Phosphatidylethanolamine Bilayers Research Articles

Why do polyarginines adsorb at neutral phospholipid bilayers and polylysines do not? An insight from density functional theory calculations and molecular dynamics simulations.

Adsorption of cell-penetrating peptides (CPPs) at cellular membranes is the first and necessary step for their subsequent translocation across cellular membranes into the cytosol. It has been experimentally shown that CPPs rich in arginine (Arg) amino acid penetrate across phospholipid bilayers more effectively than their lysine (Lys) rich counterparts. In this work, we aim to understand the differences in the first translocation step, adsorption of Arg9 and Lys9 peptides at fully hydrated neutral phosphatidylcholine (PC) and phosphatidylethanolamine (PE) lipid bilayers and evaluate in detail the energetics of the process using molecular dynamics (MD) simulations and free energy calculations of adsorption of the single peptide. We show that the adsorption of Arg9 is energetically feasible, with the free energy of adsorption being ∼-5.0 kcal mol-1 at PC and ∼-5.5 kcal mol-1 at PE bilayers. In contrast, adsorption of Lys9 is not observed at PC bilayers, and their adsorption at PE bilayers is very weak, being ∼-0.5 kcal mol-1. We show by energy decomposition and analysis of peptide hydration along the membrane that significantly stronger electrostatic interactions of Arg9 with lipid phosphate groups, together with the greater loss of peptide hydration (and in turn stronger hydrophobic interactions) along the membrane translocation path, are the main driving factors governing the adsorption of Arg-rich peptides at neutral lipid bilayers in contrast to Lys-rich peptides. Finally, we also compare the energetics in lipid/bilayer systems with the density functional theory (DFT) calculations of the corresponding model systems in the continuum water model and reveal the energetic differences in different environments.

Cholesterol's inhibition effect on entering of chlorzoxazone into phosphatidylethanolamine bilayer: Relevance to cytochrome P450 drug metabolism at endoplasmic reticulum membranes

Many drugs are metabolized by cytochrome P450 (CYP) in the endoplasmic reticulum (ER) membrane. Recent studies have shown that CYP-substrate drugs reach the CYP active site after entering the lipid hydrophobic part of the ER membrane. To clarify the role of cholesterol (Chol) in the CYP-related drug metabolic process, we investigated the lipid bilayer entry of CYP-substrate drugs using a model membrane system as follows. The model membrane system comprised palmitoyl-oleoyl-phosphatidylethanolamine (POPE) and Chol. Phosphatidylethanolamine is the second major phospholipid component of ER membranes. Chlorzoxazone (CZX) was used as the CYP-substrate drug. Calorimetric measurements showed that the addition of CZX to POPE bilayers decreased the gel–liquid crystal phase transition temperature; X-ray diffraction indicated that CZX distributes into the liquid crystal phase bilayers but not practically the gel phase POPE bilayers. In the presence of Chol, dialysis and X-ray structural analyses showed that Chol inhibited CZX entry into the bilayer with an increase in Chol concentration. The Chol concentration in the ER membrane (5–10 mol%) is much lower than that in the plasma membrane (approximately 30 mol%). This fact may allow CYP-substrate drugs to enter the hydrophobic portion of the ER membrane more easily than other organelle membranes, yielding efficient drug metabolism.

Local conformations affect the histidine tag-Ni<sup>2+</sup> binding affinity of BinA and BinB proteins

Binary toxin (Bin) is one of the bio-larvicidal toxin produced by <em>Lysinibacillus sphaericus</em>. Two component proteins, BinA and BinB toxins are required simultaneously to exert its larvicidal activity. The binary toxin has been proposed to act initially on the susceptible cell membrane. Here, the cell membrane binding of the binary toxin was imitated via specific histidine (His)-nickel ion (Ni²⁺) chelating. The N-terminal His-conjugated binary toxins (His-Bin) were attached onto the Ni²⁺-lipid bilayer surface besides its facilitating of purification process. The N-terminal conjugation of histidine tag did not interfere with the folding structure of both toxins. Subsequently, the attachment of binary toxins on the lipid membrane was successful with Ni²⁺-phosphatidylcholine (POPC)/phosphatidylethanolamine (POPE) bilayers (a model membrane that mimics the mosquito cell membrane) but not for Ni²⁺-phosphatidylcholine bilayer. However, His-BinA formed unstable attachment with Ni²⁺-POPC/POPE bilayers since it could be removed by buffer rinsing. In contrast, His-BinB required imidazole solution to detach from Ni²⁺-POPC/POPE. Particularly, His-BinB had higher binding affinity to Ni²⁺-ion than His-BinA. The lipid membrane attachment led to the initial finding that although BinA and BinB toxins share high homology structures, their capability for Ni²⁺ chelation was different. The local N-terminal structure of binary toxin seems to interfere the His-Ni²⁺ chelating of His-BinA.

