Membrane Surface Charge Density Research Articles

Physicochemical and electrical properties of DPPC bilayer membranes in the presence of oleanolic or asiatic acid

This study aimed to investigate the effect of selected compounds from the group of triterpene sapogenins on model phosphatidylcholine membranes. Two types of biological membrane model systems were used in the work, i.e., liposomes (microelectrophoresis method) and spherical bilayers (interfacial tension method). Each model was modified with the tested sapogenin compounds, and the change in their physicochemical and electrical parameters was analyzed. Parameters characterizing the equilibrium in the membrane of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)-oleanolic acid (OA) and DPPC-asiatic acid (AA) were determined). Based on the Young-Laplace equation, the interfacial tensions of spherical lipid bilayers were measured. The formation of 1:1 complexes was assumed in the DPPC-OA and DPPC-AA membrane systems, and the parameters characterizing the interactions in the formed complexes were calculated. Microelectrophoresis was used to study the surface charge density of lipid membranes. These values were obtained from electrophoretic mobility data using Smoluchowsky’s equation. The influence of pH on the electrolyte solution and the composition of the membranes was investigated. The results indicate that modifying DPPC membranes with selected triterpene sapogenins, both OA and AA, causes changes in the surface charge density and shifts of the isoelectric point. Data presented in this work, obtained through mathematical derivation and confirmed experimentally, are of great importance for interpreting phenomena occurring in lipid membranes. A quantitative description of equilibria between phosphatidylcholine and sapogenins lets us understand the processes on the membrane surface. The equilibria are particularly significant from the standpoint of cell functioning. Phosphatidylcholine-sapogenin interactions modulate a range of physicochemical properties of membranes, and they are important in the course of the multiple processes involving membranes in the living cell (e.g., transport mechanism).

Beta-Cyclodextrin-Modified Covalent Organic Framework Nanochannel for Electrochemical Chiral Recognition of Amino Acids.

The chiral recognition and separation of enantiomers are of great importance for biological research and the pharmaceutical industry. Preparing homochiral materials with adjustable size and chiral binding sites is beneficial for achieving an efficient chiral recognition performance. Here, a homochiral covalent organic framework membrane modified with β-cyclodextrin (CD-COF) was constructed, which was subsequently utilized as an electrochemical sensor for the enantioselective sensing of tryptophan (Trp) molecules. The preferential adsorption of l-Trp over d-Trp at the β-CD sites can enhance the surface charge density and hydrophilicity of the CD-COF membrane, resulting in an increased transmembrane ionic current. Trp enantiomers with concentrations down to 0.28 nM can be effectively discriminated. The l-/d-Trp recognition selectivity increases with the Trp concentration and reaches a value of 19.2 at 1 mM. The selective adsorption of l-Trp to the CD-COF membrane will also hinder its transport, resulting in a l-/d-Trp permeation selectivity of 15.3. This study offers a new strategy to construct homochiral porous membranes and achieve efficient chiral sensing and separation.

Mussel-inspired co-deposition of catechol-amine and graphene oxide (GO) modified anion exchange membranes (AEMs) for antifouling

Mussel-inspired co-deposition has many applications in surface modification of polymer membranes. Organic fouling of ion exchange membranes is one of the common issues that hinder electrodialysis treatment of industrial wastewater. In this work, anion exchange membranes were surface-modified by the mussel-inspired co-deposition of catechol-amine and graphene oxide (GO) nanosheets. Results showed that co-deposition with catechol (CA) and m-phenylenediamine (MPD) as reactant monomers rapidly generated modifying layer on the AEM surface. And the addition of GO into co-deposition helped improve the homogeneity and reduce polymer particle aggregation. The synergistic effect of catechol-amine and GO nanosheets was beneficial to form uniform modifying layer with improved hydrophilicity and negative charge density of membrane surface. In eletrodialysis experiments with sodium dodecyl benzene sulfonate (SDBS) as model foulants, GO-CA-MPD-3h modified AEM maintained high desalination rate close to that without folants, indicating good modifying antifouling performance. Mussel-inspired co-deposition of catecholamines and functional materials give new insights into membrane modificatin through a feasible and cost-effective method

High-performance and ultra-robust triboelectric nanogenerator based on hBN nanosheets/PVDF composite membranes for wind energy harvesting

