Langmuir Film Balance Research Articles

Role of Endogenous Ingredients in Meibum and Film Structures on Stability of the Tear Film Lipid Layer against Lateral Compression.

The property and structure of spread films of meibum extruded from rabbit eyelids and its fractions were investigated using the Langmuir film balance technique and Brewster angle microscopy in order to understand the influence of endogenous ingredients in meibum on the structure and stability of the tear film lipid layer against mechanical stimulus. Surface pressure (?)?film area ( A) isotherms for meibum were measured upon repetitive high-speed compression?expansion cycles and were found almost identical to each other with very small hysteresis, indicating the high stability of the meibum film. Brewster angle microcopy observation implied the spontaneous formation of condensed-phase network structures which consist primarily of wax esters and cholesteryl esters as nonpolar ingredients, coexisting with a monolayer phase of polar lipids two-dimensionally confined by the networks, which were spontaneously formed in the meibum film. The networks were gathered densely and deformed when the film was laterally compressed with barriers of Langmuir trough, but returned to the dispersed networks when expanded. The influence of temperature and salts dissolved in an aqueous subphase was also investigated. The results indicated that the temperature change (20 and 35 ?C) induced a difference of surface pressure at the same film areas in rather compressed films, and the presence of salts in the subphase expanded the films. However, the features of isotherm and surface morphology of the film, including their reversibility, were maintained. Phospholipid-removed meibum also formed a stable film, but slight changes were found in the hysteresis and film morphology compared to those in the meibum film. In contrast, in a film of phospholipid- and cholesterol-removed meibum, three-dimensional aggregates grew upon the first compression and not redispersed by the subsequent expansion, giving noticeable hysteresis between the isotherms. It is considered that high deformability upon compression and resilience upon expansion of the networks as well as reversible collapse and spreading property of the confined monolayer phase would hold the stability of the meibum film against repeated compressions.

Interpolymer interaction in insulin-chitosan complexes

ABSTRACTOur team conducted theoretical calculations and experimental studies of interpolymer interactions in insulin-chitosan complexes based on the spatial orientation of chitosan and insulin macromolecules using Langmuir film balance at various pH values. The most optimal conditions for interpolymer interactions between insulin and chitosan chains occur at pH value of 5.4, which corresponds to the isoelectric point of insulin. For the similar pH values of 4.2 and 6.0, the free energy of coordination between chitosan and insulin was estimated by molecular dynamic simulations as of 1.1 and 5.8 kcal mol−1, respectively.

Effects of spreading and subphase conditions on the interfacial behavior of an amphiphilic copolymer poly(n-butylacrylate)-b-poly(acrylic acid)

Aggregation behavior of an amphiphilic diblock polyelectrolyte poly(n-butylacrylate)-b-poly(acrylic acid) (PnBA-b-PAA) at the air/water interface and its Langmuir−Blodgett (LB) films were respectively characterized by the Langmuir film balance technique and atomic force microscopy. Effects of spreading solution concentration and volume, and subphase temperature and ionic strength on the interfacial aggregation behavior of PnBA-b-PAA under alkaline condition were systematically studied. With the increase of spreading concentration, surface pressure−molecular area isotherms shift to small molecular areas at the corresponding spreading volumes. With the rise in subphase temperature, the isotherms shift to small molecular areas due to the decreased pKa value and increased solubility of PAA blocks. With the increase of subphase ionic strength, the isotherms shift to large molecular areas because of the predominant salting out effect. The LB films transferred from alkaline subphase with different ionic strengths are more likely to exhibit isolated circular micelles. With the increase of spreading concentration, diameters of micelle cores first decrease and then increase, a behavior which is attributed to the gradually increased chain entanglements and surface polymer concentration, respectively. For small spreading volume, core diameters in the LB films first decrease and then increase with the rise in subphase temperature. For large spreading volume, however, core diameters increase with temperature due to the higher mobility and weaker entanglements of PAA blocks underwater. With the increase of subphase ionic strength, furthermore, core diameters first increase and then decrease, which is attributed to the predominant electrostatic shielding and salting out effects, respectively.