The host-defense peptide piscidin P1 reorganizes lipid domains in membranes and decreases activation energies in mechanosensitive ion channels

The host-defense peptide (HDP) piscidin 1 (P1), isolated from the mast cells of striped bass, has potent activities against bacteria, viruses, fungi, and cancer cells and can also modulate the activity of membrane receptors. Given its broad pharmacological potential, here we used several approaches to better understand its interactions with multicomponent bilayers representing models of bacterial (phosphatidylethanolamine (PE)/phosphatidylglycerol) and mammalian (phosphatidylcholine/cholesterol (PC/Chol)) membranes. Using solid-state NMR, we solved the structure of P1 bound to PC/Chol and compared it with that of P3, a less potent homolog. The comparison disclosed that although both peptides are interfacially bound and α-helical, they differ in bilayer orientations and depths of insertion, and these differences depend on bilayer composition. Although Chol is thought to make mammalian membranes less susceptible to HDP-mediated destabilization, we found that Chol does not affect the permeabilization effects of P1. X-ray diffraction experiments revealed that both piscidins produce a demixing effect in PC/Chol membranes by increasing the fraction of the Chol-depleted phase. Furthermore, P1 increased the temperature required for the lamellar-to-hexagonal phase transition in PE bilayers, suggesting that it imposes positive membrane curvature. Patch-clamp measurements on the inner Escherichia coli membrane showed that P1 and P3, at concentrations sufficient for antimicrobial activity, substantially decrease the activating tension for bacterial mechanosensitive channels. This indicated that piscidins can cause lipid redistribution and restructuring in the microenvironment near proteins. We conclude that the mechanism of piscidin's antimicrobial activity extends beyond simple membrane destabilization, helping to rationalize its broader spectrum of pharmacological effects.

Membrane States of Saturated Glycerophospholipids: A Thermodynamic Study of Bilayer Phase Transitions.

Bilayer membranes formed by phospholipids vary in their membrane states by undergoing phase transitions in response to various external environmental factors. Pressure is one of these important environmental factors, but there are very few studies on the effects of pressure on phospholipid bilayer membranes. It is possible to deepen our understanding of the membrane states of phospholipid bilayer membranes by combining information regarding temperature- and/or ligand-responsivity with that regarding pressure-responsivity. In this review, we thermodynamically characterize the bilayer phase transitions of three kinds of saturated glycerophospholipids, each with a different polar head group (phosphatidyl-ethanolamine (PE), -choline (PC) or -glycerol (PG)), and explain their various membrane states depending on temperature and pressure. Both temperature- and pressure-responsivity reveal inherent features of these bilayer membranes: the metastability of the gel phase for PE bilayer membranes, the polymorphism of the gel phases for PC bilayer membranes and morphological changes in bilayer aggregates for PG bilayer membranes.

Temperature– and Pressure–Induced Phase Transitions of Phosphatidylethanolamine Bilayer Membranes

Temperature– and Pressure–Induced Phase Transitions of Phosphatidylethanolamine Bilayer Membranes

Phospholipid-Cellulose Interactions: Insight from Atomistic Computer Simulations for Understanding the Impact of Cellulose-Based Materials on Plasma Membranes.

Cellulose is an important biocompatible and nontoxic polymer widely used in numerous biomedical applications. The impact of cellulose-based materials on cells and, more specifically, on plasma membranes that surround cells, however, remains poorly understood. To this end, here, we performed atomic-scale molecular dynamics simulations of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) bilayers interacting with the surface of a cellulose crystal. Both biased umbrella sampling and unbiased simulations clearly show the existence of strong attractive interactions between phospholipids and cellulose: the free energy of the cellulose-bilayer binding was found to be -1.89 and -1.96 kJ/mol per cellulose dimer for PC and PE bilayers, respectively. Although the values are similar, there is a pronounced difference between PC and PE bilayers. The driving force in both cases is the formation of hydrogen bonds. There are two distinct types of hydrogen bonds: (1) between the lipid head groups and the hydroxyl (hydroxymethyl) groups of cellulose, and (2) lipid-water and cellulose-water bonds. The former is the dominant component for PE systems whereas the latter dominates in PC systems. This suggests that achieving controlled binding via new cellulose modifications must pay close attention to the lipid head groups involved. The observed attractive phospholipid-cellulose interactions have a significant effect on bilayer properties: a cellulose crystal induces noticeable structural perturbations on the bilayer leaflet next to the crystal. Given that such perturbations can be undesirable when it comes to the interactions of cellulose-based materials with cell membranes, our computational studies suggest that the impact of cellulose could be reduced through chemical modification of the cellulose surface which prevents cellulose-phospholipid hydrogen bonding.