Triboelectric nanogenerator (TENG) is a novel energy technology that converts high-entropy mechanical energy from the environment into electrical energy using triboelectrification and electrostatic induction effects. However, the low surface charge density and easy wear of traditional triboelectric materials are bottlenecks for their practical applications. Here, a remarkable triboelectric properties, superior mechanical properties, exceptional wear resistance, and high thermal conductivity hexagonal boron nitride nanosheets (hBNNS)/ polyvinylidene difluoride (PVDF) composite negative triboelectric material is developed. The inclusion of hBNNS as an electron acceptor and nucleating agent not only effectively boosted the surface charge density (711 μC/m2) and mechanical characteristics of the composite membrane, but also the friction coefficient (COF) and thermal conductivity of the 2 wt% hBNNS/PVDF composite membrane decreased by 28.6 % and increased by 22.2 % compared to the PVDF matrix, respectively. The optimized TENG delivers a peak voltage of 434 V, a current of 53 μA, and a power of 4.84 mW. Furthermore, it exhibits exceptional ultra-robust, enabling continuous operation for 72 h at a wind speed of 17.5 m/s while maintaining performance above 90.6 %. Additionally, the TENG is capable of effectively powering a smart hygrothermograph and realizing wireless real-time monitoring, or directly lit up 102 white LEDs in a serial connection. This work addresses the challenges of low output performance and limited lifespan in TENG from a material perspective, providing a valuable reference method for the design of negative triboelectric materials with high surface charge density, good mechanical properties, low COF, and high thermal conductivity.

The influence of divalent ions on the osmotic energy conversion performance of 2D cation exchange membrane in reverse electrodialysis process

As reverse electrodialysis (RED) technology evolves, achieving high-performance osmotic energy conversion (OEC) necessitates porous membranes with exceptional ion selectivity and permeability. Two-dimensional membranes crafted from graphene oxide (GO) or MXene stand out for their immensely high surface charges, which facilitate superb ion selectivity during transport. Their unique layered stacking structure further yields ultra-small nanochannel radii, bolstering ion selectivity while maintaining high permeability. This makes them prime candidates for osmotic energy conversion. However, the presence of divalent ions in natural seawater readily alters the surface charge density and interlayer spacing of these 2D membranes, significantly impacting their OEC performance, resulting in a power density reduction of about 6 % -10 %. Prior research focused on the ionic properties of divalent ions but overlooked their impact on the nanochannel structure, including changes in interlayer spacing and surface charge density. This study thus delves into the OEC performance variations of GO and MXene 2D layered membranes under varying divalent ion concentrations and salt ratios. Starting from the ion properties of divalent ions and their impact on the nanochannel structure, combined with simulation research, explain their mechanism of action. This research offers theoretical foundations for the future design and application of 2D layered membranes, providing novel solutions and insights to tackle critical challenges like enhancing power density and adapting to real seawater conditions.

Determination of pore edge tension from the kinetics of rupture of giant unilamellar vesicles using the Arrhenius equation: effects of sugar concentration, surface charge and cholesterol.

The pore edge tension (Γ) of a membrane closely intertwines with membrane stability and plays a vital role in the mechanisms that facilitate membrane resealing following pore formation caused by electrical and mechanical tensions. We have explored a straightforward procedure to determine Γ by fitting the inverse of the tension-dependent logarithm of the rate constant of rupture of giant unilamellar vesicles (GUVs) using the Arrhenius equation. The GUVs were prepared using a combination of 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DOPG) and 1, 2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) in a physiological environment. The effects of sugar concentration, membrane surface charge density, and membrane cholesterol concentration on Γ have been investigated. The values of Γ increase with sugar concentration in the physiological buffer, measuring 9.6 ± 0.3, 10.4 ± 0.1, and 16.2 ± 0.1 pN for 40, 100, and 300 mM, respectively. A higher concentration of anionic lipids (70 mol% of DOPG) significantly reduces Γ. An increasing trend of Γ with cholesterol content was observed; specifically, the values of Γ were 11.9 ± 0.9, 13.9 ± 0.7, and 16.2 ± 0.4 pN for 15, 29, and 40 mol% cholesterol, respectively. Thus, the presence of higher anionic lipids in the bilayer led to a decrease in membrane stability. In contrast, the presence of higher sugar concentrations in the buffer and increased cholesterol concentration in the membranes enhanced membrane stability.

