Acyl Chain Region Research Articles

Antimicrobial Peptide Coating of TiO2 Nanoparticles for Boosted Antimicrobial Effects

AbstractThis study explores the coating of photocatalytic nanoparticles with antimicrobial peptides (AMPs) for boosted antimicrobial effects, and how such effects depend on AMP properties. For this, TiO2 nanoparticles are coated with the AMP KYE21 or its hydrophobically enhanced variant WWWKYE21. Mirroring effects of free peptides, coated nanoparticles displayed higher binding and UV‐induced degradation for bacteria‐like than for mammalian‐like membranes. In addition, they degraded bacterial lipopolysaccharides (LPS). WWWKYE21‐coated nanoparticles displayed higher binding to LPS and bacteria‐like membranes and photocatalytic degradation, although saturation effects are found at high nanoparticle binding. Neutron reflectometry showed that binding of peptide‐coated nanoparticles to bacteria‐like membranes resulted in partial lipid removal in the absence of UV, but that UV illumination caused additional degradation, featuring increases in the hydration of headgroup and acyl chain regions. For LPS, UV induced removal of its outer O‐antigen region. Analogous to findings in model systems, antimicrobial effects of peptide‐coated nanoparticles against Escherichia coli bacteria on illumination are pronounced, while toxicity against human monocytes remained low. Altogether, results show that AMP coating boosts the antimicrobial effects of photocatalytic nanoparticles without causing cell toxicity. From a broader perspective, the study points to the potential of nanoarchitectonic combination of component properties for the design of composite NP properties.

Concentration-Dependent Effects of Curcumin on Membrane Permeability and Structure.

Growing evidence suggests that many bioactive molecules can nonspecifically modulate the physicochemical properties of membranes and influence the action of embedded membrane proteins. This study investigates the interactions of curcumin with protein-free model membranes consisting of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and DOPC with cholesterol (4/1 mol ratio). The focus is on the capability of curcumin to modify membrane barrier properties such as water permeability assayed through the droplet interface bilayer (DIB) model membrane. For pure DOPC, our findings show a concentration-dependent biphasic effect: a reduction in water permeability is observed at low concentrations (up to 2 mol %), whereas at high concentrations of curcumin, water permeability increases. In the presence of cholesterol, we observed an overall reduction in water permeability. A combination of complementary experimental methods, including phase transition parameters studied by differential scanning calorimetry (DSC) and structural properties measured by attenuated total reflectance (ATR)-FTIR, provides a deeper understanding of concentration-dependent interactions of curcumin with DOPC bilayers in the absence and presence of cholesterol. Our experimental findings align with a molecular mechanism of curcumin's interaction with model membranes, wherein its effect is contingent on its concentration. At low concentrations, curcumin binds to the lipid-water interface through hydrogen bonding with the phosphate headgroup, thereby obstructing the transport of water molecules. Conversely, at high concentrations, curcumin permeates the acyl chain region, inducing packing disorders and demonstrating evidence of phase separation. Enhanced knowledge of the impact of curcumin on membranes, which, in turn, can affect protein function, is likely to be beneficial for the successful translation of curcumin into effective medicine.

Oseltamivir phosphate interaction with model membranes

Oseltamivir belongs to the neuraminidase inhibitors, developed against the influenza virus, and registered under the trademark Tamiflu. Despite its long-term acquaintance, there is limited information in the literature about its physicochemical and structural properties in a lipid-water system. We present an experimentally determined partition coefficient with structural information on the interaction of oseltamivir with the model membrane, its possible location, and its effect on the membrane thermodynamics. The hydrophobic part of the lipid bilayer is affected to a moderate extent, which was proved by slight changes in thermal and structural properties. Hereby, interaction of oseltamivir with the phospholipid bilayer induces concentration dependent decrease of lateral pressure in the bilayer acyl chain region. Oseltamivir charges the bilayer surface positively, which results in the zeta potential increase and changes in anisotropic properties studied by the polarised light microscopy. At the highest oseltamivir concentrations studied, the multilamellar structure is extensively disturbed, likely due to electrostatic repulsion between the adjacent bilayers.