Differential adsorption of a membrane skeletal protein, spectrin, in phospholipid membranes

The interaction of phospholipids with the peripheral membrane proteins like spectrin is important not only to understand the various physiological functions of cells, but also to gain insight into the mechanism involved in the self-assembly of polymer-like long chain molecules at the soft surfaces and interfaces. The lipid head-group specificity of adsorption of spectrin to supported phopsholipid bilayer model membranes has been investigated using the X-ray reflectivity (XRR) technique. Model lipid bilayers composed of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) head groups have been prepared on a soft polymer cushion and the XRR measurements have been carried out from the bilayers immersed in a water bath using high-energy synchrotron X-rays. Our results suggest that in PC-based membranes the spectrin chains form a uniform layer on top of the bilayer with their chains lying on the membrane surface, while in PE-based membranes with relatively smaller head groups, the spectrin chains are attached only through a few possible binding sites with the rest of the part projected out of the membrane surface. In addition, the reflectivity profiles reveal the penetration of spectrin polypeptide chains through the PE bilayer in its fluid phase. Pressure-area isotherm measurements on Langmuir monolayers also support similar observations on the adsorption of spectrin molecules to the membranes composed of PC and PE. The observed results were explained using a qualitative model based on the ion-mediated protein interaction in the PC-based membrane.

Calculating Membrane Area Compressibility from MD Simulations: a Novel Computational Framework

Like thin elastic sheets, lipid bilayers can bend and stretch, and are characterized by mechanical constants such as bending rigidity (Kc) and area compressibility (Ka), which define the free energy of bilayer deformation and thus have direct implications in quantifying the energetics of protein-membrane interactions. Limited experimental data for Kc and Ka of different lipid mixtures makes such calculations challenging. Existing computational methods for calculating Ka from MD simulations exhibit strong dependence on bilayer size, or require multiple simulations with constraints on the area, or tension. We present a novel computational framework that is independent of system size and requires analysis only of a single sufficiently converged simulation trajectory. The method is based on Helfrich's theory of elasticity by relating relative changes in area to relative changes in thickness. The distribution of local fluctuations in the thickness of each bilayer leaflet is used to construct a potential of mean force and recover the bilayer Ka from a quadratic fit. The rapid calculation utilizes freely available software packages. We present results for a variety of single and multicomponent bilayers of phosphatidylcholine, phosphatidylethanolamine and sphingomyelin, utilizing simulation results at different temperatures and bilayer sizes. The calculated compressibility moduli range from 170 to 320 mN/m and agree for the majority of tested cases with values reported from experiments and other computational methods. We further validate our method in a comparison with an existing polymer brush model and confirm the linear relationship between thickness and (Kc/Ka) ˆ (1/2), all calculated from the simulation trajectories.

Location and Effects of an Antitumoral Catechin on the Structural Properties of Phosphatidylethanolamine Membranes.

Green tea catechins exhibit high diversity of biological effects including antioncogenic properties, and there is enormous interest in their potential use in the treatment of a number of pathologies. It is recognized that the mechanism underlying the activity of catechins relay in part in processes related to the membrane, and many studies revealed that the ability of catechins to interact with lipids plays a probably necessary role in their mechanism of action. We present in this work the characterization of the interaction between an antitumoral synthetically modified catechin (3-O-(3,4,5-trimethoxybenzoyl)-(−)-catechin, TMCG) and dimiristoylphosphatidyl-ethanolamine (DMPE) membranes using an array of biophysical techniques which include differential scanning calorimetry, X-ray diffraction, infrared spectroscopy, atomic force microscopy, and molecular dynamics simulations. We found that TMCG incorporate into DMPE bilayers perturbing the thermotropic transition from the gel to the fluid state forming enriched domains which separated into different gel phases. TMCG does not influence the overall bilayer assembly of phosphatidylethanolamine systems but it manages to influence the interfacial region of the membrane and slightly decrease the interlamellar repeat distance of the bilayer. TMCG seems to be located in the interior of the phosphatidylethanolamine bilayer with the methoxy groups being in the deepest position and some portion of the molecule interacting with the water interface. We believe that the reported interactions are significant not only from the point of view of the known antitumoral effect of TMCG, but also might contribute to understanding the basic molecular mechanism of the biological effects of the catechins found at the membrane level.

