1,2-Dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG) monolayers: influence of temperature, pH, ionic strength and binding of alkaline earth cations

Divalent cations promote huntingtin fibril formation on endoplasmic reticulum derived and model membranes

Huntington's Disease (HD) is caused by an abnormal expansion of the polyglutamine (polyQ) domain within the first exon of the huntingtin protein (htt). This expansion promotes disease-related htt aggregation into amyloid fibrils and the formation of proteinaceous inclusion bodies within neurons. Fibril formation is a complex heterogenous process involving an array of aggregate species such as oligomers, protofibrils, and fibrils. In HD, structural abnormalities of membranes of several organelles develop. In particular, the accumulation of htt fibrils near the endoplasmic reticulum (ER) impinges upon the membrane, resulting in ER damage, altered dynamics, and leakage of Ca2+. Here, the aggregation of htt at a bilayer interface assembled from ER-derived liposomes was investigated, and fibril formation directly on these membranes was enhanced. Based on these observations, simplified model systems were used to investigate mechanisms associated with htt aggregation on ER membranes. As the ER-derived liposome fractions contained residual Ca2+, the role of divalent cations was also investigated. In the absence of lipids, divalent cations had minimal impact on htt structure and aggregation. However, the presence of Ca2+ or Mg2+ played a key role in promoting fibril formation on lipid membranes despite reduced htt insertion into and association with lipid interfaces, suggesting that the ability of divalent cations to promote fibril formation on membranes is mediated by induced changes to the lipid membrane physicochemical properties. With enhanced concentrations of intracellular calcium being a hallmark of HD, the ability of divalent cations to influence htt aggregation at lipid membranes may play a role in aggregation events that lead to organelle abnormalities associated with disease.

Effect of cationic dendrimer on membrane mimetic systems in the form of monolayer and bilayer

Interactions between a zwitterionic phospholipid, 1, 2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and four anionic phospholipids dihexadecyl phosphate (DHP), 1, 2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG), 1, 2-dipalmitoyl-sn-glycero-3-phosphate (DPP) and 1, 2-dipalmitoyl-sn-glycero-3-phospho ethanol (DPPEth) in combination with an additional amount of 30 mol% cholesterol were separately investigated at air-buffer interface through surface pressure (π) - area (A) measurements. π-A isotherm derived parameters revealed maximum negative deviation from ideality for the mixtures comprising 30 mol% anionic lipids. Besides the film functionality, structural changes of the monomolecular films at different surface pressures in the absence and presence of polyamidoamine (PAMAM, generation 4), a cationic dendrimer, were visualised through Brewster angle microscopy and fluorescence microscopic studies. Fluidity/rigidity of monolayers were assessed by surface dilatational rheology studies. Effect of PAMAM on the formation of adsorbed monolayer, due to bilayer disintegration of liposomes (DPPC:anionic lipids= 7:3 M/M, and 30 mol% cholesterol) were monitored by surface pressure (π) - time (t) isotherms. Bilayer disintegration kinetics were dependent on lipid head group and chain length, besides dendrimer concentration. Such studies are considered to be an in vitro cell membrane model where the alteration of molecular orientation play important roles in understanding the nature of interaction between the dendrimer and cell membrane. Liposome-dendrimer aggregates were nontoxic to breast cancer cell line as well as in doxorubicin treated MDA-MB-468 cell line suggesting their potential as drug delivery systems.

Analysis of the Interaction between DNA Aptamers and Cytochrome C on the Surface of Lipid Films and on the MUA Monolayer: A QCM-D Study

We analyzed the possibility of the detection of cytochrome c (cyt c) being physically adsorbed on lipid films or covalently bounded to 11-mercapto-1-undecanoic acid (MUA) chemisorbed on the gold layer using quartz crystal microbalance with dissipation monitoring (QCM-D). The negatively charged lipid film composed of a mixture of zwitterionic DMPC and negatively charged DMPG phospholipids at a molar ratio of 1:1 allowed the formation of a stable cyt c layer. Addition of DNA aptamers specific to cyt c, however, resulted in removal of cyt c from the surface. The interaction of cyt c with the lipid film and its removal by DNA aptamers were accompanied by changes in viscoelastic properties evaluated using the Kelvin-Voigt model. Cyt c covalently bound to MUA also provided a stable protein layer already at its relatively low concentrations (0.5 μM). A decrease in the resonant frequency following the addition of gold nanowires (AuNWs) modified by DNA aptamers was observed. The interaction of aptamers with cyt c on the surface can be a combination of specific and non-specific interactions due to electrostatic forces between negatively charged DNA aptamers and positively charged cyt c.

Shape-dependent gold nanoparticle interactions with a model cell membrane

Customizable gold nanoparticle platforms are motivating innovations in drug discovery with massive therapeutic potential due to their biocompatibility, stability, and imaging capabilities. Further development requires the understanding of how discrete differences in shape, charge, or surface chemistry affect the drug delivery process of the nanoparticle. The nanoparticle shape can have a significant impact on nanoparticle function as this can, for example, drastically change the surface area available for modifications, such as surface ligand density. In order to investigate the effects of nanoparticle shape on the structure of cell membranes, we directly probed nanoparticle-lipid interactions with an interface sensitive technique termed sum frequency generation (SFG) vibrational spectroscopy. Both gold nanostars and gold nanospheres with positively charged ligands were allowed to interact with a model cell membrane and changes in the membrane structure were directly observed by specific SFG vibrational modes related to molecular bonds within the lipids. The SFG results demonstrate that the +Au nanostars both penetrated and impacted the ordering of the lipids that made up the membrane, while very little structural changes to the model membrane were observed by SFG for the +Au nanospheres interacting with the model membrane. This suggests that the +Au nanostars, compared to the +Au nanospheres, are more disruptive to a cell membrane. Our findings indicate the importance of shape in nanomaterial design and provide strong evidence that shape does play a role in defining nanomaterial-biological interactions.