Effect of Host Cholesterol on the Membrane Dynamics of Outer Membrane Lipids of Mycobacteria.

The ability of Mycobacterium tuberculosis to remain dormant after primary infection represents the prime cause of new TB cases throughout the world. Hence, diagnosis and treatment of individuals hosting dormant mycobacterium is one of the crucial strategies to be adopted for the prevention of Tuberculosis. Among many strategies unleashed by the latent bacterium, one of them is scavenging host cholesterol for carbon source. Cholesterol modifies lipid membranes over many scales and here, its effect on mycobacterial membrane biophysics and the subsequent effect on partitioning of antibiotics into cholesterol- enriched mycobacterial membranes was investigated. Our research showed that cholesterol alters the phase state behavior of mycobacterial outer membrane lipids by enhancing the overall membrane order at the headgroup and acyl chain region and is integrated into both ordered and disordered domains/phases, with a preference for the latter. Exogenous cholesterol further alters the drug partitioning behavior of structurally different drugs, pointing to a larger clinical potential of using more hydrophobic medications to target dormant bacteria.

Phosphatidylethanolamine and Cholesterol Promote Hemifusion Formation: A Tug of War between Membrane Interfacial Order and Intrinsic Negative Curvature of Lipids.

Membrane fusion is an important process for the survival of eukaryotes. The shape of lipids plays an important role in fusion by stabilizing nonlamellar fusion intermediates. Lipids with intrinsic positive curvature such as lysophosphatidylcholine and others inhibit hemifusion formation, whereas lipids having intrinsic negative curvature, e.g., phosphatidylethanolamine and cholesterol (CH), are known to promote hemifusion formation. In this work, we have measured the effect of dioleoylphosphatidylethanolamine (DOPE) and CH on the depth-dependent organization, dynamics, and fusion of dioleoylphosphatidylcholine membranes. Both DOPE and CH promote hemifusion formation despite their ability to order the interfacial and acyl chain region of the membrane and block water percolation at these regions. Generally, membrane ordering and inhibition of water percolation at the acyl chain region are detrimental to membrane fusion. This clearly emphasizes the importance of intrinsic negative curvature of lipids in membrane fusion. Interestingly, DOPE and CH show differential effects on the rate of hemifusion formation, which might be owing to their ability to induce order at the interfacial region and intrinsic negative curvature. Overall, our result is significant in understanding the role of lipidic shape in membrane fusion.

Is lipophilicity the main factor determining the effective drug penetration through lipid membranes? Combined GIXD and PM-IRRAS studies on the example of anthracyclines

Cell membranes constitute the first barrier for the drugs on their way to the cell. Therefore, their penetration determines the effectivity of the treatment and is defined by the drug-lipid interactions. A systematic, comprehensive study to assess the impact of different aspects influencing the effective penetration of model lipid membranes by the drugs including lipophilicity, charge, ability to form dimers as well as stearic hindrance was performed. Four anthracyclines differing in the structure and with lipophilicity increasing in the following order: doxorubicin (DOx) < pirarubicin (THP) < daunorubicin (DNR) < idarubicin (IDA) were chosen to assess their impact on 1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (DMPS) monolayers mimicking cancer cell membranes. The investigation performed at the air–water interface involved the application of multiple surface-sensitive techniques to assess the drug interactions with both polar heads of the phospholipid and the penetration of the acyl chain region. The obtained results allowed us to state that although electrostatic interactions and lipophilicity of the drug are one of the key factors, the ability to form dimers as well as stearic hinderance may strongly prevent from the effective penetration of the phospholipid monolayers even for drugs with higher lipophilicity and therefore have to be also taken into account in the development of new therapeutic agents.