Zinc Modulates Skeletal Ryanodine Receptor Function Resulting in Altered Sarcoplasmic Reticulum Calcium Release

Impaired Ca2+-homeostasis due to dysfunction in the principal sarcoplasmic reticulum Ca2+-release channel, the type-1 ryanodine receptor (RyR1) contributes to the pathophysiology of muscular dystrophy. Dynamic alterations in intracellular Zn2+ and Ca2+-homeostasis are associated with dystrophinopathies, but the mechanism by which Zn2+ modulates RyR1 is unknown. We have investigated how Zn2+ modulates RyR1-channel gating at the single-channel level. Native RyR1-channels were isolated from guinea-pig skeletal muscle and incorporated into planar phosphatidylethanolamine bilayers under voltage-clamp conditions as described previously (1). We reveal that Zn2+ is a high affinity effector of RyR1. Sequential addition of cytosolic Zn2+ in the range 100 pM to 1 μM significantly increased RyR1-channel open probability (Po) in a dose dependent manner. The increase in channel activity resulted from an increase in the frequency of openings with no significant effect on mean channel open times. These data indicate that Zn2+ increases RyR1 Po by sensitizing the channel to cytosolic Ca2+. In skeletal muscle, Mg2+ is an endogenous inhibitor of RyR1 and helps to prevent inappropriate release of Ca2+. We therefore wanted to investigate the activatory effects of Zn2+ in the presence of physiological levels of Mg2+. Under resting conditions (100 nM cytosolic Ca2+) we observed that the presence of 1 mM cytosolic Mg2+ resulted in complete closure of the channel (Po=0). Unexpectedly, RyR1-channel openings were observed following the addition of pathophysiological levels of Zn2+ (10 nM) where Po increased from 0 to 0.14 (n=2). Dysregulated Zn2+ levels may therefore lead to abnormally active or “leaky” RyR1-channels, suggesting that altered Zn2+-homeostasis could perturb intracellular Ca2+-dynamics and contribute to the progression of muscular dystrophy.(1) Woodier et al.,(2015) JBC;290(28):17599-610.Funded by the BHF

Molecular structures of fluid phosphatidylethanolamine bilayers obtained from simulation-to-experiment comparisons and experimental scattering density profiles.

Following our previous efforts in determining the structures of commonly used PC, PG, and PS bilayers, we continue our studies of fully hydrated, fluid phase PE bilayers. The newly designed parsing scheme for PE bilayers was based on extensive MD simulations, and is utilized in the SDP analysis of both X-ray and neutron (contrast varied) scattering measurements. Obtained experimental scattering form factors are directly compared to our simulation results, and can serve as a benchmark for future developed force fields. Among the evaluated structural parameters, namely, area per lipid A, overall bilayer thickness DB, and hydrocarbon region thickness 2DC, the PE bilayer response to changing temperature is similar to previously studied bilayers with different headgroups. On the other hand, the reduced hydration of PE headgroups, as well as the strong hydrogen bonding between PE headgroups, dramatically affects lateral packing within the bilayer. Despite sharing the same glycerol backbone, a markedly smaller area per lipid distinguishes PE from other bilayers (i.e., PC, PG, and PS) studied to date. Overall, our data are consistent with the notion that lipid headgroups govern bilayer packing, while hydrocarbon chains dominate the bilayer's response to temperature changes.

Effect of Electric Field on Structure and Dynamics of Bilayers Formed From Anionic Phospholipids

The effect of molecular structure on ensemble structure and dynamics of phospholipid bilayers has been investigated. Bilayers of dimyristoyl phosphatidylserine (DMPS) supported on Au(111) surfaces were prepared by Langmuir-Blodgett and Langmuir-Schaeffer deposition and studied with a combination of electrochemical measurements and in situ Polarisation Modulation Infrared Reflection Absorption Spectroscopy (PM-IRRAS). DMPS bilayers have relatively large capacitance when compared with those formed from similar molecules and this is attributed to a high solvent content within the bilayer, resulting from the need for solvation of the negatively charged lipid headgroups. Infrared spectra show that the ensemble of molecules is in a gel state, with extended and ordered hydrocarbon chains, similarly to bilayers of dimyristoyl phosphatidylethanolamine (DMPE) molecules, which are of similar shape. The infrared spectra also show that, in contrast to DMPE, the headgroups of DMPS are very strongly hydrated and have higher mobility. This higher mobility allows the re-orientation of the molecules under the influence of an applied electric field: re-orientation both of headgroups and hydrocarbon tail groups is observed. Thus the shape and charge of the molecules in an ensemble have a strong influence on both their structure and dynamics in the presence of an externally applied electric field.