Thermodynamics and In-Plane Viscoelasticity of Anionic Phospholipid Membranes Modulated by an Ionic Liquid

This article presents the effects of an imidazolium-based ionic liquid (IL) on the thermodynamics and in-plane viscoelastic properties of model membranes of anionic phospholipids. The negative Zeta potential of multilamellar vesicles of 14 carbon lipid 1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DMPG) is observed to reduce due to the presence of few mole % of an IL 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]). The effect was found to be stronger on enhancing the chain length of the lipid. The surface pressure-area isotherms of lipid monolayer formed at air-water interface are modified by the IL reducing the effective area per molecule. Further, the equilibrium elasticity of the film is altered depending upon the thermodynamic phase of the lipids. While the presence of the IL in the DMPG lipid makes it ordered in the gel phase by reducing the entropy, the effect is opposite in the fluid phase. The in-plane viscoelastic parameters of the lipid film is quantified by dilation rheology using the oscillatory barriers of a Langmuir trough. Even though the low chain lipid DMPG does not show any effect of IL on its storage and loss moduli, the longer chain lipids exhibit a prominent effect in the liquid extended (LE) phase. Further, the dynamic response of the lipid film is found to be distinctly different in the liquid condensed (LC) phase from that of the LE phase.

La3+ and Y3+ interactions with the carboxylic acid moiety at the liquid/vapor interface: Identification of binding complexes, charge reversal, and detection limits

Specific interactions of yttrium and lanthanum ions with a fatty acid Langmuir monolayer were investigated using vibrational sum frequency spectroscopy. The trivalent ions were shown to interact with the charged form of the carboxylic acid group from nanomolar concentrations (<300 nM). Analysis of the spectral features from both the symmetric and the asymmetric carboxylate modes reveals the presence of at least three distinct coordination structures linked to specific binding configurations. Although the same species were identified for both La³⁺ and Y³⁺, they display a different concentration dependence, highlighting the ion-specificity of the interaction. From the analysis of the response of interfacial water molecules, the reversal of the surface charge, as well as the formation of yttrium hydroxide complexes, were detected upon increasing the amount of salt in solution. The binding interaction and kinetics of absorption are sensitive to the solution pH, showing a distinct ion speciation in the interfacial region when compared to the bulk. Changing the subphase pH or adding a monovalent background electrolyte that promotes deprotonation of the carboxylic acid headgroup could further improve the detection limit of La³⁺ and Y³⁺ to concentrations < 100 nM. These findings demonstrate that nM concentrations of trace metals contaminants, typically found on monovalent salts, can significantly influence the binding structure and kinetics in Langmuir monolayers.

Albumin displacement at the air-water interface by Tween (Polysorbate) surfactants

Tween (polysorbate) 20 and 80 are surfactants used for the development of parenteral protein drugs, due to their beneficial safety profile and stabilisation properties. To elucidate the mechanism by which Tween 20 and 80 stabilise proteins in aqueous solutions, either by a "direct" protein to surfactant interaction and/or by an interaction with the protein film at the air-water interface, we used spectroscopic (Infrared Reflection Absorption Spectroscopy, IRRAS) and microscopic techniques (Brewster Angle Microscopy, BAM) in combination with surface pressure measurements. To this end, the impact of both types of Tweens with regard to the displacement of the protein from the air-water interface was studied. As a model protein, human serum albumin (HSA) was used. The results for the displacement of the adsorbed HSA films by Tweens 20 and 80 can partially be understood on the basis of an orogenic displacement mechanism, which depends on the critical surface pressure of the adsorbed protein film. With increasing concentration of Tween in the sub-phase, BAM images showed the formation of different domain morphologies. IRRA-spectra supported the finding that at high protein concentration in the sub-phase, the protein film could not be completely displaced by the surfactants. Comparing the impact of both surfactants, we found that Tween 20 adsorbed faster to the protein film than Tween 80. The adsorption kinetics of both Tweens and the speed of protein displacement increased with rising surfactant concentration. Tween 80 reached significant lower surface pressures than Tween 20, which led to an incomplete displacement of the observed HSA film.

Lasalocid Acid Antibiotic at a Membrane Surface Probed by Sum Frequency Generation Spectroscopy

Carboxyl polyether ionophores (CPIs) are widely used as veterinary antibiotics and to increase food utilization in ruminating animals. Furthermore, CPIs can target drug-resistant bacteria, but detailed knowledge about their mode-of-action is needed to develop agents with a reasonable therapeutic index. It has been suggested that ionophores bind to membranes and incur large structural changes to shield a bound ion from the hydrophobic environment of the lipid bilayer for transport. One crucial piece of information is missing, however: Is it necessary for the free ionophore to adsorb on the membrane surface before interacting with a cation to facilitate cross-membrane ion transport? To answer this question, we applied sum-frequency generation (SFG) vibrational spectroscopy and surface tensiometry to identify the interaction between the prototypical CPI lasalocid acid (LA) and a model membrane. Observed changes in the surface pressure demonstrate that the free LA undergoes a self-assembly process with the lipid monolayer. Spectra taken from the lipid monolayer show that the free acid inserts partially into the lipid monolayer and then after complexation with sodium chloride disrupts the lipid monolayer. Overall, this study strongly suggests that this must be the crucial step of LA and metal ion complexation that allows the ionophore to traverse a lipid membrane.