Assessment of DPPC Liposome Disruption by Embedded Tocopheryl Malonate

In this study, the effect of α-tocopheryl malonate (TM) on physical and structural properties of DPPC liposomes was investigated using ANS fluorescence, DPH, and TMA–DPH anisotropy fluorescence and differential scanning calorimetry (DSC) methods. The presence of embedded TM in DPPC liposomes caused alteration in its phase transition temperatures, structural order, dynamics, and hydration of head groups increasingly with growing TM concentration. The ANS fluorescence results demonstrated that increasing TM presence in the DPPC gel phase due to interrupted membrane structure caused the formation of new binding sites. Temperature investigations in the range of 20 °C to 60 °C showed that increasing temperature rises ANS fluorescence which reaches local and global maxima at 36 °C and 42 °C, respectively. The rising TM concentration at the phase transition temperature of DPPC led to the lowering of ANS fluorescence, indicating a decreased binding of ANS. Simultaneously, during heating, a roughly 10-nm shift of ANS emission maximum was observed. The results indicated that in the fluid phase, the observed quenching appears as a result of increasing accessibility of water molecules into ANS in this region. The DPH results indicated that in the gel phase presence of TM introduced disorder in the hydrophobic acyl chain region led to its fluidization. The TMA–DPH results indicated an increasing disorder in the interface region and an increasing hydration of head group atoms at the surface of the membrane. The increasing concentration of TM results in the formation of multicomponent DSC traces, suggesting the formation of another structural phase. The applied methods proved that the incorporation of TM into DPPC membrane results in the interaction of malonate moiety with DPPC head group atoms in the interphase layer and induces the interruption in the membrane packing order, leading to its structural changes. The presented results show that TM presence could regulate the membrane properties, thus it may indicate one of the possible mechanisms responsible for the effective disruption of cell membranes by TM. The knowledge of molecular mechanism how TM interacts with the membrane will help to elucidate its possible pharmacological activity.

Surfactant Proteins SP-B and SP-C in Pulmonary Surfactant Monolayers: Physical Properties Controlled by Specific Protein-Lipid Interactions.

The lining of the alveoli is covered by pulmonary surfactant, a complex mixture of surface-active lipids and proteins that enables efficient gas exchange between inhaled air and the circulation. Despite decades of advancements in the study of the pulmonary surfactant, the molecular scale behavior of the surfactant and the inherent role of the number of different lipids and proteins in surfactant behavior are not fully understood. The most important proteins in this complex system are the surfactant proteins SP-B and SP-C. Given this, in this work we performed nonequilibrium all-atom molecular dynamics simulations to study the interplay of SP-B and SP-C with multicomponent lipid monolayers mimicking the pulmonary surfactant in composition. The simulations were complemented by z-scan fluorescence correlation spectroscopy and atomic force microscopy measurements. Our state-of-the-art simulation model reproduces experimental pressure-area isotherms and lateral diffusion coefficients. In agreement with previous research, the inclusion of either SP-B and SP-C increases surface pressure, and our simulations provide a molecular scale explanation for this effect: The proteins display preferential lipid interactions with phosphatidylglycerol, they reside predominantly in the lipid acyl chain region, and they partition into the liquid expanded phase or even induce it in an otherwise packed monolayer. The latter effect is also visible in our atomic force microscopy images. The research done contributes to a better understanding of the roles of specific lipids and proteins in surfactant function, thus helping to develop better synthetic products for surfactant replacement therapy used in the treatment of many fatal lung-related injuries and diseases.