Advanced electrokinetic characterization of layer-by-layer assembled polyelectrolyte membranes: Consideration and use of the electrokinetic leakage phenomenon

The sensitivity of zeta potential to the sign of the surface charge of membranes makes it an interesting parameter for the characterization of layer-by-layer (Lbl) modified polyelectrolytes membranes. However, during tangential electrokinetic measurements, a non-negligible parasitic phenomenon referred to as “electrokinetic leakage” may occur. If not taken into account, this phenomenon can lead to dramatic quantitative and qualitative errors, especially when it comes to characterize the surface charge of layer-by-layer assembled polyelectrolyte membranes. In this work, we show that it can lead to substantial errors when interpreting experimental data, including misinterpreting the sign of the membrane surface charge density. An advanced protocol based on the measurement of the streaming current by varying the spacing between the two samples needed for the measurements allows to (i) correct the raw data for the electrokinetic leakage phenomenon and thus to correctly and accurately determine the zeta potential of the membrane surface and (ii) to detect whether polyelectrolytes are deposited only on the membrane surface or whether they can also penetrate into the membrane pores. The approach followed and the conclusions drawn in this study can be straightforwardly applied to any kind of membrane functionalization.

Presence of bicontinuous microemulsion-type domains and dielectrically inert interfacial water layers in lamellar gel-stabilized oil-in-water emulsions

HypothesisLamellar gels are widely formulated in household and cosmetic products because of their eminent ability to improve long-term stability of thermodynamically unstable oil-in-water emulsions. However, despite long study, how and why membrane internal structure and membrane-membrane interactions are modified by the presence of polar and nonpolar oils remains elusive. ExperimentsUsing small- and wide-angle X-ray scattering, dielectric spectroscopy, and field-emission transmission electron microscope, we investigate intermembrane interactions and water-mediated microscopic interfacial properties in lamellar gels and lamellar gel-stabilized oil-in-water emulsions based on cetyltrimethylammonium chloride and 1-hexadecanol. FindingsReducing membrane surface charge density enhances undulation fluctuation disorder, resulting in a crossover of dominant interactions from electrostatic double-layer repulsion to Helfrich interaction. Oil-emulsification induces similar structural impacts to the reduced 1-hexadecanol ratio, confirming preferential dissolution of higher-alcohol in oil phases. An emerging Teubner-Stray scattering component upon emulsification of nonpolar oil evidences that oil droplets and lamellar gels are indirectly connected via bicontinuous microemulsion-type domains. Dielectric spectra reveal strikingly small water permittivity in the lamellar gel and emulsion samples, which is quantitatively explained by a cumulative effect of a dielectrically inert interfacial thin water layer (<1nm) and a highly polarizable bulk-like water layer. This phenomenon appears to be intrinsic to diverse lamellar stack architectures.

Charged Lipids Modulate the Phase Separation in Multicomponent Membranes.

Phase separation in lipid membranes controls the organization of membrane components and thus regulates membrane-mediated processes. Membrane phase behavior is influenced by the molecular properties of its components and their relative concentrations. Charged lipid species are among the most essential components of lipid membranes, and their impact on the membrane phase behavior is yet to be fully understood. Aiming to provide insight into this impact, this paper investigates how the presence and amount of anionic and cationic lipids affect the phase behavior of multicomponent membranes. Membranes of ternary composition DOPC:DPPC:Chol with two distinct molar ratios were used to test the hypothesis that inclusion of charged lipids with saturated tails, beyond a certain concentration, would impede phase separation in an otherwise phase-separating membrane. Fluorescence microscopy examination of electroformed giant liposomes revealed that when more than half of DOPC in the examined mixtures was replaced with DOPA or DOTAP, phase separation in liposomes was somewhat suppressed, and this effect increased with increasing charged lipid content. This effect depended on the membrane surface charge density as the half-maximal effect was observed at around 0.0072 C Å-2 in all examined cases. The phase-separation suppressing effect of DOPA was neutralized when oppositely charged lipid DOTAP was included in the mixture. Likewise, presence of divalent cation Ca2+ in the solution neutralized the impact of negatively charged DOPA. These results underline the detrimental influence of surface charge density on membrane phase behavior. More importantly, these findings suggest that the charged lipid content in membranes may be a regulator of their phase behavior and open new opportunities for the design of synthetic lipid membranes.