Influence of Ca2+ on the surface behavior of phosphatidic acid and its mixture with diacylglycerol pyrophosphate at different pHs

The signaling lipids phosphatidic acid (PA) and diacylglycerol pyrophosphate (DGPP) are involved in regulating the stress response in plants. PA and DGPP are anionic lipids consisting of a negatively charged phosphomonoester or pyrophosphate group attached to diacylglycerol, respectively. Changes in the pH modulate the protonation of their head groups modifying the interaction with other effectors. Here, we examine in a controlled system how the presence of Ca²⁺ modulates the surface organization of dioleyl diacylglycerol pyrophosphate (DGPP) and its interaction with dioleoyl phosphatidic acid (DOPA) at different pHs. Both lipids formed expanded monolayers at pH 5 and 8. At acid and basic pHs, monolayers formed by DOPA or DGPP became denser when Ca²⁺ was added to the subphase. At pH 5, Ca²⁺ also induced an increase of surface potential of both lipids. Conversely, at pH 8 the effects induced by the presence of Ca²⁺ on the surface potential were reversed. Mixed monolayers of DOPA and DGPP showed a non-ideal behavior. The addition of even tiny amounts of DGPP to DOPA films caused a reduction of the mean molecular area. This effect was more evident at pH 8 compared to pH 5. Our finding suggests that low amounts of DGPP in an film enriched in DOPA could lead to a local increase in film packing with a concomitant change in the local polarization, further regulated by local pH. This fact may have implications for the assigned role of PA as a pH-sensing phospholipid or during its interaction with proteins.

Interactions between Mutant Bacterial Lipopolysaccharide (LPS-Ra) Surface Layers: Surface Vesicles, Membrane Fusion, and Effect of Ca2+and Temperature

Lipopolysaccharides (LPS) are a major component of the protective outer membrane of Gram-negative bacteria. Understanding how the solution conditions may affect LPS-containing membranes is important to optimizing the design of antibacterial agents (ABAs) which exploit electrostatic and hydrophobic interactions to disrupt the bacteria membrane. Here, interactions between surface layers of LPS (Ra mutants) in aqueous media have been studied using a surface force apparatus (SFA), exploring the effects of temperature and divalent Ca²⁺ cations. Complementary dynamic light scattering (DLS) characterization suggests that vesicle-like aggregates of diameter ∼28-80 nm are formed by LPS-Ra in aqueous media. SFA results show that LPS-Ra vesicles adsorb weakly onto mica in pure water at room temperature (RT) and the surface layers are readily squeezed out as the two surfaces approach each other. However, upon addition of calcium (Ca²⁺) cations at near physiological concentration (2.5 mM) at RT, LPS multilayers or deformed LPS liposomes on mica are observed, presumably due to bridging between LPS phosphate groups and between LPS phosphates and negatively charged mica mediated by Ca²⁺, with a hard wall repulsion at surface separation D₀ ∼ 30-40 nm. At 40 °C, which is above the LPS-Ra β-α acyl chain melting temperature (T_m = 36 °C), fusion events between the surface layers under compression could be observed, evident from δD ∼ 8-10 nm steps in the force-distance profiles attributed to LPS-bilayers being squeezed out due to enhanced fluidity of the LPS acyl-chain, with a final hard wall surface separation D₀ ∼ 8-10 nm corresponding to the thickness of a single bilayer confined between the surfaces. These unprecedented SFA results reveal intricate structural responses of LPS surface layers to temperature and Ca²⁺, with implications to our fundamental understanding of the structures and interactions of bacterial membranes.

Electrostatically Driven Encapsulation of Hydrophilic, Non-Conformational Peptide Epitopes into Liposomes

Since the first use of liposomes as carriers for antigens, much work has been done to elucidate the mechanisms involved in the encapsulation of vaccine-relevant biomolecules. However, only a few studies have specifically investigated the encapsulation of hydrophilic, non-conformational peptide epitopes. We performed comprehensive and systematic screening studies, in order to identify conditions that favor the electrostatic interaction of such peptides with lipid membranes. Moreover, we have explored bi-terminal sequence extension as an approach to modify the isoelectric point of peptides, in order to modulate their membrane binding behavior and eventually shift/expand the working range under which they can be efficiently encapsulated in an electrostatically driven manner. The findings of our membrane interaction studies were then applied to preparing peptide-loaded liposomes. Our results show that the magnitude of membrane binding observed in our exploratory in situ setup translates to corresponding levels of encapsulation efficiency in both of the two most commonly employed methods for the preparation of liposomes, i.e., thin-film hydration and microfluidic mixing. We believe that the methods and findings described in the present studies will be of use to a wide audience and can be applied to address the ongoing relevant issue of the efficient encapsulation of hydrophilic biomolecules.