Influence of Membrane-Fusogen Distance on the Secondary Structure of Fusogenic Coiled Coil Peptides.

Liposomal membrane fusion is an important tool to study complex biological fusion mechanisms. We use lipidated derivatives of the specific heterodimeric coiled coil pair E: (EIAALEK)3 and K: (KIAALKE)3 to study and control the fusion of liposomes. In this model system, peptides are tethered to their liposomes via a poly(ethylene glycol) (PEG) spacer and a lipid anchor. The efficiency of the fusion mechanism and function of the peptides is highly affected by the PEG-spacer length and the lipid anchor type. Here, the influence of membrane–fusogen distance on the peptide–membrane interactions and the peptide secondary structures is studied with Langmuir film balance and infrared reflection absorption spectroscopy. We found that the introduction of a spacer to monolayer-tethered peptide E changes its conformation from solvated random coils to homo-oligomers. In contrast, the described peptide–monolayer interaction of peptide K is not affected by the PEG-spacer length. Furthermore, the coexistence of different conformations when both lipopeptides E and K are present at the membrane surface is demonstrated empirically, which has many implications for the design of effective fusogenic recognition units and the field of artificial membrane fusion.

DNA Delivery Systems Based on Peptide-Mimicking Cationic Lipids-The Effect of the Co-Lipid on the Structure and DNA Binding Capacity.

In continuation of previous work, we present a new promising DNA carrier, OO4, a highly effective peptide-mimicking lysine-based cationic lipid. The structural characteristics of the polynucleotide carrier system OO4 mixed with the commonly used co-lipid DOPE and the saturated phospholipid DPPE have been studied in two-dimensional and three-dimensional model systems to understand their influence on the physical–chemical properties. The phase behavior of pure OO4 and its mixtures with DOPE and DPPE was studied at the air–water interface using a Langmuir film balance combined with infrared reflection-absorption spectroscopy. In bulk, the self-assembling structures in the presence and absence of DNA were determined by small-angle and wide-angle X-ray scattering. The amount of adsorbed DNA to cationic lipid bilayers was measured using a quartz crystal microbalance. The choice of the co-lipid has an enormous influence on the structure and capability of binding DNA. DOPE promotes the formation of nonlamellar lipoplexes (cubic and hexagonal structures), whereas DPPE promotes the formation of lamellar lipoplexes. The correlation of the observed structures with the transfection efficiency and serum stability indicates that OO4/DOPE 1:3 lipoplexes with a DNA-containing cubic phase encapsulated in multilamellar structures seem to be most promising.

A comparative study on the effect of structural variations of co-solvent on the Tetronic® micelles with widely different hydrophobicity

Here we report, a comparative study on the micellization and phase behavior of two commercially available ethylene oxide – propylene oxide (EO-PO) octablock copolymers, Tetronics® 1304 and 1307 (hereafter written as T1304 and T1307), in the presence of 2,2,2‑trifluoroethanol (TFE) and ethanol (EtOH) using cloud point (CP), dynamic light scattering (DLS), high sensitivity differential scanning calorimetry (HSDSC), relative viscosity and Langmuir film balance. These copolymers fabricated with almost the same PO block length vary in their EO chain lengths and hence display different characteristics. T1304 shows a more hydrophobic character and forms micelles at ambient temperature while T1307, due to longer EO chain length, exhibits high hydrophilicity and faces difficulty in forming micelles which results into high polydispersity in the aqueous system. In the presence of co-solvents TFE and EtOH, however a more interesting behavior is noted. T1304, as evident from DLS and viscosity measurements, displays micellar growth in the presence of TFE, while the presence of EtOH induces micellar breakdown. Though, in case of T1307, presence of TFE improves micellization and a single micellar peak becomes evident in the DLS stacks, no micellar growth can be noticed both through DLS and relative viscosity, which is possibly due to its highly hydrophilic character. Likewise, presence of TFE depresses the CMT in both the copolymers while EtOH increases it. A similar trend is also justified by π-A isotherms form Langmuir monolayers.

