Biophysical Properties of Lipid Membranes from Barley Roots during Low-Temperature Exposure and Recovery

Glycerolipid remodeling, a dynamic mechanism for plant subsistence under cold stress, has been posited to affect the biophysical properties of cell membranes. In barley roots, remodeling has been observed to take place upon exposure to chilling stress and to be partially reverted during stress relief. In this study, we explored the biophysical characteristics of membranes formed with lipids extracted from barley roots subjected to chilling stress, or during a subsequent short- or long-term recovery. Our aim was to determine to what extent barley roots were able to offset the adverse effects of temperature on their cell membranes. For this purpose, we analyzed the response of the probe Laurdan inserted in bilayers of different extracts, the zeta potential of liposomes, and the behavior of Langmuir monolayers upon compression. We found important changes in the order of water molecules, which is in agreement with the changes in the unsaturation index of lipids due to remodeling. Regarding Langmuir monolayers, we found that films from all the extracts showed a reorganization at a surface pressure that depends on temperature. This reorganization occurred with an increase in entropy for extracts from control plants and without entropy changes for extracts from acclimated plants. In summary, some membrane properties were recovered after the stress, while others were not, suggesting that the membrane biophysical properties play a role in the mechanism of plant acclimation to chilling. These findings contribute to our understanding of the impact of lipid remodeling on biophysical modifications in plant roots.

Out-of-plane deformability and its coupling with electrostatics in biomembranes

Cell membranes are quasi-bidimensional soft systems formed by multipoles in an ordered array that can be polarized in an electric field. Consequently, electrostatic potentials emerge inside membranes, and membranes respond to external electric fields. From a mechanical perspective, membranes can be easily compressed-expanded, laterally deformed, and curved. Bending is particularly easy, and this kind of deformation translates to changes in the relative positions of the negative and positive charges, leading to strain gradient-induced polarization. Conversely, an external electric field gradient will exert a bending stress that translates to mechanical membrane deformation. These phenomena are described through membrane flexoelectricity. Here, we describe this property in lipid bilayers and cell membranes and summarize the studies in the field with emphasis on the effects promoted by membrane asymmetry.

Dexamethasone and Dexamethasone Phosphate: Effect on DMPC Membrane Models

Dexamethasone (Dex) and Dexamethasone phosphate (Dex-P) are synthetic glucocorticoids with high anti-inflammatory and immunosuppressive actions that gained visibility because they reduce the mortality in critical patients with COVID-19 connected to assisted breathing. They have been widely used for the treatment of several diseases and in patients under chronic treatments, thus, it is important to understand their interaction with membranes, the first barrier when these drugs get into the body. Here, the effect of Dex and Dex-P on dimyiristoylphophatidylcholine (DMPC) membranes were studied using Langmuir films and vesicles. Our results indicate that the presence of Dex in DMPC monolayers makes them more compressible and less reflective, induces the appearance of aggregates, and suppresses the Liquid Expanded/Liquid Condensed (LE/LC) phase transition. The phosphorylated drug, Dex-P, also induces the formation of aggregates in DMPC/Dex-P films, but without disturbing the LE/LC phase transition and reflectivity. Insertion experiments demonstrate that Dex induces larger changes in surface pressure than Dex-P, due to its higher hydrophobic character. Both drugs can penetrate membranes at high lipid packings. Vesicle shape fluctuation analysis shows that Dex-P adsorption on GUVs of DMPC decreases membrane deformability. In conclusion, both drugs can penetrate and alter the mechanical properties of DMPC membranes.