Monitoring Molecular Structure Changes of Cow and Camel Milk Mixtures during Coagulation: A Study Based on TPA and 2DCOS-MIR Spectroscopy

The aim of the present study is to evaluate the potential of mid-infrared spectroscopy in combination with two-dimensional correlation spectroscopy (2DCOS) and partial least squares discriminant analysis (PLS-DA) to monitor molecular structure evolution of milk mixtures of two species (cow and camel) during enzymatic coagulation. Molecular structure changes and texture characteristics were investigated on five different milk formulations by mixing camel milk (CaM) and cow milk (CM) (i.e., CaM, CaM3:CM1, CaM1:CM1, CaM1:CM3, and CM, (v/v)). Regarding MIR spectroscopy, two spectral regions were considered to monitor milk coagulation, the fatty acid acyl-chain region (3000–2800 cm−1) and the protein region (1700–1500 cm−1). Different formulation dissimilarities were revealed through the synchronous 2DCOS spectra and their respective auto-peaks. Moreover, using the cross-peak symbols of the synchronous and asynchronous spectra helps to establish the sequence of molecular structure modifications during coagulation. Finally, the analysis of the 2DCOS-MIR synchronous and asynchronous maps by PLS-DA gave good discriminant accuracy (88 to 100%) between each formulation. Those results highlighted that different synchronous and asynchronous chemical phenomena occurred during milk coagulation depending on the milk formulation. Texture attributes were influenced by camel milk ratio in mixtures based on the initial composition of milks. Samples containing more than 50% of CM showed significantly higher hardness compared to pure CaM and CaM3:CM; however, CM gel displayed lower springiness contrary to CaM. Textural characteristics of the resulted gels also confirmed that 2DCOS MIR observations are consistent with textural observations of the gels obtained after milk mixtures coagulation.

Antioxidant and Anti-inflammatory Activity of Sea Cucumber (Holothuria scabra) Active Compounds against KEAP1 and iNOS Protein.

Oxidative stress and inflammation have a role in the development of various diseases. Oxidative stress and inflammation are associated with many proteins, including Kelch ECH associating protein 1 (KEAP1) and inducible nitric oxide synthase (iNOS) proteins. The active compounds contained in Holothuria scabra have antioxidant and anti-inflammatory properties. This study aimed to evaluate the antioxidant and anti-inflammatory activity of sea cucumber's active compounds by targeting KEAP1 and iNOS proteins. 2,2-Diphenyl-1-picrylhydrazyl (DPPH) and nitric oxide (NO) scavenging activity of H. scabra extract were measured spectrophotometrically. The 3-dimensional (3D) structures of sea cucumber's active compounds and proteins were obtained from the PubChem and Research Collaboratory for Structural Bioinformatics Protein Data Bank (RCSB PDB) databases. Molecular docking was performed using AutoDock Vina software. Molecular dynamics simulations were carried out using Yet Another Scientific Artificial Reality Application (YASARA) software with environmental parameters according to the cell's physiological conditions. The membrane permeability test was performed using the PerMM web server. The methanol extract of H. scabra had a weak antioxidant activity against DPPH and strong activity against NO radical. Scabraside and holothurinoside G had the most negative binding affinity values when interacting with the active site of KEAP1 and iNOS proteins. Molecular dynamics simulations also showed that both compounds were stable when interacting with KEAP1 and iNOS. However, scabraside and holothurinoside G were difficult to penetrate the cell plasma membrane, which is seen from the high energy transfer value in the lipid acyl chain region of phospholipids. Scabraside and holothurinoside G are predicted to act as antioxidants and anti-inflammations, but in their implementation to in vitro and in vivo study, it is necessary to have liposomes or nanoparticles, or other delivery methods to help these 2 compounds enter the cell.

The Influence of Cationic Nitrosyl Iron Complex with Penicillamine Ligands on Model Membranes, Membrane-Bound Enzymes and Lipid Peroxidation.