Nonlinear Viscoelasticity of Highly Ordered, Two-Dimensional Assemblies of Metal Nanoparticles Confined at the Air/Water Interface.

In this study, we investigated the viscoelastic properties of metal nanoparticle monolayers at the air/water interface by dilational rheology under periodic oscillation of surface area. Au nanoparticles capped with oleylamine form a stable, dense monolayer on a Langmuir film balance. The stress response function of a nanoparticle monolayer was first analyzed using the classical Kelvin-Voigt model, yielding the spring constant and viscosity. The obtained results suggest that the monolayer of nanoparticles is predominantly elastic, forming a two-dimensional physical gel. As the global shape of the signal exhibited a clear nonlinearity, we further analyzed the data with the higher modes in the Fourier series expansion. The imaginary part of the higher mode signal was stronger than the real part, suggesting that the dissipative term mainly causes the nonlinearity. Intriguingly, the response function measured at larger strain amplitude became asymmetric, accompanied by the emergence of even modes. The significance of interactions between nanoparticles was quantitatively assessed by calculating the potential of mean force, indicating that the lateral correlation could reach up to the distance much larger than the particle diameter. The influence of surface chemical functions and core metal has also been examined by using Au nanoparticles capped with partially fluorinated alkanethiolate and Ag nanoparticles capped with myristic acid. The combination of dilational rheology and correlation analyses can help us precisely control two-dimensional colloidal assembly of metal nanoparticles with fine-adjustable localized surface plasmon resonance.

Physicochemical characterization of colored soluble protein fractions extracted from Spirulina ( Spirulina platensis).

The aim of this study was to evaluate the physicochemical properties of Spirulina colored soluble protein fractions extracted from spray-dried Spirulina powder. Three fractions, including the blue soluble Spirulina protein, the green soluble Spirulina protein, and the total soluble Spirulina protein fractions were obtained. Investigations on their behavior at air/water interface were carried out using dynamic methods of drop volume and bubble pressure surface tension measurements. Evaluation of their monolayer films' mechanical behavior was done via compression isotherms using Langmuir film balance. The protein contents of the fractions were 82.76, 82.29, and 74.53% for the blue, the green, and the total fractions, respectively. Surface tension decay increased with increasing concentration for all the fractions. No significant difference in surface tension decay was observed between the samples at 0.3% (w/w). Surface tension decay was less important at pH 3 for all the fractions. The total fraction and the blue fraction appeared to form more elastic films than the green fraction. The blue soluble fraction also presented the highest collapse pressure and initial expansion area.

Novel Colloidal Microstructures of β-Escin and the Liposomal Components Cholesterol and DPPC.

The discovery of immunostimulating complex formation by the saponin Quil A from the plant Quillaja saponaria with cholesterol and a phospholipid opened up new avenues for the development of drug delivery systems for vaccine application with additional adjuvant properties. In this study, β-escin, a monodesmosidic triterpene saponin from horse chestnut, was investigated in terms of its interaction with liposomal components (cholesterol, dipalmitoylphosphatidylcholine) by Langmuir film balance studies and with regard to particle formation visualized by transmission electron microscopy. Astrong interaction of β-escin with cholesterol was observed by Langmuir isotherms due to the intercalation of the saponin into the monolayer, whereas no interaction occurred with dipalmitoylphosphatidylcholine. Transmission electron microscopy studies also confirmed the strong interaction of β-escin with cholesterol. In aqueous pseudo-ternary systems (β-escin, dipalmitoylphosphatidylcholine, cholesterol) and in pseudo-binary systems (β-escin, cholesterol), new colloidal structures built up from ring-like and worm-like subunits were observed with a size of about 100 - 200 nm. These colloidal structures are formed in pseudo-binary systems by aggregation of the subunits, whereas in pseudo-ternary systems, they are formed among others by attacking the liposomal membrane. The rehydration of the liposomal dispersions in NANOpure water or Tris buffer pH 7.4 (140 mM) resulted in the same particle formation. In contrast, the sequence of the dispersions' production process affected the particle formation. Unless adding the saponin to the other components from the beginning, just a liposomal dispersion was formed without any colloidal aggregates of the subunits mentioned above.