Interactions of Monovalent and Divalent Cations with Cardiolipin Monolayers

Cardiolipin is a mitochondrial phospholipid with four alkyl chains and two phosphate moieties. Tetramyristoyl cardiolipin (TMCL, (14:0)₄CL) monolayers at the air-water interface are characterized by compression isotherms, which show a liquid expanded/liquid condensed phase transition. The phase transition surface pressure π_c depends on the composition of the aqueous solution. In a calculation, this is attributed to the electrostatic double layer, which is induced by the head groups of the model membrane, and competitive ion binding. The intrinsic binding constant is large for protons ( K_H = 10 L/mol) and small for monovalent cations ( K_M (Na⁺, K⁺, Cs⁺) = 10^-3 L/mol). The different intrinsic binding constants explain the non-monotonic behavior of π_c on increasing the salt concentration: raising the monovalent salt concentration increases π_c by charging the TMCL monolayer until 0.1 mol/L, then screening effects dominate and decrease π_c by reducing the electrostatic repulsion between lipid head groups. When at fixed 0.15 mol/L NaCl concentration, the concentration of divalent cations is increased, π_c decreases. The intrinsic binding constants of divalent cations follow the sequence Sr²⁺ < Mg²⁺ < Mn²⁺ ≈ Zn²⁺ ≈ Ca²⁺ ( K_D,Ca = 1.2 L/mol). The predictive power of the calculations was tested with different solutions.

Do carnitine and extra trace elements change stability of paediatric parenteral nutrition admixtures?

INTRODUCTION

High concentrations of trace elements (TE), in particular zinc and selenium, along with carnitine, are often added to parenteral admixtures in paediatric patients on long-term Parenteral Nutrition (PN). We aim to evaluate whether lipid droplet diameters of these admixtures maintain the recommended range of 0.4-1.0 μm.

MATERIALS AND METHODS

Stability studies were carried out on six parenteral admixtures with carnitine, trace elements and electrolytes added in different amounts. Each admixture was formulated with five different lipid emulsions with or without fish oil. Analyses were performed at time 0 (t = 0) and 24, 48, 72, 96 (t = 96) hours after compounding. Droplet diameters were determined by Light Scattering-Reverse Fourier Optics Technique. Samples, stored at 4 °C, were triple tested for a total of 450 analyses. Regression analyses were performed using panel-data techniques.

RESULTS

During the 4 days, lipid droplet diameters were in the expected range of 0.4-1.0 μm regardless of trace element and carnitine amounts in all admixtures apart from those containing fish-oil based emulsions and calcium concentrations equal to 4.5 mmol/L. In these latter admixtures, 12% of droplet diameters were larger than 1.0 μm and 2% exceeded 5.0 μm immediately after compounding.

CONCLUSION

Carnitine and high concentrations of trace elements do not affect PN admixtures stability and can be safely infused in long-term home-PN paediatric patients and prematures. Only high calcium concentrations in compresence with fish oil based lipid emulsions seem to change PN stability.

Surface organization of a DPPC monolayer on concentrated SrCl2 and ZnCl2 solutions

Transition metals are known to be enriched in organic-coated marine aerosols, but the impact these cations have on their surface properties is not well understood. Here the effect of Zn²⁺ enrichment on the surface properties of a dipalmitoylphosphatidylcholine (DPPC) monolayer was investigated and compared to that of the alkaline earth metal Sr²⁺, an ion not enriched in aerosols. Phase behavior of the DPPC film on concentrated aqueous solutions was probed with surface pressure-area isotherms while domain morphology was monitored with Brewster angle microscopy (BAM). Infrared reflection-absorption spectroscopy (IRRAS) and vibrational sum frequency generation (VSFG) spectroscopy were used to assess the impact of cations on the conformation and orientation of alkyl chains as well as the hydration state of the carbonyl and phosphatidylcholine (PC) moieties. Results of compression isotherms and BAM show that Zn²⁺ strongly interacts with DPPC molecules, and induces condensation of the monolayer while Sr²⁺ only weakly interacts with the monolayer in expanded phases. Conformational order and orientation of alkyl chains in the condensed phase are not significantly altered by either cation. IRRAS indicates that Sr²⁺ has weak interactions with the PC headgroup. Zn²⁺ ions cause dehydration of carbonyl groups and binds to the phosphate group in a 2 : 1 bridging complex. Findings here suggest that Sr²⁺ is not enriched in aerosols because it behaves similar to a monovalent ion and only weakly interacts with the monolayer, while enrichment of Zn²⁺ is due to strong binding to the lipid film.

Spectroscopic evidence for hydration and dehydration of lipid bilayers upon interaction with metal ions: a new physical insight

In this manuscript, we investigate the interactions of different metal ions with zwitterionic phospholipid bilayers of different chain lengths using the well-known membrane probe PRODAN and steady state and time resolved fluorescence spectroscopy.