Comparing Symmetric and Asymmetric Bilayer-Protein Interactions using Molecular Dynamics

Gram-negative bacteria have an outer membrane (OM) bilayer composed of extracellular lipopolysaccharides (LPS) and periplasmic phospholipids, providing an effective barrier against the permeation of water, ions and other molecules. To circumvent this barrier, bacteria have evolved proteins that span the OM, which permit the transport of essential nutrients. Most previous molecular dynamics (MD) simulation studies examined OM protein dynamics in a symmetric phospholipid bilayer to determine substrate binding affinity, ion conductance, ligand interaction and various other properties. Given the advancement in computational resources, we are now able to look at larger system sizes and hence more realistic membrane-protein systems for OM proteins. Here, we compare simulations of the Escherichia coli, OM cobalamin transporter BtuB embedded in an asymmetric LPS containing bilayer to BtuB embedded in a symmetric 1-palmitoyl 2-oleoyl phosphatidylethanolamine (POPE) bilayer. The simulations show that the characteristically long, charged extracellular loops of BtuB become stabilized when placed in the LPS-containing bilayer whereas previous simulations showed that BtuB needed two bound calcium ions to stabilize these loops. Further properties of the interactions between the membrane and protein in each simulation are also explored.

Interactions of the amphiphiles arbutin and tryptophan with phosphatidylcholine and phosphatidylethanolamine bilayers in the dry state

BackgroundWater is essential for life, but some organisms can survive complete desiccation, while many more survive partial dehydration during drying or freezing. The function of some protective molecules, such as sugars, has been extensively studied, but much less is known about the effects of amphiphiles such as flavonoids and other aromatic compounds. Amphiphiles may be largely soluble under fully hydrated conditions, but will partition into membranes upon removal of water. Little is known about the effects of amphiphiles on membrane stability and how amphiphile structure and function are related. Here, we have used two of the most intensively studied amphiphiles, tryptophan (Trp) and arbutin (Arb), along with their isolated hydrophilic moieties glycine (Gly) and glucose (Glc) to better understand structure-function relationships in amphiphile-membrane interactions in the dry state.ResultsFourier-transform infrared (FTIR) spectroscopy was used to measure gel-to-liquid crystalline phase transition temperatures (Tm) of liposomes formed from phosphatidylcholine and phosphatidylethanolamine in the presence of the different additives. In anhydrous samples, both Glc and Arb strongly depressed Tm, independent of lipid composition, while Gly had no measurable effect. Trp, on the other hand, either depressed or increased Tm, depending on lipid composition. We found no evidence for strong interactions of any of the compounds with the lipid carbonyl or choline groups, while all additives except Gly seemed to interact with the phosphate groups. In the case of Arb and Glc, this also had a strong effect on the sugar OH vibrations in the FTIR spectra. In addition, vibrations from the hydrophobic indole and phenol moieties of Trp and Arb, respectively, provided evidence for interactions with the lipid bilayers.ConclusionsThe two amphiphiles Arb and Trp interact differently with dry bilayers. The interactions of Arb are dominated by contributions of the Glc moiety, while the indole governs the effects of Trp. In addition, only Trp-membrane interactions showed a strong influence of lipid composition. Further investigations, using the large structural diversity of plant amphiphiles will help to understand how their structure determines the interaction with membranes and how that influences their biological functions, for example under freezing or dehydration conditions.

Effect of Headgroup on the Physicochemical Properties of Phospholipid Bilayers in Electric Fields: Size Matters

The effect of molecular structure on ensemble structure of phospholipid films has been investigated. Bilayers of dimyristoyl phosphatidylethanolamine (DMPE) were prepared on Au(111) electrodes using Langmuir-Blodgett and Langmuir-Schaeffer deposition. Capacitance and charge density measurements were used to investigate the adsorption behavior and barrier properties of the lipid bilayers. In situ polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was employed to investigate the organization of the molecules within the bilayer. DMPE bilayers exhibit lower capacitance than bilayers formed from the related lipid, dimyristoyl phosphatidylcholine (DMPC). The infrared data show that these results can be explained by structural differences between the bilayers formed from each molecule. DMPE organizes into bilayers with hydrocarbon chains tilted at a smaller angle to the surface normal, which results in a thicker film. The hydrocarbon chains contain few conformational defects. Spectra in the carbonyl and phosphate stretching mode regions indicate low solvent content of DMPE films. Both of these effects combine to produce films with lower capacitance and enhanced barrier properties. The results are explained in terms of the differences in structure between the constituent molecules.