Urea induced changes in self-assembly and aggregate microstructures of amphiphilic star block copolymers with widely different hydrophobicity

The presence of excess of denaturants like urea unfolds proteins and disfavours micellization of surfactants; the mode of interaction of urea has always been a matter of debate. Inspired by this need, the effect of urea on the monolayer and micellar characteristics of three polyethylene oxide/propylene oxide (PEO/PPO) starblock copolymers Tetronics® viz. T1301, T1304 and T1307 (with almost same molecular weight of PPO but varying% PEO as 10, 40 and 70%, respectively) in aqueous urea solutions was investigated through scattering/spectral technique and Langmuir film balance.The three copolymers showed markedly different behaviour. In the absence of urea, the most hydrophobic T1301 forms vesicular structures (few hundred nm), T1304 with moderate hydrophobicity forms spherical micelles (∼10–20nm) with low polydispersity while very hydrophilic T1307 exists as a highly polydispersed system with unimers (∼3–5nm), loose aggregates (∼10–20nm) and clusters (with few hundred nm size). However, the addition of urea (0–8M) to these copolymer solutions leads to the disruption of any loose aggregates formed which is probably due to its binding with the copolymer chains that enhances the solubility of Tetronics® and overall polydispersity of the copolymer solutions. In support to our hypothesis, the binding of urea was validated using fluorescence measurements/Langmuir film balance and it was established that urea follows the direct mechanism (as proposed in earlier reports) and the intercalation of urea between the PEO chains leads to the extended conformations as observed by Langmuir film balance. Although, urea had a common denaturing effect on all the three copolymers, the hydrating effect of urea was most evident for copolymers with more hydrophilic character.

The interfacial properties of various milk fat globule membrane components using Langmuir isotherms

Abstract In an attempt to understand the interfacial properties of various milk fat globule membrane (MFGM) components, their monolayers were characterized using a Langmuir film balance. MFGM fractions were separated into lipids (total and polar lipids) and defatted mass. The interfacial properties of these components were characterized at the air/water interface. The interfacial properties of MFGM were characterized at both the air/water and oil/water interfaces. Using the compression isotherm, the surface elasticity of MFGM components was also calculated. It was found that MFGM polar lipids showed relatively high surface elasticity which confirms their potential to help stabilize food systems.

Surface Self-Assembly and Properties of Monolayers Formed by Reverse Poly(butylene oxide)-poly(ethylene oxide)-poly(butylene oxide) Triblock Copolymers with Lengthy Hydrophilic Blocks

The surface behavior and properties of several reverse poly(butylene oxide)-poly(ethylene oxide)-poly(butylene oxide) block copolymers, BO8EO90BO8, BO12EO227BO12, BO14EO378BO14, BO20EO411BO20, and BO21EO385BO21 at the air/water interface have been analyzed by drop tensiometry, Langmuir film balance, and atomic force microscopy (AFM). The kinetic adsorption process of block copolymer chains at the air/water interface is a diffusion-controlled process at short times. Structural rearrangements of the copolymer backbones are progressively more important as the adsorption carries on. The adsorption layers formed at the interface display evident solid-like behavior in the whole range of frequencies analyzed even at the lowest frequencies used probably as a result of the interconnection between hydrophobic ends of polymeric chains. All the copolymer display adsorption isotherm profiles are composed of four different regions in which the different characteristic regimes (“pancake”, mushroom”, “brush”, and collaps...

Effects of Membrane PEGylation on Entry and Location of Antifungal Drug Itraconazole and Their Pharmacological Implications.