An interlayer-based positive charge compensation strategy for the preparation of highly selective Mg2+/Li+ separation nanofiltration membranes

The Mg2+/Li+ screening selectivity determines the practical application of nanofiltration technology in lithium extraction from salt-lake brines and industrial lithium production. In this work, a membrane structure design strategy based on the interlayers was developed to prepare nanofiltration (NF) membranes with high Mg2+/Li+ separation selectivity and high permeability. Through investigating the influence of the interlayer charge on the structure and performance of the NF membrane, the interlayer for the preparation of the NF membrane with high Mg2+/Li + separation selectivity was screened. Relevant characterizations indicate that the positively charged interlayer can endow the NF membrane with appropriate pore size and surface charge density to achieve high Mg2+/Li+ separation selectivity through the synergistic effect of size sieving and Donnan exclusion. Moreover, the pore size of the NF membrane is controlled by the release rate of the piperazine (PIP) from the interlayer, while the surface charge density of the NF membrane is jointly determined by the charge of the interlayer and PA layer. The performance evaluation suggests that the NF membrane prepared with the best screened interlayer has a Mg2+/Li+ separation factor up to 88.6 while maintaining a high permeability (22.5 L m−2 h−1·bar−1), which is better than that of the most membranes reported in the literature. Overall, this work is expected to promote the practical application of NF membranes in lithium extraction from salt-lake brines and industrial lithium production.

Membrane composition drives sidechain ionization and assembly of transmembrane protein domains.

Ionization states of amino acid residues play significant roles in membrane protein assembly and function; however, they are difficult to decipher experimentally. The analysis is further complicated by the dearth of data on gradients in polarity, electric potentials, and hydration at the protein-membrane interface. Here we examine how electrostatic interactions, suggested to be essential for T-cell receptor (TCR) membrane assembly, could be manipulated by modifying the membrane composition. Novel pH-sensitive ionizable EPR labels were employed to profile heterogeneous dielectric environment along the transmembrane protein-lipid interfaces. Transmembrane α-helical WALP peptide was used for obtaining the profile of effective pK(a) as a function of membrane depth. It was found that effective pK(a) of membrane-buried sidechains can be significantly shifted by varying the membrane surface charge density. A peptide corresponding to the transmembrane domain of TCRα was labeled with pH-sensitive nitroxide and incorporated into liposomes. EPR of this label reported on the sidechain protonation state, membrane insertion, and association of the helices within the membrane. It was found that an increase in negative change density at the membrane surface alters the protonation state of membrane-buried ionizable sidechain in the transmembrane domain of TCRα. Hyperfine Correlation Spectroscopy (HYSCORE) was used to assess effects of sidechain ionization state on water penetration at membrane-peptide interface while Double Electron-Electron Resonance (DEER) quantified the domain agglomeration. Turning the charge of the sidechain “off” increases tendency of the transmembrane domain to agglomerate - a phenomenon that could be critical for the TCR assembly and its degradation. Overall, EPR of ionizable probes is informative for uncovering mechanisms as to how the changes in local environment along the protein-lipid interface, can result in a “tunable” pKa of the ionizable sidechains and drive the structural changes in membrane proteins.

Characterization of the surface charge property and porosity of track-etched polymer membranes.

As an important property of porous membranes, the surface charge property determines many ionic behaviors of nanopores, such as ionic conductance and selectivity. Based on the dependence of electric double layers on bulk concentrations, ionic conductance through nanopores at high and low concentrations is governed by the bulk conductance and surface charge density, respectively. Here, through the investigation of ionic conductance inside track-etched single polyethylene terephthalate (PET) nanopores under various concentrations, the surface charge density of PET membranes is extracted as ∼-0.021C/m2 at pH 10 over measurements with 40 PET nanopores. Simulations show that surface roughness can cause underestimation in surface charge density due to the inhibited electroosmotic flow. Then, the averaged pore size and porosity of track-etched multipore PET membranes are characterized by the developed ionic conductance method. Through coupled theoretical predictions in ionic conductance under high and low concentrations, the averaged pore size and porosity of porous membranes can be obtained simultaneously. Our method provides a simple and precise way to characterize the pore size and porosity of multipore membranes, especially for those with sub-100nm pores and low porosities.

An efficient finite element iterative method for solving a nonuniform size modified Poisson-Boltzmann ion channel model

In this paper, a nonuniform size modified Poisson-Boltzmann ion channel (nuSMPBIC) model is presented as a nonlinear system of an electrostatic potential and multiple ionic concentrations. It mixes nonlinear algebraic equations with a Poisson boundary value problem involving Dirichlet-Neumann mixed boundary value conditions and a membrane surface charge density to reflect the effects of ion sizes and membrane charges on electrostatics and ionic concentrations. To overcome the difficulties of strong singularities and exponential nonlinearities, it is split into three submodels with a solution of Model 1 collecting all the singular points and Models 2 and 3 much easier to solve numerically than the original nuSMPBIC model. A damped two-block iterative method is then presented to solve Model 3, along with a novel modified Newton iterative scheme for solving each related nonlinear algebraic system. To this end, an effective nuSMPBIC finite element solver is derived and then implemented as a program package that works for an ion channel protein with a three-dimensional molecular structure and a mixture of multiple ionic species. Numerical results for a voltage-dependent anion channel (VDAC) in a mixture of four ionic species demonstrate a fast convergence rate of the damped two-block iterative method, the high performance of the software package, and the importance of considering nonuniform ion sizes. Moreover, the nuSMPBIC model is validated by the anion selectivity property of VDAC.