Identifying the selectivity of antimicrobial peptides to cell membranes by sum frequency generation spectroscopy

Cationic amphiphilic peptides have been engineered to target both Gram-positive and Gram-negative bacteria while avoiding damage to other cell types. However, the exact mechanism of how these peptides target, bind, and disrupt bacterial cell membranes is not understood. One specific peptide that has been engineered to selectively capture bacteria is WLBU2 (sequence: RRWVRRVRRWVRRVVRVVRRWVRR). It has been suggested that WLBU2 activity stems from the fact that when interacting with bacterial cell membranes the peptide assumes an α-helical structure and inserts itself into the membrane. Alternatively, in the presence of mammalian cell membranes, the peptide assumes an inert β-sheet structure. To test this hypothesis, the authors applied sum frequency generation (SFG) spectroscopy and surface tensiometry to identify the structure of WLBU2 as it interacts with model lipid monolayers that mimic mammalian and bacterial cell membranes. Model mammalian cell membranes were built upon zwitterionic 1,2-dipalmitoyl-sn-glycero-3-phosphocholine lipids while bacterial cell membranes were constructed with negatively charged 1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol) lipids. Observed changes in surface pressure at the peptide-lipid-air interface demonstrate that the peptide has a clear binding preference toward negatively charged bacteria-like lipids. The structure of both the lipids and peptides were characterized by SFG spectra collected at the monolayer interface. Changes in monolayer structure as the peptide binds were observed by tracking the intensities of SFG vibrational modes related to the acyl chains within the lipids. Peptide structures when bound to both types of lipids were determined by SFG spectra collected within the amide I vibrational band. The SFG spectra of WLBU2 interacting with the model mammalian lipid monolayer contain two peaks near 1642 and 1678 cm^-1 indicative of an inactive β-sheet structure. SFG spectra collected from the peptide bound to a bacteria-like lipid monolayer contains just a single peak near 1651 cm^-1 which corresponds to an active α-helix structure. Combined, the tensiometry and SFG results demonstrate that WLBU2 both possesses a higher binding affinity toward and is in an active α-helix structure when bound to bacterial cell membranes.

The complex nature of calcium cation interactions with phospholipid bilayers

Understanding interactions of calcium with lipid membranes at the molecular level is of great importance in light of their involvement in calcium signaling, association of proteins with cellular membranes, and membrane fusion. We quantify these interactions in detail by employing a combination of spectroscopic methods with atomistic molecular dynamics simulations. Namely, time-resolved fluorescent spectroscopy of lipid vesicles and vibrational sum frequency spectroscopy of lipid monolayers are used to characterize local binding sites of calcium in zwitterionic and anionic model lipid assemblies, while dynamic light scattering and zeta potential measurements are employed for macroscopic characterization of lipid vesicles in calcium-containing environments. To gain additional atomic-level information, the experiments are complemented by molecular simulations that utilize an accurate force field for calcium ions with scaled charges effectively accounting for electronic polarization effects. We demonstrate that lipid membranes have substantial calcium-binding capacity, with several types of binding sites present. Significantly, the binding mode depends on calcium concentration with important implications for calcium buffering, synaptic plasticity, and protein-membrane association.

Model Lung Surfactant Films: Why Composition Matters

Lung surfactant replacement therapies, Survanta and Infasurf, and two lipid-only systems both containing saturated and unsaturated phospholipids and one containing additional palmitic acid were used to study the impact of buffered saline on the surface activity, morphology, rheology, and structure of Langmuir monolayer model membranes. Isotherms and Brewster angle microscopy show that buffered saline subphases induce a film expansion, except when the cationic protein, SP-B, is present in sufficient quantities to already screen electrostatic repulsion, thus limiting the effect of changing pH and adding counterions. Grazing incidence X-ray diffraction results indicate an expansion not only of the liquid expanded phase but also an expansion of the lattice of the condensed phase. The film expansion corresponded in all cases with a significant reduction in the viscosity and elasticity of the films. The viscoelastic parameters are dominated by liquid expanded phase properties and do not appear to be dependent on the structure of the condensed phase domains in a phase separated film. The results highlight that the choice of subphase and film composition is important for meaningful interpretations of measurements using model systems.

Surface free energy tuning of supported mixed lipid layers

The results of investigation of parameters influencing the surface free energy of supported mixed lipid layers and means for its wide range tuning enable wettability control and design of a more efficient host layers for various applications.

Binding of Short Cationic Peptides (KX)4K to Negatively Charged DPPG Monolayers: Competition between Electrostatic and Hydrophobic Interactions

The influence of the peptide sequence on the binding of short cationic peptides composed of five lysines alternating with uncharged amino acids within the series (KX)4K to negatively charged monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG) was investigated by adsorption experiments in combination with epifluorescence microscopy. To evaluate the impact of electrostatic and hydrophobic contributions, different uncharged amino acids X with increasing hydrophobicity, where X = G (glycine), A (alanine), Abu (α-aminobutyric acid), V (valine), or L (leucine) were introduced into the peptide sequence to tune the peptide hydrophobicity. The adsorption kinetics of these peptides to a DPPG monolayer always showed two superimposed processes, one leading to an increase and another to a decrease of the surface pressure Π. Thus, the plots of the change in Π after peptide binding vs initial surface pressure of the monolayer showed an unusual behavior with maxima and negative changes in Π at high initial Π values. Epifluorescence microscopy confirmed that electrostatic binding of the peptides with a concomitant decrease in Π leads to a condensation of the lipid monolayer and the formation of liquid-condensed (LC) domains even at Π values where the monolayer is supposedly in the liquid-expanded (LE) state. An increase in hydrophobicity of the amino acid X was found to counteract the condensation and an increase in Π upon peptide binding is observed at low Π values, also concomitant with the formation of LC-domains. Compression of monolayers after peptide adsorption at low surface pressure for 4 h leads to a change of the isotherms compared to pure DPPG isotherms. The phase transition of DPPG from LE to LC state is smeared out or is shifted to higher surface pressure. Considerable changes in the shapes of LC-domains were observed after peptide binding. Growth of the LC-domains was hindered in most cases and regular domain patterns were formed. Binding of (KL)4K leads to a decrease in line tension and the formation of extended filaments protruding from initially circular domains.