Itraconazole (ITZ) is an antifungal agent used clinically to treat mycotic infections. However, its therapeutic effects are limited by low solubility in aqueous media. Liposome-based delivery systems (LDS) have been proposed as a delivery mechanism for ITZ to alleviate this problem. Furthermore, PEGylation, the inclusion in the formulation of a protective "stealth sheath" of poly(ethylene glycol) around carrier particles, is widely used to increase circulation time in the bloodstream and hence efficacy. Together, these themes highlight the importance of mechanistic and structural understanding of ITZ incorporation into liposomes both with and without PEGylation because it can provide a potential foundation for the rational design of LDS-based systems for delivery of ITZ, using alternate protective polymers or formulations. Here we have combined atomistic simulations, cryo-TEM, Langmuir film balance, and fluorescence quenching experiments to explore how ITZ interacts with both pristine and PEGylated liposomes. We found that the drug can be incorporated into conventional and PEGylated liposomes for drug concentrations up to 15 mol % without phase separation. We observed that, in addition to its protective properties, PEGylation significantly increases the stability of liposomes that host ITZ. In a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer without PEGylation, ITZ was found to reside inside the lipid bilayer between the glycerol and the double-bond regions of POPC, adopting a largely parallel orientation along the membrane surface. In a PEGylated liposome, ITZ partitions mainly to the PEG layer. The results provide a solid basis for further development of liposome-based delivery systems.

Membrane Binding and Remodelling by the COPII Coat

Dynamic remodelling of cellular membranes in conjunction with protein complex assembly and disassembly constitutes a key component of intracellular trafficking pathways. Reconstitution of selected steps outside the cell using artificial membranes facilitates the use of biophysical techniques for investigating structure and dynamics of protein-lipid assemblies. COPII-coated membrane vesicles are responsible for transporting cargo from the ER towards the Golgi apparatus in the secretory pathway. The small GTPase Sar1, which belongs to the Sar1/Arf family of GTPases and the Ras-superfamily, is an essential component in COPII-vesicle formation. Upon activation by GTP, Sar1 binds to membranes, embedding an amphipathic helix into the proximal leaflet of the bilayer. The role of GTP presence versus GTP hydrolysis in the fission of COPII vesicles is not completely clear. We study COPII vesicle formation in a bottom-up fashion on baker's yeast proteins and model membranes using cryo-electron microscopy techniques (cryo-EM), confocal fluorescence microscopy and fluorescence correlation spectroscopy (FCS). Cryo-EM shows strongly different COPII coated vesicle morphologies under GTP hydrolyzing versus non-hydrolyzing conditions. More subtle differences are observed by cryo-EM among reconstituted COPII samples under non-hydrolyzing conditions. Membrane binding of the small GTPases Sar1 and Arf1 is studied with biophysical techniques: By combining FCS with a Langmuir film balance, the protein's footprint in the proximal membrane leaflet is revealed. Dual-color fluorescence cross-correlation spectroscopy (FCCS) analysis has been developed into a method for obtaining binding curves and testing binding models of protein binding to freely diffusing vesicles. The higher specificity makes fluorescence cross-correlation advantageous compared to previous fluorescence autocorrelation analysis also for small vesicle binding studies.

Saponin Interactions with Model Membrane Systems – Langmuir Monolayer Studies, Hemolysis and Formation of ISCOMs

Saponins are used in medicine due to their pharmacological and immunological effects. To better understand interactions of saponins with model membranes and natural membranes of, for example, erythrocytes, Langmuir film balance experiments are well established. For most saponins, a strong interaction with cholesterol was demonstrated in dependence of both the aglycone part and the sugar moieties and is suggested to be correlated with a strong hemolytic activity, high toxicity, and high surface activity, as was demonstrated for the steroid saponin digitonin. In general, changes in the sugar chain or in substituents of the aglycone result in a modification of the saponin properties. A promising saponin with regard to fairly low hemolytic activity and high adjuvant effect is α-tomatine, which still shows a high affinity for cholesterol. An interaction with cholesterol and lipids has also been proven for the Quillaja saponin from the bark of Quillaja saponaria Molina. This triterpene saponin was approved in marketed vaccines as an adjuvant due to the formation of immunostimulating complexes. Immunostimulating complexes consist of a Quillaja saponin, cholesterol, phospholipids, and a corresponding antigen. Recently, another saponin from Quillaja brasiliensis was successfully tested in immunostimulating complexes, too. Based on the results of interaction studies, the formation of drug delivery systems such as immunostimulating complexes or similar self-assembled colloids is postulated for a variety of saponins.