Quaternary ammonium engineered polyamide membrane with high positive charge density for efficient Li+/Mg2+separation

The efficient separation of Li+/Mg2+ is the key to lithium extraction from salt lake brine. Herein, a multi-amino quaternary ammonium salt (3,5-dimethylhydrazide-benzyl trimethyl ammonium bromide, DHTAB) was designed and synthesized to modify pristine polyamide (PA) nanofiltration membranes. DHTAB was grafted on the surface of the PA nanofiltration membrane to construct a positively selective layer for Li+/Mg2+ separation. The introduction of DHTAB increased the membrane surface charge density to +5.16 mC m−2, which was nearly three times that of the pristine PA membrane. The increased charge density of the modified membrane led to a stronger Donnan exclusion for Mg2+, and accordingly a high MgCl2 rejection of 99.2%. The modified membranes exhibited a high Li+/Mg2+ separation factor of up to 60.1, when applied for MgCl2 and LiCl binary solution, which exceeded all known PA-based nanofiltration membranes.

Analysis of Ionic Domains on a Proton Exchange Membrane Using a Numerical Approximation Model Based on Electrostatic Force Microscopy.

Understanding the ionic channel network of proton exchange membranes that dictate fuel cell performance is crucial when developing proton exchange membrane fuel cells. However, it is difficult to characterize this network because of the complicated nanostructure and structure changes that depend on water uptake. Electrostatic force microscopy (EFM) can map surface charge distribution with nano-spatial resolution by measuring the electrostatic force between a vibrating conductive tip and a charged surface under an applied voltage. Herein, the ionic channel network of a proton exchange membrane is analyzed using EFM. A mathematical approximation model of the ionic channel network is derived from the principle of EFM. This model focusses on free charge movement on the membrane based on the force gradient variation between the tip and the membrane surface. To verify the numerical approximation model, the phase lag of dry and wet Nafion is measured with stepwise changes to the bias voltage. Based on the model, the variations in the ionic channel network of Nafion with different amounts of water uptake are analyzed numerically. The mean surface charge density of both membranes, which is related to the ionic channel network, is calculated using the model. The difference between the mean surface charge of the dry and wet membranes is consistent with the variation in their proton conductivity.

Effect of Selected Anionic and Cationic Drugs Affecting the Central Nervous System on Electrical Properties of Phosphatidylcholine Liposomes: Experiment and Theory.

Interactions between phospholipid membranes and selected drugs affecting the central nervous system (CNS) were investigated. Small, unilamellar liposomes were used as biomimetic cell membrane models. Microelectrophoretic experiments on two-component liposomes were performed using the electrophoretic light scattering technique (ELS). The effect of both positively (perphenazine, PF) and negatively (barbituric acid, BA) charged drugs on zwitterionic L-α-phosphatidylcholine (PC) membranes were analyzed. Experimental membrane surface charge density (δ) data were determined as a function of pH. Quantitative descriptions of the adsorption equilibria formed due to the binding of solution ions to analyzed two-component membranes are presented. Binding constants of the solution ions with perphenazine and barbituric acid-modified membranes were determined. The results of our research show that both charged drugs change surface charge density values of phosphatidylcholine membranes. It can be concluded that perphenazine and barbituric acid are located near the membrane surface, interacting electrostatically with phosphatidylcholine polar heads.