Self-Organisation, Thermotropic and Lyotropic Properties of Glycolipids Related to their Biological Implications

Glycolipids are amphiphilic molecules which bear an oligo- or polysaccharide as hydrophilic head group and hydrocarbon chains in varying numbers and lengths as hydrophobic part. They play an important role in life science as well as in material science. Their biological and physiological functions are quite diverse, ranging from mediators of cell-cell recognition processes, constituents of membrane domains or as membrane-forming units. Glycolipids form an exceptional class of liquid-crystal mesophases due to the fact that their self-organisation obeys more complex rules as compared to classical monophilic liquid-crystals. Like other amphiphiles, the supra-molecular structures formed by glycolipids are driven by their chemical structure; however, the details of this process are still hardly understood. Based on the synthesis of specific glycolipids with a clearly defined chemical structure, e.g., type and length of the sugar head group, acyl chain linkage, substitution pattern, hydrocarbon chain lengths and saturation, combined with a profound physico-chemical characterisation of the formed mesophases, the principles of the organisation in different aggregate structures of the glycolipids can be obtained. The importance of the observed and formed phases and their properties are discussed with respect to their biological and physiological relevance. The presented data describe briefly the strategies used for the synthesis of the used glycolipids. The main focus, however, lies on the thermotropic as well as lyotropic characterisation of the self-organised structures and formed phases based on physico-chemical and biophysical methods linked to their potential biological implications and relevance.

Antimicrobial Peptides from the Aurein Family Form Ion-Selective Pores in Bacillus subtilis

The mechanism of action of aurein 2.2 and aurein 2.3, antimicrobial peptides from the frog Litoria aurea, was investigated. Proteomic profiling of the Bacillus subtilis stress response indicates that the cell envelope is the main target for both aureins. Upon treatment, the cytoplasmic membrane depolarizes and cellular ATP levels decrease. Global element analysis shows that intracellular concentrations of certain metal ions (potassium, magnesium, iron, and manganese) strongly decrease. Selective translocation of some ions over others was demonstrated in vitro. The same set of ions also leaks from B. subtilis cells treated with sublethal concentrations of gramicidin S, MP196, and nisin. Aureins do not permeabilize the cell membrane for propidium iodide thus excluding formation of large, unspecific pores. Our data suggest that the aureins acts by forming small pores thereby causing membrane depolarization, and by triggering the release of certain metal ions thus disturbing cellular ion homeostasis.

Mechanical and electrokinetic effects of polyamines/phospholipid interactions in model membranes

The mechanical and electrical properties of phospholipids layers influenced by interaction with polyamines were determined by measuring surface pressure and compression modulus of monolayers and zeta potential of liposomes. The saturated derivative of phosphatidic acid (DPPA) formed layers of the organization varying with compression degree. Contact of DPPA layers with polyamines present in the subphase resulted in changing their mechanical properties and the conditions in which the layer reorganization appears. The parameters corresponding to the layer reorganization depended on the size and charge of polyamines' molecules. The values of: area per DPPA molecule, surface pressure at the point of layer structure reorganization, and surface pressure at the point of collapse characterizing of DPPA layers in the studied systems were determined. It was found that polyamines influenced to a much lesser extent the mechanical properties of monolayers formed from unsaturated derivative of phosphatidic acid slightly increasing its mechanical resistance in the range of higher molecular packing. The results of electrokinetic measurements revealed that surface charge of phosphatidic acid liposomes was effectively neutralized in the presence of polyamines. A similar effect was observed for phosphatidyl glycerol and for negatively charged polystyrene latex particles used as a reference. The influence of polyamines on the mechanical properties of DPPA layers was interpreted assuming a possibility of penetration of the lipid layer by polyamines' molecules. Comparison of action of putrescine and calcium ions and effects of polyamines on phosphatidyl glycerol provided additional justification for the proposed interpretation of the observed effects.

Penetration of milk-derived antimicrobial peptides into phospholipid monolayers as model biomembranes

Three antimicrobial peptides derived from bovine milk proteins were examined with regard to penetration into insoluble monolayers formed with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or 1,2-dipalmitoyl-sn-glycero-3-phospho-rac-(1-glycerol) sodium salt (DPPG). Effects on surface pressure (Π) and electric surface potential (ΔV) were measured, Π with a platinum Wilhelmy plate and ΔV with a vibrating plate. The penetration measurements were performed under stationary diffusion conditions and upon the compression of the monolayers. The two type measurements showed greatly different effects of the peptide-lipid interactions. Results of the stationary penetration show that the peptide interactions with DPPC monolayer are weak, repulsive, and nonspecific while the interactions with DPPG monolayer are significant, attractive, and specific. These results are in accord with the fact that antimicrobial peptides disrupt bacteria membranes (negative) while no significant effect on the host membranes (neutral) is observed. No such discrimination was revealed from the compression isotherms. The latter indicate that squeezing the penetrant out of the monolayer upon compression does not allow for establishing the penetration equilibrium, so the monolayer remains supersaturated with the penetrant and shows an under-equilibrium orientation within the entire compression range, practically.