Surface and Aggregation Behavior of Pentablock Copolymer PNIPAM7-F127-PNIPAM7 in Aqueous Solutions.

The triblock Pluronic F127 was modified by introducing poly(N-isopropylacrylamide) (PNIPAM) at both the poly(ethylene oxide) ends, and the pentablock copolymer so-prepared was characterized by gel permeation chromatography and (1)H NMR. The degree of polymerization of NIPAM blocks at the two ends was 7. The solution behavior and microstructure of copolymer aggregates in water and aqueous salt solution were examined and compared with F127 by UV-visible absorption spectroscopy, microdifferential scanning calorimetry, dynamic light scattering (DLS), and small-angle neutron scattering (SANS). The behavior of the pentablock copolymer at the air/water interface was determined by Langmuir film balance. Two lower critical solution temperatures were observed for pentablock copolymer, corresponding to poly(propylene oxide) and PNIPAM blocks, respectively. DLS studies show that micelle size increased with increase in temperature and in the presence of salt. SANS measurements provided temperature-dependent structural evolution of copolymer micelles in water and salt solution. The copolymer displays an isotherm with four classical regions (pancake, mushroom, brush, and condensed state). The study has potential applications in controlled drug delivery due to the tunable phase behavior and biocompatibility of the copolymer.

Interfacial molecular interactions based on the conformation recognition between the insoluble antitumor drug AD-1 and DSPC

In this study, molecular interactions between the anti-cancer agent 20(R)-25-methoxyl-dammarane-3β, 12β, 20-triol (AD-1) and phospholipid 1,2-Distearoyl-sn-glycero-3-phosphocholine (DSPC) were investigated using the Langmuir film balance technique. The characteristics of binary Langmuir monolayers consisting of DSPC and AD-1 were conducted on the basis of the surface pressure⿿area per molecule (Ͽ⿿A) isotherms. It was found that the drug was able to become efficiently inserted into preformed DSPC monolayers, indicating a preferential interaction between AD-1 and DSPC. For the examined lateral pressure at 20mN/m, the largest negative values of οGex were found for the AD-1/DSPC monolayer, which should be the most stable. Based on the calculated values of οGex, we found that the AD-1/DSPC systems exhibited the best mixed characteristics when the molar fraction of the AD-1 was 0.8; at that relative concentration, the AD-1 molecules can mix better and interact with the phospholipid molecules. In addition, the drug-DSPC binary supramolecular structure was also deposited on the mica plates as shown by atomic force microscopy (AFM). Finally, molecular docking calculations explained satisfactorily that, based on the conformations interactions (conformation recognition), even at an AD-1/DSPC molar ratio as high as 8:2, the interfacial stabilization of the AD-1/DSPC system was fairly strong due to hydrophobic interactions. A higher loading capacity of DSPC might be possible, as it is associated with a more flexible geometrical environment, which allows these supramolecular structures to accept larger increases in drug loading upon steric binding.

Interaction of the Antimicrobial Peptides Rhesus θ-Defensin and Porcine Protegrin-1 with Anionic Phospholipid Monolayers.