Hopanoid Hopene Locates in the Interior of Membranes and Affects Their Properties

Hopanoids are proposed as sterol surrogates in some bacteria, and it has been proved that some hopanoids are able to induce a liquid-order phase state in lipid membranes. The members of this group of molecules have diverse structures, and not all of them have been studied in detail yet. Here, we study membranes with the hopanoid hopene (hop-22 (29)-ene or diploptene), which is the product of the cycling of squalene by squalene-hopene cyclase, and thus is present in the first step of hopanoid biosynthesis. Hopene is particularly interesting because it lacks a polar head group, which opens the question of how does this molecule accommodate in a lipid membrane, and what are the effects promoted by its presence. In order to get an insight into this, we prepared monolayers and bilayers of a phospholipid with hopene and studied their properties in comparison with pure phospholipid membranes, and with the sterol cholesterol or the hopanoid diplopterol. Film stiffness, shear viscosity, and bending dynamics were very affected by the presence of hopene, while zeta-potential, generalized polarization of Laurdan, and conductivity were affected moderately by this molecule. The results suggest that at very low percentages, hopene locates parallel to the phospholipid molecules, while the excess of the hopene molecules stays between leaflets, as previously proposed using molecular dynamics simulations.

Recovery from chilling modulates the acyl-editing of phosphatidic acid molecular species in barley roots (Hordeum vulgare L.)

In plants, lipid metabolism and remodelling are key mechanisms for survival under temperature stress. The present study attempted to compare the lipid profile in barley roots both under chilling stress treatment and in the subsequent recovery to stress. Lipids were obtained through a single-extraction method with a polar solvent mixture, followed by mass spectrometry analysis. The results indicate that lipid metabolism was significantly affected by chilling. Most of the glycerolipids analysed returned to control values during short- and long-term recovery, whereas several representative phosphatidic acid (PA) molecular species were edited during long-term recovery. Most of the PA molecular species that increased in the long-term had the same acyl chains as the phosphatidylcholine (PC) species that decreased. C34:2 and C36:4 underwent the most remarkable changes. Given that the mechanisms underlying the acyl-editing of PC in barley roots remain elusive, we also evaluated the contribution of lysophosphatidylcholine acyltransferases (HvLPCAT) and phospholipase A (HvPLA). In line with the aforementioned results, the expression of the HvLPCAT and HvPLA genes was up-regulated during recovery from chilling. The differential acyl-editing of PA during recovery, which involves the remodelling of PC, might therefore be a regulatory mechanism of cold tolerance in barley.

Triglyceride Lenses at the Air-Water Interface as a Model System for Studying the Initial Stage in the Biogenesis of Lipid Droplets

Lipid droplets (LD) are intracellular structures consisting of an apolar lipid core, composed mainly of triglycerides (TG) and steryl esters, coated by a lipid-protein mixed monolayer. The mechanisms underlying LD biogenesis at the endoplasmic reticulum membrane are a matter of many current investigations. Although models explaining the budding-off of protuberances of phase-segregated TG inside bilayers have been proposed recently, the assumption of such initial blisters needs further empirical support. Here, we study mixtures of egg phosphatidylcholine (EPC) and TG at the air-water interface in order to describe some physical properties and topographic stability of TG bulk structures in contact with interfaces. Brewster angle microscopy images revealed the appearance of microscopic collapsed structures (CS) with highly reproducible lateral size (∼1 μm lateral radius) not varying with lateral packing changes and being highly stable at surface pressures (π) beyond collapse. By surface spectral fluorescence microscopy, we were able to characterize the solvatochromism of Nile Red both in monolayers and inside CS. This allowed to conclude that CS corresponded to a phase of liquid TG and to characterize them as lenses forming a three-phase (oil-water-air) system. Thereby, the thicknesses of the lenses could be determined, observing that they were dramatically flattened when EPC was present (6-12 nm compared to 30-50 nm for lenses on EPC/TG and TG films, respectively). Considering the shape of lenses, the interfacial tensions, and the Neumann's triangle, this experimental approach allows one to estimate the oil-water interfacial tension acting at each individual microscopic lens and at varying compression states of the surrounding monolayer. Thus, lenses formed on air-water Langmuir films can serve to assess variables of relevance to the initial step of LD biogenesis, such as the degree of dispersion of excluded-TG phase and shape, spatial distribution, and oil-water interfacial tension of lenses.