Structural characterization of a model gram-negative bacterial surface using lipopolysaccharides from rough strains of Escherichia coli

Lipopolysaccharides (LPS) make up approximately 75% of the Gram-negative bacterial outer membrane (OM) surface, but because of the complexity of the molecule, there are very few model OMs that include LPS. The LPS molecule consists of lipid A, which anchors the LPS within the OM, a core polysaccharide region, and a variable O-antigen polysaccharide chain. In this work we used RcLPS (consisting of lipid A plus the first seven sugars of the core polysaccharide) from a rough strain of Escherichia coli to form stable monolayers of LPS at the air-liquid interface. The vertical structure RcLPS monolayers were characterized using neutron and X-ray reflectometry, while the lateral structure was investigated using grazing incidence X-ray diffraction and Brewster angle microscopy. It was found that RcLPS monolayers at surface pressures of 20 mN m(-1) and above are resolved as hydrocarbon tails, an inner headgroup, and an outer headgroup of polysaccharide with increasing solvation from tails to outer headgroups. The lateral organization of the hydrocarbon lipid chains displays an oblique hexagonal unit cell at all surface pressures, with only the chain tilt angle changing with surface pressure. This is in contrast to lipid A, which displays hexagonal or, above 20 mN m(-1), distorted hexagonal packing. This work provides the first complete structural analysis of a realistic E. coli OM surface model.

Temperature-induced transitions in the structure and interfacial rheology of human meibum

Meibomian lipids are the primary component of the lipid layer of the tear film. Composed primarily of a mixture of lipids, meibum exhibits a range of melt temperatures. Compositional changes that occur with disease may alter the temperature at which meibum melts. Here we explore how the mechanical properties and structure of meibum from healthy subjects depend on temperature. Interfacial films of meibum were highly viscoelastic at 17°C, but as the films were heated to 30°C the surface moduli decreased by more than two orders of magnitude. Brewster angle microscopy revealed the presence of micron-scale inhomogeneities in meibum films at higher temperatures. Crystalline structure was probed by small angle x-ray scattering of bulk meibum, which showed evidence of a majority crystalline structure in all samples with lamellar spacing of 49 Å that melted at 34°C. A minority structure was observed in some samples with d-spacing at 110 Å that persisted up to 40°C. The melting of crystalline phases accompanied by a reduction in interfacial viscosity and elasticity has implications in meibum behavior in the tear film. If the melt temperature of meibum was altered significantly from disease-induced compositional changes, the resultant change in viscosity could alter secretion of lipids from meibomian glands, or tear-film stabilization properties of the lipid layer.

Phase transitions and spatially ordered counterion association in ionic-lipid membranes: theory versus experiment

Aqueous dispersions of phosphatidylglycerol (PG) lipids may present an anomalous chain-melting transition at low ionic strengths, as seen by different experimental techniques such as calorimetry or light scattering. The anomaly disappears at high ionic strengths or for longer acyl-chain lengths. In this article, we use a statistical model for the bilayer that distinguishes both lipid chain and headgroup states in order to compare model and experimental thermotropic and electrical properties. The effective van der Waals interactions among hydrophobic chains compete with the electrostatic repulsions between polar headgroups, which may be ionized (counterion dissociated) or electrically neutral (associated with counterions). Electric degrees of freedom introduce new thermotropic charge-ordered phases in which headgroup charges may be spatially ordered, depending on the electrolyte ionic strength, introducing a new rationale for experimental data on PGs. The thermal phases presented by the model for different chain lengths, at fixed ionic strength, compare well with an experimental phase diagram constructed on the basis of differential scanning calorimetry profiles. In the case of dispersions of DMPG (dimyristoyl phosphatidylglycerol) with added monovalent salt, the model properties reproduce the main features displayed by data from differential scanning calorimetry as well as the characteristic profile for the degree of ionization of the bilayer surface across the anomalous transition region, obtained from the theoretical interpretation of electrokinetic (conductivity and electrophoretic mobility) measurements.

Phase transitions and spatially ordered counterion association in ionic-lipid membranes: a statistical model

We propose a statistical model to account for the gel-fluid anomalous phase transitions in charged bilayer- or lamellae-forming ionic lipids. The model Hamiltonian comprises effective attractive interactions to describe neutral-lipid membranes as well as the effect of electrostatic repulsions of the discrete ionic charges on the lipid headgroups. The latter can be counterion dissociated (charged) or counterion associated (neutral), while the lipid acyl chains may be in gel (low-temperature or high-lateral-pressure) or fluid (high-temperature or low-lateral-pressure) states. The system is modeled as a lattice gas with two distinct particle types--each one associated, respectively, with the polar-headgroup and the acyl-chain states--which can be mapped onto an Ashkin-Teller model with the inclusion of cubic terms. The model displays a rich thermodynamic behavior in terms of the chemical potential of counterions (related to added salt concentration) and lateral pressure. In particular, we show the existence of semidissociated thermodynamic phases related to the onset of charge order in the system. This type of order stems from spatially ordered counterion association to the lipid headgroups, in which charged and neutral lipids alternate in a checkerboard-like order. Within the mean-field approximation, we predict that the acyl-chain order-disorder transition is discontinuous, with the first-order line ending at a critical point, as in the neutral case. Moreover, the charge order gives rise to continuous transitions, with the associated second-order lines joining the aforementioned first-order line at critical end points. We explore the thermodynamic behavior of some physical quantities, like the specific heat at constant lateral pressure and the degree of ionization, associated with the fraction of charged lipid headgroups.