This paper shows the biological effects of cationic binuclear tetranitrosyl iron complex with penicillamine ligands (TNIC-PA). Interaction with a model membrane was assessed using a fluorescent probes technique. Antioxidant activity was studied using a thiobarbituric acid reactive species assay (TBARS) and a chemiluminescence assay. The catalytic activity of monoamine oxidase (MAO) was determined by measuring liberation of ammonia. Antiglycation activity was determined fluometrically by thermal glycation of albumine by D-glucose. The higher values of Stern-Volmer constants (KSV) obtained for the pyrene located in hydrophobic regions (3.9 × 104 M-1) compared to KSV obtained for eosin Y located in the polar headgroup region (0.9 × 104 M-1) confirms that TNIC-PA molecules prefer to be located in the hydrophobic acyl chain region, close to the glycerol group of lipid molecules. TNIC-PA effectively inhibited the process of spontaneous lipid peroxidation, due to additive contributions from releasing NO and penicillamine ligand (IC50 = 21.4 µM) and quenched luminol chemiluminescence (IC50 = 3.6 μM). High activity of TNIC-PA in both tests allows us to assume a significant role of its radical-scavenging activity in the realization of antioxidant activity. It was shown that TNIC-PA (50-1000 μM) selectively inhibits the membrane-bound enzyme MAO-A, a major source of ROS in the heart. In addition, TNIC-PA is an effective inhibitor of non-enzymatic protein glycation. Thus, the evaluated biological effects of TNIC-PA open up the possibility of its practical application in chemotherapy for socially significant diseases, especially cardiovascular diseases.

Collective dynamics of domain structures in liquid crystalline lipid bilayers

Objectives. Numerous studies of biosystems indicate the distinct role of quasi-one-dimensional molecular structures in the transport of energy, charges, and information. Of particular interest are the studies on the collective dynamics of quasi-one-dimensional lateral structures in liquid crystalline membranes and the possibility of local excitation transfer through such structures. In this paper, we developed a model for the collective dynamics of quasi-one-dimensional domain structures in lipid bilayers interacting with the environment. The objective is to study the mechanisms of the directed energy transport in liquid crystalline lipid membranes.Methods. In this paper, the percolation domain structures formed as a result of phase separation in multicomponent lipid membranes are considered to be quasi-one-dimensional domain structures. The model distinguishes two subsystems interacting with each other and differing in their structural and dynamic properties, i.e., the membrane surface formed by polar groups of lipid molecules and the internal hydrophilic region of the membrane formed by acyl chains of lipids. The acyl chain subsystem is simulated using the Ginzburg-Landau Hamiltonian which considers the dependence of its dynamics on temperature close to the lipid melting phase transition temperature Tc.Results. Analysis of dynamic states has shown that elastic excitations moving at constant rate in the form of solitons may exist near temperatures Tc in the considered quasi-one-dimensional domain structures. In addition, motion of the elastic excitation region (kink) along domain structures in the acyl chain region causes the formation of acoustic soliton, i.e., the compression region in the polar group subsystem moving in concert with the kink displacement. The soliton localization region covers about 10 molecules and depends significantly on the interaction parameter of the polar group and acyl chain subsystems. Soliton moves at a subsonic speed determined, in particular, by the magnitude of an external force.Conclusions. The model developed in this paper shows that liquid crystalline domain structures in lipid membranes exhibit properties of active media, wherein the formation and displacement of localized elastic excitations on macroscopic spatial and temporal scales may occur. The proposed molecular mechanism of the soliton transport along quasi-one-dimensional domain structures may be used for describing the directed energy transfer along lateral domain channels in biomembranes and the cooperative functioning of the membrane bioenergetic and receptor complexes.

Membrane Potentials Trigger Molecular-Scale Rearrangements in the Outer Membrane of Gram-Negative Bacteria.