On the Coupling between Mechanical Properties and Electrostatics in Biological Membranes

Cell membrane structure is proposed as a lipid matrix with embedded proteins, and thus, their emerging mechanical and electrostatic properties are commanded by lipid behavior and their interconnection with the included and absorbed proteins, cytoskeleton, extracellular matrix and ionic media. Structures formed by lipids are soft, dynamic and viscoelastic, and their properties depend on the lipid composition and on the general conditions, such as temperature, pH, ionic strength and electrostatic potentials. The dielectric constant of the apolar region of the lipid bilayer contrasts with that of the polar region, which also differs from the aqueous milieu, and these changes happen in the nanometer scale. Besides, an important percentage of the lipids are anionic, and the rest are dipoles or higher multipoles, and the polar regions are highly hydrated, with these water molecules forming an active part of the membrane. Therefore, electric fields (both, internal and external) affects membrane thickness, density, tension and curvature, and conversely, mechanical deformations modify membrane electrostatics. As a consequence, interfacial electrostatics appears as a highly important parameter, affecting the membrane properties in general and mechanical features in particular. In this review we focus on the electromechanical behavior of lipid and cell membranes, the physicochemical origin and the biological implications, with emphasis in signal propagation in nerve cells.

The antimicrobial peptide Polybia-MP1 differentiates membranes with the hopanoid, diplopterol from those with cholesterol

Polybia-MP1 is an antimicrobial peptide that shows a decreased activity in membranes with cholesterol (CHO). Since it is now accepted that hopanoids act as sterol-surrogates in some sterol-lacking bacteria, we here inquire about the impact of Polybia-MP1 on membranes containing the hopanoid diplopterol (DP) in comparison to membranes with CHO. We found that, despite the properties induced on lipid membranes by DP are similar to those induced by CHO, the effect of Polybia-MP1 on membranes with CHO or DP was significantly different. DP did not prevent dye release from LUVs, nor the insertion of Polybia-MP1 into monolayers, and peptide-membrane affinity was higher for those with DP than with CHO. Zeta potentials ( ζ ) for DP-containing LUVs showed a complex behavior at increasing peptide concentration. The effect of the peptide on membrane elasticity, investigated by nanotube retraction experiments, showed that peptide addition softened all membrane compositions, but membranes with DP got stiffer at long times. Considering this, and the ζ results, we propose that peptides accumulate at the interface adopting different arrangements, leading to a non-monotonic behavior. Possible correlations with cell membranes were inquired testing the antimicrobial activity of Polybia-MP1 against hopanoid-lacking bacteria pre-incubated with DP or CHO. The fraction of surviving cells was lower in cultures incubated with DP compared to those incubated with CHO. We propose that the higher activity of Polybia-MP1 against some bacteria compared to mammalian cells is not only related to membrane electrostatics, but also the composition of neutral lipids, particularly the hopanoids, could be important.

Surface charge density and fatty acids enhance the membrane permeation rate of CPP-cargo complexes

The CPP-effect makes reference to the process by which the membrane translocation rate of a cargo is enhanced by chemical functionalization with cell-penetrating peptides (CPPs). In this work we combine a simple kinetic model with free-energy calculations to explore the energetic basis of the CPP-effect. Two polyglicines are selected as model hydrophilic cargoes, and nona-arginine as a prototypical CPP. We assess the cargo carrying efficiency of nona-arginine by comparing the adsorption and insertion energies of the cargoes, the cargo-free CPPs, and the CPP-cargo complexes, into lipid membranes of varying composition. We also analyze the effect of modifying the type and concentration of anionic lipids, and the implication of these factors on the translocation rate of the CPP-cargo complex. Of particular interest is the evaluation of the catalytic role of palmitic acid (palmitate) as a promoter of the CPP-effect. We also analyse the influence of the size of the cargo on the membrane adsorption and insertion energies. Our results show that the efficiency of nona-arginine as a transmembrane carrier of simple hydrophilic molecules is modulated by the size of the cargo, and is strongly enhanced by increasing the concentration of anionic lipids and of ionized fatty acids in the membrane.

