given to two areas where these films offer unique opportunities, namely, in mimicking cell membrane models and exploiting nanoarchitectonics concepts to produce sensors, investigate molecular recognitions, and assemble molecular machines. The most promising topics for the short- and long-term prospects of the LB technique are also highlighted.

What can we learn about amphiphile-membrane interaction from model lipid membranes?

Surface-active amphiphiles find applications in a wide range of areas of industry such as agrochemicals, personal care, and pharmaceuticals. In many of these applications, interaction with cell membranes is a key factor for achieving their purpose. How do amphiphiles interact with lipid membranes? What are their bases for membrane specificity? Which biophysical properties of membranes are susceptible to modulation by amphiphilic membrane-effectors? What aspects of this interaction are important for performing their function? In our work on membrane biophysics over the years, questions like these have arisen and we now share some of our findings and discuss them in this review. This topic was approached focusing on the membrane properties and their alterations rather than on the amphiphile structure requirements for their interaction. Here, we do not aim to provide a comprehensive list of the modes of action of amphiphiles of biological interest but to help in understanding them.

Insect RDL Receptor Models for Virtual Screening: Impact of the Template Conformational State in Pentameric Ligand-Gated Ion Channels

The RDL receptor is one of the most relevant protein targets for insecticide molecules. It belongs to the pentameric ligand-gated ion channel (pLGIC) family. Given that the experimental structures of pLGICs are difficult to obtain, homology modeling has been extensively used for these proteins, particularly for the RDL receptor. However, no detailed assessments of the usefulness of homology models for virtual screening (VS) have been carried out for pLGICs. The aim of this study was to evaluate which are the determinant factors for a good VS performance using RDL homology models, specially analyzing the impact of the template conformational state. Fifteen RDL homology models were obtained based on different pLGIC templates representing the closed, open, and desensitized states. A retrospective VS process was performed on each model, and their performance in the prioritization of active ligands was assessed. In addition, the three best-performing models among each of the conformations were subjected to molecular dynamics simulations (MDS) in complex with a representative active ligand. The models showed variations in their VS performance parameters that were related to the structural properties of the binding site. VS performance tended to improve in more constricted binding cavities. The best performance was obtained with a model based on a template in the closed conformation. MDS confirmed that the closed model was the one that best represented the interactions with an active ligand. These results imply that different templates should be evaluated and the structural variations between their channel conformational states should be specially examined, providing guidelines for the application of homology modeling for VS in other proteins of the pLGIC family.

Mechanistic Understanding from Molecular Dynamics in Pharmaceutical Research 2: Lipid Membrane in Drug Design

We review the use of molecular dynamics (MD) simulation as a drug design tool in the context of the role that the lipid membrane can play in drug action, i.e., the interaction between candidate drug molecules and lipid membranes. In the standard "lock and key" paradigm, only the interaction between the drug and a specific active site of a specific protein is considered; the environment in which the drug acts is, from a biophysical perspective, far more complex than this. The possible mechanisms though which a drug can be designed to tinker with physiological processes are significantly broader than merely fitting to a single active site of a single protein. In this paper, we focus on the role of the lipid membrane, arguably the most important element outside the proteins themselves, as a case study. We discuss work that has been carried out, using MD simulation, concerning the transfection of drugs through membranes that act as biological barriers in the path of the drugs, the behavior of drug molecules within membranes, how their collective behavior can affect the structure and properties of the membrane and, finally, the role lipid membranes, to which the vast majority of drug target proteins are associated, can play in mediating the interaction between drug and target protein. This review paper is the second in a two-part series covering MD simulation as a tool in pharmaceutical research; both are designed as pedagogical review papers aimed at both pharmaceutical scientists interested in exploring how the tool of MD simulation can be applied to their research and computational scientists interested in exploring the possibility of a pharmaceutical context for their research.

Influence of amphotericin B on the DPPC/DOPC/sterols mixed monolayer in the presence of calcium ions

Amphotericin B, an acquainted antifungal drug, has reattracted the attention of most scholars due to its one important advantage of making the fungus less resistant. Amphotericin B's antifungal properties are derived from its ability to interact with ergosterols on the fungal cells' membrane to form pores. However, the cholesterol in the human cell membranes is similar in structure to ergosterol, which cause the drug to produce certain toxicity and make the clinical use of amphotericin B limited. The study of the interaction between amphotericin B and lipid monolayer in the presence of cholesterol or ergosterol is crucial to understanding the mechanism of effect of the drug on cell membranes. Langmuir monolayer as a model for half of cell membranes can precisely control the proportion of components and the solution environment, which has been used to do a lot of research about the interaction of amphotericin B with lipids. It is noteworthy that some ions associated with life activities play an important role in it, such as calcium ions. In this work, the surface pressure-mean molecular area isotherms, elastic modulus and the surface pressure-time curves of DPPC/DOPC/sterol mixed monolayer with or without amphotericin B were studied in the different concentration of calcium ions. The morphology of the Langmuir-Blodgett films transferred on the mica were observed by atomic force microscopy. The results shown that AmB changed the elastic modulus and surface morphology of DPPC/DOPC/sterol mxied monolayer, which was significantly different with different types of sterols. Calcium ions can regulate the effect of this drug, which was clearly different due to different types of sterols. This work provides useful information to further understand the influence mechanism of calcium ions on the interaction between AmB and phospholipid/sterol monolayer, which is helpful to find out the effect mechanism of calcium ion on the interaction between AmB and phospholipid monolayer containing ergosterol or cholesterol and to understand the mechanism of AmB influencing on the membrane of fungal or human cells.

Interaction between polyene antifungal drug and saturated phospholipid monolayer regulated by calcium ions at the air-water interface

Amphotericin B (AmB) is a polyene antifungal drug, which could directly form pores on the sterol-free phospholipid monolayers. The interaction between AmB and phospholipid can be affected by calcium ions, but the mechanism is still unclear. DPPC is a saturated phospholipid with -PC group, which is often used to simulate the outer cell membrane leaflet. And DPPC is also the main constituent of pulmonary surfactants. In this work, the DPPC monolayer was used as a model membrane to study the effect of calcium ions on the interaction between AmB and phospholipid. The influence of different concentration of calcium ions on the elastic modulus, mean molecular area increment, excess Gibbs free energy and stability of the AmB/DPPC mixed monolayer has been researched at the surface pressure of 7.5 mN/m, 12.5 mN/m and 22.5 mN/m. The AmB/DPPC monolayers at the air-water interface have been observed in real-time by Brewster angle microscope and the microstructure of the Langmuir-Blodgett monolayer films transferred onto the mica have been researched by scanning electron microscope and atomic force microscope. The results showed that calcium ions had a significant influence on the elastic modulus, mean molecular area increment, excess Gibbs free energy, stability and microstructure of the AmB/DPPC monolayer. It has been indicated that the influence of calcium ions on the interaction between AmB and DPPC molecules mainly depended on the effect of calcium ions on the orientation of AmB molecules. The calcium ions could regulate the effect of AmB to the stability of the DPPC monolayer. This regulatory role changed with the different concentrations of calcium ions and the different phase states of the monolayer. This work provides useful information to further understand the influence mechanism of calcium ions on the interaction between AmB and saturated phospholipid with -PC group, which is helpful to find out the effect mechanism of calcium ion on the interaction between AmB and the outer layer of cell membrane or pulmonary surfactants in different phase states and to understand the toxicity mechanism of AmB on the cell membrane or lungs.