The structural complexity of the cell envelope of Gram-negative bacteria limits the fabrication of realistic models of bacterial cell membranes. A vertical Langmuir-Blodgett withdrawing was used to deposit a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) monolayer on the Au(111) surface. The second leaflet composed of di[3-deoxy-D-manno-octulosonyl]-lipid A (KLA) was deposited using Langmuir-Schaefer transfer. The use of an electrode material as a support for the POPE-KLA bilayer allowed electrochemical control of the membrane's stability, compactness, and structure. Capacitance-potential curves showed a typical pattern for the supported lipid bilayers electrochemical characteristic. The minimum membrane capacitance was ∼4 μF cm-2 and did not change in the following desorption-adsorption cycles, indicating the presence of a stable bilayer structure with an asymmetric composition of both leaflets. However, at a molecular scale, as elucidated in spectroelectrochemical experiments, large differences in the response of both leaflets to electric potentials were observed. The acyl chains in POPE and KLA existed in a liquid state. The quantitative analysis of the CH stretching modes indicated potential-driven reorientations in the hydrophobic fragment of the bilayer, already in the adsorbed state. To assign observed rearrangements to POPE and KLA lipids in both leaflets, per-deuterated d31-POPE was transferred into the inner leaflet. Since no potential-dependent changes of the CD2 stretching modes in the d31-POPE-KLA bilayer were observed, reorientations in the acyl chain region were assigned to the KLA molecules. Mg2+ ions were bound to the polar head groups of KLA. The strength of electrostatic interactions in the polar head group region of KLA was dependent on the direction of the electric field. At negative electric potentials, the binding of divalent cations weakened, which gave the KLA molecules increased orientational flexibility. This behavior in electric fields is peculiar for the outer membrane and indicates that the microbial cell membranes have different electrochemical properties than phospholipid bilayers.

Influence of lipophilicity of anthracyclines on the interactions with cholesterol in the model cell membranes – Langmuir monolayer and SEIRAS studies

The interactions of anthracyclines with biological membranes strongly depend on the drug lipophilicity, which might also determine the specific affinity to cholesterol molecules. Therefore, in this work we show the studies concerning the effect of two selected anthracyclines, daunorubicin (DNR) and idarubicin (IDA) on simple models of healthy (DMPC:Chol 7:3) and cancer cells membranes with increased level of cholesterol (DMPC:Chol 3:7) as well as pure cholesterol monolayers prepared at the air-water interface and supported on gold surface. It has been shown that more lipophilic IDA is able to penetrate cholesterol monolayers more effectively than DNR due to the formation of IDA-cholesterol arrangements at the interface, as proved by the thermodynamic analysis of compression-expansion cycles. The increased interactions of IDA were also confirmed by the time measurements of pre-compressed monolayers exposed to drug solutions as well as grazing incidence X-ray diffraction studies demonstrating differences in the 2D organization of cholesterol monolayers. Langmuir studies of mixed DMPC:Chol membranes revealed the reorganization of molecules in the cancer cell models at the air-water interface at higher surface pressures due to the removal of DNR, while increased affinity of IDA towards cholesterol allowed this drug to penetrate the layer more efficiently without its removal. The SEIRAS spectra obtained for supported DMPC:Chol bilayers proved that IDA locates both in the ester group and in the acyl chain region of the bilayer, while DNR does not penetrate the membranes as deeply as IDA. The increased penetration of the mixed phospholipid layers by idarubicin might be attributed to the higher lipophilicity caused by the lack of methoxy group and resulting in a specific affinity towards cholesterol.

Ceramide-mediation of diffusion in supported lipid bilayers

The fluidity and compositional heterogeneity of the mammalian plasma membrane play deterministic roles in a variety of membrane functions. Designing model bilayer systems allows for compositional control over these properties. Ceramide is a phospholipid capable of extensive headgroup-region hydrogen bonding, and we report here on the role of ceramide in planar model bilayers. We use fluorescence recovery after photobleaching (FRAP) to obtain translational diffusion constants of two chromophores in supported model bilayers composed of cholesterol, 1,2-dioleoyl-sn-phosphatidylcholine (DOPC), sphingomyelin, and ceramide. FRAP data for perylene report on the acyl chain region of the model bilayer and FRAP data for 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(lissamine rhodamine B sulfonyl) sense diffusional dynamics in the bilayer headgroup region. Dynamics in the headgroup region exhibit anomalous diffusion behavior that is characteristic of spatially heterogeneous media.