N-terminal acetylation of a mastoparan-like peptide enhances PE/PG segregation in model membranes

The synthetic peptides L1A and its acetylated analog (acL1A) display potent Gram-negative bactericidal activities without being hemolytic. We have gathered evidence that the N-terminal acetylation of L1A enhances the lytic activity in anionic vesicles with high capability to insert into and disturb lipid packing of model membranes. Here, the impact of L1A and acL1A was evaluated on a model membrane that mimics the cytoplasmic membrane of Gram-negative bacteria, which is rich in phosphatidylethanolamine (PE) and phosphatidylglycerol (PG), using 3:1 mixture of POPE/DOPG and a variety of techniques. We followed peptide adsorption and penetration by zeta potential determination of large unilamellar vesicles, accessibility of tryptophan residue to acrylamide by quenching assays, and Gibbs isotherms. The secondary structure of the peptide on the membranes was assessed using circular dichroism. Peptide mixing ability with the lipids and phase segregation was assessed by the observation of Langmuir monolayers with fluorescence microscopy, as well as with differential scanning calorimetry thermograms of multilamellar vesicles. All in all, the results indicate that both peptides adsorb and penetrate POPE/DOPG membranes with similar affinities, decreasing the surface charge, and adopting alpha structures. Both peptides mix with DOPG and demix from POPE, and consequently, persist at the interface to larger surface pressures in the presence of PG than in pure PE monolayers. This selective degree of mixing of the peptides with PE and PG leads to peptide-induced segregation of PG from PE, being the less charged peptide, acL1A, able to segregate the lipids more efficiently.

Somuncurins: Bioactive Peptides from the Skin of the Endangered Endemic Patagonian Frog Pleurodema somuncurense

The skin glands of amphibian species hold a major component of their innate immunity, namely a unique set of antimicrobial peptides (AMPs). Although most of them have common characteristics, differences in AMP sequences allow a huge repertoire of biological activity with varying degrees of efficacy. We present the first study of the AMPs from Pleurodema somuncurence (Anura: Leptodactylidae: Leiuperinae). Among the 11 identified mature peptides, three presented antimicrobial activity. Somuncurin-1 (FIIWPLRYRK), somuncurin-2 (FILKRSYPQYY), and thaulin-3 (NLVGSLLGGILKK) inhibited Escherichia coli growth. Somuncurin-1 also showed antimicrobial activity against Staphylococcus aureus. Biophysical membrane model studies revealed that this peptide had a greater permeation effect in prokaryotic-like membranes and capacity to restructure liposomes, suggesting fusogenic activity, which could lead to cell aggregation and disruption of cell morphology. This study contributes to the characterization of peptides with new sequences to enrich the databases for the design of therapeutic agents. Furthermore, it highlights the importance of investing in nature conservation and the power of genetic description as a strategy to identify new compounds.

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.

The challenge of post-mortem GHB analysis: storage conditions and specimen types are both important

BACKGROUND

For the interpretation of concentrations of gamma-hydroxybutyrate (GHB) in post-mortem specimens, a possible increase due to post-mortem generation in the body and in vitro has to be considered. The influence of different storage conditions and the specimen type was investigated.

METHOD AND MATERIAL

Post-mortem GHB concentrations in femoral venous blood (VB), heart blood (HB), serum (S) from VB, urine (U), cerebrospinal fluid (CSF) and vitreous humour (VH) were determined by gas chromatography-mass spectrometry after derivatisation. Various storage conditions, that is 4 °C or room temperature (RT) and the addition of sodium fluoride (NaF), were compared during storage up to 30 days. Additionally, bacterial colonisation was determined by mass spectrometry fingerprinting.