Interaction of the cationic peptide bactenecin with mixed phospholipid monolayers at the air-water interface

The initial mechanism by which antimicrobial peptides target microbes occurs via electrostatic interactions; however, the mechanism is not well understood. We investigate the interaction of the antimicrobial peptide bactenecin with a 50:50 w:w% 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DMPG) phospholipid mixture at the air-water interface with different NaCl concentrations (0.01, 0.05, 0.1, 0.5 M) in the subphase. A larger shift of DPPC:DMPG isotherms was obtained for 0.1 M salt concentration at lower and higher pressures, demonstrating the influence of the negative charge of DMPG molecules and the screening of the electrostatic interaction by the salt concentration. Raman spectroscopy of monolayers demonstrated the presence of cysteine-cysteine bridges in bactenecin loops. The peptide adsorption in DPPC:DMPG monolayers observed by AFM images suggests a self-assembled aggregation process, starting with filament-like networks. Domains similar to carpets were formed and pore structures were obtained after a critical peptide concentration, according to the carpet model.

Sum-frequency spectroscopic study of Langmuir monolayers of lipids having oppositely charged headgroups

Sum-frequency vibrational spectroscopy, with the help of surface pressure-area (π-A) isotherm, was used to study lipid Langmuir monolayers composed of molecules with positively and negatively charged headgroups as well as a 1:1 neutral mixture of the two. The spectral profiles of the CH(x) stretch vibrations are similar for all monolayers in the liquid-condensed (LC) phase. They suggest a monolayer structure of closely packed alkyl chains that are nearly all-trans and well oriented along the surface normal. In the liquid-expanded (LE) phase, the spectra of all monolayers appear characteristic of loosely packed chains with significant gauche defects. The OH stretch spectra of interfacial water for both positively and negatively charged monolayers are significantly enhanced in comparison with a neutral water interface, but the phase measurement of SFVS indicates that OH in the two cases points toward the bulk and the interface, respectively. The enhancement results mainly from surface-field-induced polar ordering of interfacial water molecules. For a charge-neutral monolayer composed of an equal number of positively and negatively charged lipid molecules, no such enhancement is observed. This mixed monolayer exhibits a wide range of LC/LE coexistence region extended to very low surface pressure and its CH(x) spectral profile in the coexistence region resembles that of the LC phase. This result suggests that in the LC/LE coexistence region, the mixed monolayer consists of coexisting LC and LE patches in which oppositely charged lipid molecules are homogeneously mixed and dispersed.

Stoichiometry and Structure of Poly(amidoamine) Dendrimer-Lipid Complexes

The energetics, stoichiometry, and structure of poly(amidoamine) (PAMAM) dendrimer-phospholipid interactions were measured with isothermal titration calorimetry (ITC), transmission electron microscopy (TEM), atomic force microscopy (AFM), dynamic light scattering (DLS), and molecular dynamics (MD) simulations. Dendrimers of sixth-generation and smaller interacted with the lipids at an average stoichiometry and enthalpy proportional to the number of primary amines per dendrimers (4.5 ± 0.1 lipids/primary amine and 6.3 ± 0.3 kJ/mol of primary amines, respectively). Larger dendrimers, however, demonstrated a decreased number of bound lipids and heat release per primary amine, presumably due to the steric restriction of dendrimer deformation on the lipid bilayer. For example, eighth-generation PAMAM dendrimers bound to 44% fewer lipids per primary amine and released 63% less heat per primary amine as compared to the smaller dendrimers. These differences in binding stoichiometry support generation-dependent models for dendrimer-lipid complexation, which are consistent with previously observed generation-dependent differences in dendrimer-induced membrane disruption. Dendrimers of seventh-generation and larger bound to lipids with an average stoichiometry consistent with each dendrimer having been wrapped by a bilayer of lipids, whereas smaller dendrimers did not.

Morphology modifications in negatively charged lipid monolayers upon mitochondrial creatine kinase binding

Mitochondrial creatine kinase (mtCK) may participate to membrane organization at the mitochondrial level by modulating lipid state and fluidity. The effect of the protein on lipid phase behaviour of different acyl chain length phosphatidylglycerol monolayers was analyzed from pressure-area isotherms and from the compressional modulus variation with respect to the surface pressure. Monolayer morphology was visualized by Brewster angle microscopy. No condensation effect was visible on dimyristoylphosphatidylglycerol (DMPG). For the other PG monolayers tested, dipalmitoylphosphatidylglycerol (DPPG) and distearoylphosphatidylglycerol (DSPG), mtCK facilitated the formation of a liquid condensed phase. The effect depended on the surface pressure at which transition phase occurred. The effect of mtCK was more pronounced for tetramyristoylcardiolipin (TMCL) monolayers, as liquid condensed regions appeared 10 mN/m below the transition phase of the pure TMCL monolayer. The observed domains were circular and rather uniform, indicating a stabilization of the condensed phase. The same effect, namely an overall condensation of the monolayer with formation of circular domains, was observed upon protein injection beneath TMCL monolayers in different condensation states at constant area. MtCK ability to induce and stabilize a LC phase on monolayers could have important consequences in membrane organization and emphasize its structural role at mitochondrial level.