Effect of cholecalciferol on unsaturated model membranes

Vitamin D plays an important role in many physiological processes, particularly calcium and phosphorous homeostasis. The biochemistry of vitamin D is also complex, encompassing a range of active molecules that may be either endogenous or dietary in origin. The role of lipids and fats in the production, processing and use of vitamin D is an interesting one, with a relative paucity of model studies into the interactions of vitamin D with lipidic systems such as micelles and vesicles. Here, we have studied the effect of vitamin D3 in simple unsaturated phospholipid systems. We used NMR and FTIR spectroscopy to investigate the effect of increasing vitamin D concentration on the structure and dynamics of the lipid chains and interfacial region. In order to link these model studies with more complex biomimetic environments, we compare results in the presence of buffer and vitamin D binding protein. We have also used DLS to determine that vitamin D3-DOPC vesicles can retain their size distribution for varying amounts of time in different conditions. We find that the acyl chain region of vitamin D3-DOPC membranes are generally disordered, and that the addition of buffer and/or protein alters the properties of the interfacial region.

Exploring membrane viscosity at the headgroup region utilizing a hemicyanine-based fluorescent probe

Membrane viscosity is an important determinant for several cellular functions. In spite of having vast literature on the interfacial and acyl chain region viscosity of the membrane, there is hardly any report on the membrane viscosity at the headgroup region. In this present work, we have exploited the photophysical properties of a hemicyanine dye covalently linked with a C16 alkyl chain (HC16) to explore the membrane viscosity at the headgroup region. The chromophoric group of HC16 locates at the headgroup region due to its overall positive charge. Methanol-glycerol mixture of known composition and viscosity were used to develop the standard curve of apparent rotational correlation time against viscosity, and utilized this standard curve to evaluate viscosity of dimyristoylphosphatidylcholine (DMPC) membrane as a function of temperature. The result exhibited that there is a 4-fold change in headgroup viscosity of the DMPC membrane between gel and fluid phase; with a sharp change during the phase transition. Moreover, we have monitored the effect of cholesterol on the viscosity of the headgroup region of a biologically relevant lipid, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), keeping in mind the role of cholesterol on membrane organization, dynamics, and cellular functions. Interestingly, the headgroup viscosity of POPC membrane does not show a significant change in presence of various concentrations of cholesterol, though literature showed remarkable change in interfacial and acyl chain region viscosity in presence of similar concentrations of cholesterol.

Polycation-Anionic Lipid Membrane Interactions.

Natural or synthetic polycations are used as biocides or as drug/gene carriers. Understanding the interactions between these macromolecules and cell membranes at the molecular level is therefore of great importance for the design of effective polymer biocides or biocompatible polycation-based delivery systems. Until now, details of the processes at the interface between polycations and biological systems have not been fully recognized. In this study, we consider the effect of strong polycations with quaternary ammonium groups on the properties of anionic lipid membranes that we use as a model system for protein-free cell membranes. For this purpose, we employed experimental measurements and atomic-scale molecular dynamics (MD) simulations. MD simulations reveal that the polycations are strongly hydrated in the aqueous phase and do not lose the water shell after adsorption at the bilayer surface. As a result of strong hydration, the polymer chains reside at the phospholipid headgroup and do not penetrate to the acyl chain region. The polycation adsorption involves the formation of anionic lipid-rich domains, and the density of anionic lipids in these domains depends on the length of the polycation chain. We observed the accumulation of anionic lipids only in the leaflet interacting with the polymer, which leads to the formation of compositionally asymmetric domains. Asymmetric adsorption of the polycation on only one leaflet of the anionic membrane strongly affects the membrane properties in the polycation–membrane contact areas: (i) anionic lipid accumulates in the region near the adsorbed polymer, (ii) acyl chain ordering and lipid packing are reduced, which results in a decrease in the thickness of the bilayer, and (iii) polycation–anionic membrane interactions are strongly influenced by the presence and concentration of salt. Our results provide an atomic-scale description of the interactions of polycations with anionic lipid bilayers and are fully supported by the experimental data. The outcomes are important for understanding the correlation of the structure of polycations with their activity on biomembranes.