RESULTS

Twenty-six cases without involvement of exogenous GHB were examined. GHB concentrations (by specimen) at day 0 were 3.9-22.1 mg/L (VB), 6.6-33.3 mg/L (HB), < 0.5-18.1 mg/L (U), 1.1-10.4 mg/L (CSF) and 1.7-22.0 mg/L (VH). At 4 °C, concentrations increased at day 30 to 5.6-74.5 mg/L (VB), 4.6-76.5 mg/L (HB) and < 0.5-21.3 mg/L (U). At RT, concentrations rose to < 0.5-38.5 mg/L (VB), 1.2-94.6 mg/L (HB) and < 0.5-37.5 mg/L (U) at day 30. In CSF, at RT, an increase up to < 0.5-21.2 mg/L was measured, and at 4 °C, a decrease occurred (< 0.5-6.5 mg/L). GHB concentrations in VH remained stable at both temperatures (1.2-20.9 mg/L and < 0.5-26.2 mg/L). The increase of GHB in HB samples with NaF was significantly lower than that without preservation. No correlation was found between the bacterial colonisation and extent of GHB concentration changes.

CONCLUSION

GHB concentrations can significantly increase in post-mortem HB, VB and U samples, depending on storage time, temperature and inter-individual differences. Results in CSF, VH, S and/or specimens with NaF are less affected.

Hopanoids Like Sterols Form Compact but Fluid Films

Hopanoids are pentacyclic molecules present in membranes from some bacteria, recently proposed as sterol surrogates in these organisms. Diplopterol is an abundant hopanoid that, similar to sterols, does not self-aggregate in lamellar structures when pure, but forms monolayers at the air-water interface. Here, we analyze the interfacial behavior of pure diplopterol and compare it with sterols from different organisms: cholesterol from mammals, ergosterol from fungi, and stigmasterol from plants. We prepared Langmuir monolayers of the compounds and studied their surface properties using different experimental approaches and molecular dynamics simulations. Our results indicate that the films formed by diplopterol, despite being compact with low mean molecular areas, high surface potentials, and high refractive index, depict shear viscosity values similar to that for fluid films. Altogether, our results reveal that hopanoids have similar interfacial behavior than that of sterols, and thus they may have the capacity of modulating bacterial membrane properties in a similar way sterols do in eukaryotes.

The interfacial electrostatic potential modulates the insertion of cell-penetrating peptides into lipid bilayers

Cell-penetrating peptides (CPP) are short sequences of cationic amino-acids that show a surprising ability to traverse lipid bilayers. CPP are considered to be some of the most effective vectors to introduce membrane-impermeable cargos into cells, but the molecular basis of the membrane translocation mechanisms and its dependence on relevant membrane physicochemical properties have yet to be fully determined. In this paper we resort to Molecular Dynamics simulations and experiments to investigate how the electrostatic potential across the lipid/water interface affects the insertion of hydrophilic and amphipathic CPP into two-dimensional lipid structures. Simulations are used to quantify the effect of the transmembrane potential on the free-energy profile associated with the transfer of the CPP across a neutral lipid bilayer. It is found that the electrostatic bias has a relatively small effect on the binding of the peptides to the membrane surface, but that it significantly lowers the permeation barrier. A charge compensation mechanism, arising from the segregation of counter-ions while the peptide traverses the membrane, determines the shape and symmetry of the free-energy curves and underlines relevant mechanistic considerations. Langmuir monolayer experiments performed with a variety of amphiphiles model the incorporation of the CPP into the external membrane leaflet. It is shown that the dipole potential of the monolayer controls the extent of penetration of the CPP into the lipid aggregate, to a greater degree than its surface charge.

Mechanical Stability of Lipid Membranes Decorated with Dextran Sulfate

Lipid vesicles decorated with polysaccharides have been proposed as vehicles for drug delivery because the polymers confer to the vesicles an enhanced stability, increasing the probability of the drug for reaching the target cell. Here, we first test the affinity of dextran sulfate (DS) for two different vesicle composition, and afterward, we study the effect of DS on the liposome mechanical properties. We found that DS binds to both tested membrane compositions. The interaction of DS with the anionic membranes studied here is mediated by the metal ions present in the aqueous solution (Na⁺ and Ca²⁺), being higher in the presence of Ca²⁺. Binding occurs preferentially in regions of closely packed lipids. Strikingly, DS did not affect the stability against detergent and the membrane rigidity of none of the vesicles. Thus, the proposed stability increase induced by this kind of polymers in drug delivery systems is not related with a modulation of the membrane thermodynamic properties but to other biochemical factors.