A combination of Langmuir isotherm, Brewster angle microscopy (BAM), and neutron reflectivity studies have been performed to gain insight into the effects on model bacterial cell membranes of the antimicrobial peptides, Rhesus θ-defensin 1 (RTD-1), and porcine protegrin 1 (PG-1). The peptides were interacted with monolayers spread at the air-water interface and prepared from a 3:1 molar mixture of phosphatidylethanolamine and phosphatidylglycerol used to approximate the cell membranes of Gram positive bacteria. The Langmuir film balance measurements show that both peptides perturb the lipid monolayers causing an increase in surface pressure, and the BAM studies show that each results in the formation of small domains within the lipid films, around 5 μm diameter. The overall change in monolayer surface pressure caused by PG-1, however, is a little more pronounced than that due to RTD-1 (+8.5 mN·m(-1) vs +5.5 mN·m(-1)), and the rate of its initial interaction with the monolayer is a little more rapid than that for RTD-1. The neutron reflectivity studies also show differences for PG-1 and RTD-1, with the model fits to these data showing that the more amphiphilic PG-1 becomes fully embedded within the lipid film-causing an extension of the lipid acyl chains but leaving the thickness of the lipid headgroup layer unaffected-while RTD-1 is seen to insert less deeply-causing the same extension of the lipid acyl chains as PG-1 but also causing a significant increase in thickness of the lipid headgroup layer. The various differing effects of the two peptides on anionic lipid monolayers are discussed in the context of their differing hemolytic activities, and their proposed differing propensities to form transmembrane pores.

Specific Ion Effects in Cholesterol Monolayers.

The interaction of ions with interfaces and, in particular, the high specificity of these interactions to the particular ions considered, are central questions in the field of surface forces. Here we study the effect of different salts (NaI, NaCl, CaCl2 and MgCl2) on monolayers made of cholesterol molecules, both experimentally (surface area vs. lateral pressure isotherms measured by a Langmuir Film Balance) and theoretically (molecular dynamics (MD) all-atomic simulations). We found that surface isotherms depend, both quantitatively and qualitatively, on the nature of the ions by altering the shape and features of the isotherm. In line with the experiments, MD simulations show clear evidences of specific ionic effects and also provide molecular level details on ion specific interactions with cholesterol. More importantly, MD simulations show that the interaction of a particular ion with the surface depends strongly on its counterion, a feature ignored so far in most theories of specific ionic effects in surface forces.

Comparative study between chemostat and batch reactors to quantify membrane permeability changes on bacteria exposed to silver nanoparticles

Continuous and batch reactors were used to assess the effect of the exposure of casein-coated silver nanoparticles (AgNPs) on Escherichia coli (E. coli). Additionally, E. coli membrane extracts, membrane permeability and Langmuir film balance assays were used to determine integrity and changes in lipid composition in response to AgNPs exposure.Results showed that batch conditions were not appropriate for the tests due to the production of exopolymeric substances (EPS) during the growth phase. After 5h of contact between AgNPs and the used growth media containing EPS, the nanoparticles increased in size from 86nm to 282nm reducing the stability and thus limiting cell-nanoparticle interactions. AgNPs reduced E. coli growth by 20% at 1mg/L, in terms of Optical Density 670 (OD670), while no effect was detected at 15mg/L. At 50mg/L of AgNPs was not possible to perform the test due to aggregation and sedimentation of the nanoparticles. Membrane extract assays showed that at 1mg/L AgNPs had a greater change in area (−4.4cm2) on bacteria compared to 15mg/L (−4.0cm2). This area increment suggested that membrane disruption caused by AgNPs had a stabilizing/rigidifying effect where the cells responded by shifting their lipid composition to more unsaturated lipids to counteract membrane rigidification.In chemostats, the constant inflow of fresh media and aeration resulted in less AgNPs aggregation, thus increased the AgNPs-bacteria interactions, in comparison to batch conditions. AgNPs at 1mg/L, 15mg/L, and 50mg/L inhibited the growth (OD670 reduction) by 0%, 11% and 16.3%, respectively. Membrane extracts exposed to 1mg/L, 15mg/L, and 50mg/L of AgNPs required greater changes in area by −0.5cm2, 2.7cm2 and 3.6cm2, respectively, indicating that the bacterial membranes were disrupted and bacteria responded by synthesizing lipids that stabilize or strengthen membranes.This study showed that the chemostat is more appropriate for the testing of nanotoxicological effects when testing bacteria at growing conditions.