Switchable ionic selectivity of membranes with electrically conductive surface: Theory and experiment

Nanoporous membranes with electrically conductive surface represent an important class of stimuli–responsive materials. The variation of surface potential provides a powerful tool for adjusting ionic selectivity, conductivity, and rejection. This work is devoted to the theoretical and experimental study of switchable ionic selectivity. The one–dimensional Space charge (SC) and two–dimensional Uniform potential (UP) models are first generalized to constant surface potential case taking into account the Stern layer with inner (iHp) and outer (oHp) Helmholtz planes. The ionic selectivity is investigated experimentally by measuring the membrane potential at zero current for C–Nafen membranes prepared from alumina nanofibers with conductive carbon coating. The evolution of charging current is used to determine the dependence of surface charge density and differential capacitance on the applied potential. These data are fitted by the UP and SC models to find the Stern layer capacitance. It is shown that the variation of surface potential results in a continuous change of ionic selectivity from anion to cation. The membrane potential data are fitted by the UP and SC models using the chemical charge density and concentration boundary layer thickness as fitting parameters. It allows to determine the potential, at which the membrane becomes non–selective. The SC and UP models provide close results for membrane potential and surface charge density and demonstrate a good agreement with the experimental data. The UP model overestimates the solution velocity and ion concentrations at the membrane surface, while it underestimates the ion fluxes and iHp/oHp potentials. This work essentially extends our understanding of ion transport in stimuli–responsive membranes operated by the electric field. The results can be applied in the area of nanofiltration, (reverse) electrodialysis, electrochemical sensors, and nanofluidic devices.

Robust antifouling anion exchange membranes modified by graphene oxide (GO)-enhanced Co-deposition of tannic acid and polyethyleneimine

Three polyphenol-polyamine co-deposition systems containing different polyphenols (catechol (CA), gallic acid (GA) and tannic acid (TA)) and Polyethyleneimine (PEI) were established and used to construct adhesive layer on the surface of anion exchange membranes (AEMs). And graphene oxide (GO) was introduced into co-depositions to construct an antifouling modifying layer. The modified AEMs were characterized and applied in electrodialysis for antifouling experiments. The results showed that the TA-PEI-M with a large depositing amount exhibited more hydrophilic surface than those of CA-PEI-M and GA-PEI-M, but still low value in the negative charge density. The introduction of GO increased the negative charge density of membrane surface and endowed AEMs with enhanced antifouling ability significantly. Furthermore, GO-TA-PEI-M could maintain surface properties and antifouling performance after immersing in NaOH solutions, indicating good alkaline-stability. It was attributed to that the abundant phenolic groups in TA played a vital role in the formation of co-deposition layer. The abundant phenolic groups in TA could provide stronger adhesion and multiple reactive sites to generate more stable covalent structures with PEI. In addition, the hydrogel products produced by the electrostatic interaction of TA and PEI helped increase the depositing amount. This work proposed a novel approach to fabricate surface-modified antifouling AEMs with alkaline stability by polyphenol-polyamine co-deposition.

Membrane Binding of Antimicrobial Peptides Is Modulated by Lipid Charge Modification.

Peptide interactions with lipid bilayers play a key role in a range of biological processes and depend on electrostatic interactions between charged amino acids and lipid headgroups. Antimicrobial peptides (AMPs) initiate the killing of bacteria by binding to and destabilizing their membranes. The multiple peptide resistance factor (MprF) provides a defense mechanism for bacteria against a broad range of AMPs. MprF reduces the negative charge of bacterial membranes through enzymatic conversion of the anionic lipid phosphatidyl glycerol (PG) to either zwitterionic alanyl-phosphatidyl glycerol (Ala-PG) or cationic lysyl-phosphatidyl glycerol (Lys-PG). The resulting change in the membrane charge is suggested to reduce the binding of AMPs to membranes, thus impeding downstream AMP activity. Using coarse-grained molecular dynamics to investigate the effects of these modified lipids on AMP binding to model membranes, we show that AMPs have substantially reduced affinity for model membranes containing Ala-PG or Lys-PG. More than 5000 simulations in total are used to define the relationship between lipid bilayer composition, peptide sequence (using five different membrane-active peptides), and peptide binding to membranes. The degree of interaction of a peptide with a membrane correlates with the membrane surface charge density. Free energy profile (potential of mean force) calculations reveal that the lipid modifications due to MprF alter the energy barrier to peptide helix penetration of the bilayer. These results will offer a guide to the design of novel peptides, which addresses the issue of resistance via MprF-mediated membrane modification.

Calculating Ceramic NF Membrane Surface Charge

The main objective in this work was to determine the active membrane surface fixed charge; through the zeta potential for different salt types at different concentrations and pH values. In order to provide a relation between the membranes active surface charge (Xm) and the ion type and concentration. The membrane surface charge density (σs) was estimated by using the simplified Grahame equation and the Gouy-Chapman equation, using the measure zeta potential. Afterwards the membranes active surface charge (Xm) was calculated.