Negative Dipole Potentials and Carboxylic Polar Head Groups Foster the Insertion of Cell-Penetrating Peptides into Lipid Monolayers

Cell-penetrating peptides (CPPs) are polycationic sequences of amino acids recognized as some of the most effective vehicles for delivering membrane-impermeable cargos into cells. CPPs can traverse cell membranes by direct translocation, and assessing the role of lipids on the membrane permeation process is important to convene a complete model of the CPP translocation. In this work, we focus on the biophysical basis of peptide-fatty acid interactions, analyzing how the acid-base and electrostatic properties of the lipids determine the CPP adsorption and incorporation into a Langmuir monolayer, focusing thus on the first two stages of the direct translocation mechanism. We sense the binding and insertion of the peptide into the lipid structure by measuring the changes in the surface pressure, the surface potential, and the reflectivity of the interface. We show that, beyond the presence of anionic moieties, negative dipole potentials and carboxylic polar head groups significantly promote the insertion of the peptide into the monolayer. On the basis of our results, we propose the appearance of stable CPP-lipid complexes whose kinetics of formation depends on the length of the lipids' hydrocarbon chains.

Regulation of phase boundaries and phase-segregated patterns in model membranes

Demixing of components has long been described in model membranes. It is a consequence of non-ideal lateral interactions between membrane components, and it causes the presence of segregated phases, forming patches (domains) of different properties, thus introducing heterogeneity into the membrane. In the present review we first describe the processes through which domains are generated, how they grow, and why they are rounded, striped or fractal-like, as well as why they get distributed forming defined patterns. Next, we focus on the effect of an additive on a lipid mixture, which usually induces shifts in demixing points, thus stabilizing or destabilizing the phase-segregated state. Results found for different model membranes are summarized, detailing the ways in which phase segregation and the generated patterns may be modulated. We focus on which are, from our viewpoint, the most relevant regulating factors affecting the surface texture observed in model membranes. This article is part of a Special Issue entitled: Emergence of Complex Behavior in Biomembranes edited by Marjorie Longo.

Effect of N-terminal acetylation on lytic activity and lipid-packing perturbation induced in model membranes by a mastoparan-like peptide

L1A (IDGLKAIWKKVADLLKNT-NH2) is a peptide that displays a selective antibacterial activity to Gram-negative bacteria without being hemolytic. Its lytic activity in anionic lipid vesicles was strongly enhanced when its N-terminus was acetylated (ac-L1A). This modification seems to favor the perturbation of the lipid core of the bilayer by the peptide, resulting in higher membrane lysis. In the present study, we used lipid monolayers and bilayers as membrane model systems to explore the impact of acetylation on the L1A lytic activity and its correlation with lipid-packing perturbation. The lytic activity investigated in giant unilamellar vesicles (GUVs) revealed that the acetylated peptide permeated the membrane at higher rates compared with L1A, and modified the membrane's mechanical properties, promoting shape changes. The peptide secondary structure and the changes in the environment of the tryptophan upon adsorption to large unilamellar vesicles (LUVs) were monitored by circular dichroism (CD) and red-edge excitation shift experiments (REES), respectively. These experiments showed that the N-terminus acetylation has an important effect on both, peptide secondary structure and peptide insertion into the bilayer. This was also confirmed by experiments of insertion into lipid monolayers. Compression isotherms for peptide/lipid mixed films revealed that ac-L1A dragged lipid molecules to the more disordered phase, generating a more favorable environment and preventing the lipid molecules from forming stiff films. Enthalpy changes in the main phase transition of the lipid membrane upon peptide insertion suggested that the acetylated peptide induced higher impact than the non-acetylated one on the thermotropic behavior of anionic vesicles.