Computer Simulation of Water-Mediated Adhesion Between Phospholipid Bilayer and Solid Support Functionalized with Self-Assembled Monolayers

An attempt is made to estimate, via computer simulation of the force-distance relation, the free energy of adhesion between a phosphatidylethanolamine bilayer and an alkanethiolate self-assembled monolayer (SAM) in aqueous medium. The simulations are performed using the grand canonical Monte Carlo technique and atomistic force fields. The bilayer adhesion free energy is predicted to be -22 ± 3 mJ/m(2) (-1.4 ± 0.2 kcal/mol) on a hydrophilic carboxyl-terminated SAM and -1 ± 1 mJ/m(2) (-0.06 ± 0.06 kcal/mol) on a hydrophobic methyl-terminated SAM.

Molecular Dynamics Study of the Interaction of Arginine with Phosphatidylcholine and Phosphatidylethanolamine Bilayers

In this work, the differential interaction of zwitterionic arginines with fully hydrated dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE) bilayers was analyzed by molecular dynamics simulations. In both systems, arginine binds to lipids with the carboxylate moiety oriented toward the aqueous phase, in agreement with previous experimental determinations of ζ potential of DMPC and DMPE liposomes. The guanidinium groups are found at different depths within the bilayers indicating that some arginines are buried, especially in DMPE. We observe, in the DMPE system, that the strongest interaction occurs between the guanidinium group and the carbonyl oxygen of the lipid. In the case of DMPC membranes, the strongest interaction is found between the guanidinium groups of the arginines and the phosphate groups of the lipids. Unexpectedly, arginine zwitterions are stabilized through the creation of hydrogen bonds (HB), either with water or with polar groups of the lipids. The mechanisms of interaction seem to be different in both membranes. In DMPE bilayers, arginines insert by breaking the inner HB network of the polar head groups, consequently increasing the occupied area per lipid molecule. In the DMPC bilayers the arginines insert by replacing the already present water molecules within the membrane, without significant effects on the area per lipid.

A Differential Scanning Calorimetry Study of the Effects and Interactions of Antimicrobial Peptide LS3 on Phosphatidylethanolamine Bilayers

Differential Scanning Calorimetry studies of a synthetic peptide revealed the peptide decreased the temperature of the lamellar-hexagonal phase transition of cis-trans mixtures of phosphatidylethanolamine. The transition enthalpy varied significantly with lipid composition. The findings are discussed with reference to peptide saturation on the bilayer surface, bilayer thinning and peptide orientation.

Effects of the antimalarial drug primaquine on the dynamic structure of lipid model membranes

Primaquine (PQ) is a potent therapeutic agent used in the treatment of malaria and its mechanism of action still lacks a more detailed understanding at a molecular level. In this context, we used differential scanning calorimetry (DSC), pressure perturbation calorimetry (PPC), and electron spin resonance (ESR) to investigate the effects of PQ on the lipid phase transition, acyl chain dynamics, and on volumetric properties of lipid model membranes. DSC thermograms revealed that PQ stabilizes the fluid phase of the lipid model membranes and interacts mainly with the lipid headgroups. This result was revealed by the great effect on the pretransition of phosphatidylcholines and the destabilization of the inverted hexagonal phase of a phosphatidylethanolamine bilayer. Spin probes located at different positions along the lipid chain were used to monitor different membrane regions. ESR results indicated that PQ is effective in changing the acyl chain ordering and dynamics of the whole chain of dimyristoylphosphatidylcholine (DMPC) phospholipid in the rippled gel phase. The combined ESR and PPC results revealed that the slight DMPC volume changes at the main phase transition induced by the presence of PQ is probably due to a less dense lipid gel phase. At physiological pH, the cationic amphiphilic PQ strongly interacts with the lipid headgroup region of the bilayers, causing considerable disorganization in the hydrophobic core. These results shed light on the molecular mechanism of primaquine–lipid interaction, which may be useful in the understanding of the complex mechanism of action and/or the adverse effects of this antimalarial drug.