Octenidine: Novel insights into the detailed killing mechanism of Gram-negative bacteria at a cellular and molecular level

Octenidine (OCT) is a widely used antiseptic molecule with an antimicrobial spectrum covering a broad range of bacteria and fungi. However, the detailed molecular mechanism of killing has not yet been elucidated. The objective of our study was to investigate the mode of action of OCT's potent effect on Gram-negative bacteria using Escherichia coli as a model organism as well as corresponding model membranes. The effects of OCT on cellular morphology were observed by electron microscopy, changes affecting membrane integrity (surface charge, fluidity, permeabilisation and depolarisation) by zeta potential, fluorescence microscopy and spectroscopy. Specific interactions of OCT with membrane phospholipids were addressed using differential scanning calorimetry, X-ray scattering and fluorescence techniques. OCT neutralises the surface charge of E. coli leading to disruption of the outer membrane and dramatic loss of the cell wall and further penetrates through the periplasmic space reaching the inner membrane. Model membranes showed that OCT inserts into the hydrophobic fatty acyl chain region of the bilayer, inducing complete lipid disorder. The loss of membrane integrity is also reflected by membrane depolarisation and changes in membrane fluidity as shown by electron microscopy. Insertion of OCT into the outer and inner membrane of E. coli results in a chaotic lipid arrangement that leads to rapid disruption of the cell envelope. We propose that this unspecific mode of action based on purely physical interactions is the basis of the very broad antimicrobial profile and makes it unlikely that resistance to OCT will develop.

Interactions of anticancer drugs doxorubicin and idarubicin with lipid monolayers: New insight into the composition, structure and morphology

We quantify directly here for the first time the extents of interactions of two different anthracycline drugs with pure and mixed lipid monolayers with respect to the surface pressure and elucidate differences in the resulting interaction mechanisms. The work concerns interactions of doxorubicin (DOx) and idarubicin (IDA) with monolayers of the zwitterionic DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) and negatively charged DMPS (1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (sodium salt)) as well as a 7:3 mixture of the two lipids. These drugs are used in current cancer treatments, while the lipid systems were chosen as phosphocholines are the major lipid component of healthy cell membranes, and phosphoserines are the major lipid component that is externalized into the outer leaflet of cancerous cell membranes. It is shown that DOx interacts with DMPS monolayers to a greater extent than with DMPC monolayers by lower limits of a factor of 5 at a surface pressure of 10 mN/m and a factor of 12 at 30 mN/m. With increasing surface pressure, the small amount of drug (~0.3 µmol/m2) bound to DMPC monolayers is excluded from the interface, yet its interaction with DMPS monolayers is enhanced until there is even more drug (~3.2 µmol/m2) than lipid (~2.6 µmol/m2) at the interface. Direct evidence is presented for all systems studied that upon surface area compression lipid is reproducibly expelled from the monolayer, which we infer to be in the form of drug-lipid aggregates, yet the nature of adsorption of material back to the monolayer upon expansion is system-dependent. At 30 mN/m, most relevant to human physiology, the interactions of DOx and IDA are starkly different. For DOx, there is a conformational change in the interfacial layer driven by aggregation, resulting in the formation of lateral domains that have extended layers of drug. For the more lipophilic IDA, there is penetration of the drug into the hydrophobic acyl chain region of the monolayer and no indication of lateral segregation. In addition to the Langmuir technique, these advances were made as a result of direct measurements of the interfacial composition, structure and morphology using two different implementations of neutron reflectometry and Brewster angle microscopy. The results provide new insight into key processes that determine the uptake of drugs such as limited drug penetration through cell membranes by passive diffusion as well as activation of drug removal mechanisms related to multidrug resistance.