Wrinkled labyrinths in critical demixing ferrofluid

A thin film of a critical ferrofluid mixture undergoes a sequence of transitions in a magnetic field. First the application of a field induces a critical demixing of the fluid into cylindrical droplets of the minority phase immersed in an extended majority phase. At a second critical field the cylindrical shape is destabilized and transforms into a labyrinth pattern. A third wrinkling transition occurs at even higher field if the liquid has a liquid/air interface. The wrinkling is absent if the droplet has a cover-slide on top. We explain the wrinkling by the wetting behavior of the liquid/air interface that shifts the surface region away from a critical demixing point.

The insertion of Polybia-MP1 peptide into phospholipid monolayers is regulated by its anionic nature and phase state

Polybia-MP1 or simply MP1 (IDWKKLLDAAKQIL-NH₂) is a peptide with broad-spectrum bactericidal activity and a strong inhibitory effect against cancer cells. The aim of this work was to evaluate the effect of biophysical properties such as membrane texture and film thickness on MP1 interaction with neutral and anionic lipid membranes. For this purpose, we first explored the peptide's surface behavior. MP1 showed high surface activity, adsorbing onto bare air/aqueous interfaces up to higher surface pressures than the collapse pressure of MP1 Langmuir films. The MP1-lipid membrane interaction was studied using Langmuir phosphatidylcholine and phosphatidylserine (PS) monolayers as model membrane systems. PS was chosen since this negatively charged lipid was found predominantly on the outer leaflet of tumor cells, and it enhances MP1 activity for PS-containing membranes to a greater extent than for other negatively charged lipids. MP1 incorporated into anionic PS monolayers, which show a liquid-expanded (LE) phase or LE-liquid-condensed (LC) phase coexistence, up to lipid-packing densities higher than those of cell membranes. The mixed lipid/MP1 films were explored by Brewster angle microscopy and atomic force microscopy. MP1 partitioned preferentially into the LE phase state of PS films, and were thus excluded from the coexisting LC phase. This interaction had strong electrostatic bases: in pure water, the lipid-peptide interaction was strong enough to induce formation of reversible lipid-peptide 3D structures associated with the interface. MP1 incorporation into the LE phase was accompanied by a shift of the phase transition pressure to higher values and a thinning of the lipid film. These results showed a clear correlation between peptide penetration capacity and the presence or induction of the thin LE phase. This capacity to regulate membrane physical properties may be of relevance in the binding, incorporation and membrane selectivity of this promising antitumor peptide.

Molecular Explanation for the Abnormal Flux of Material into a Hot Spot in Ester Monolayers

Langmuir monolayers of certain surfactants show a negative derivative of the surface pressure with respect to temperature. In these monolayers, a local temperature gradient leads to local yielding of the solid phase to a kinetically flowing liquid, so that the material flows toward the hotter regions that act as sinks. The accumulation of material leads to the formation of nonequilibrium multilamellar bubbles of different sizes. Here we investigate the molecular factors leading to such a peculiar behavior. First, we identify the required structural molecular moieties, and second we vary the composition of the subphase in order to analyze its influence. We conclude that esters appear to be unique in two key aspects: they form monolayers whose compression isotherms shift to lower areas as the temperature increases, and thus collapse into a hot spot; and they bind weakly to the aqueous subphase, i.e., water does not attach to the monolayer at the molecular level, but only supports it. Molecular simulations for a selected system confirm and help explain the observed behavior: surfactant molecules form a weak hydrogen bonding network, which is disrupted upon heating, and also the molecular tilting changes with temperature, leading to changes in the film density.

Electrostatic interactions at the microscale modulate dynamics and distribution of lipids in bilayers

For decades, it has been assumed that electrostatic long-range (micron distances) repulsions in lipid bilayers are negligible due to screening from the aqueous milieu. This concept, mostly derived from theoretical calculations, is broadly accepted in the biophysical community. Here we present experimental evidence showing that domain-domain electrostatic repulsions in charged and also in neutral lipid bilayers regulate the diffusion, in-plane structuring and merging of lipid domains in the micron range. All the experiments were performed on both, lipid monolayers and bilayers, and the remarkable similarity in the results found in bilayers compared to monolayers led us to propose that inter-domain repulsions occur mainly within the plane of the membrane. Finally, our results indicate that electrostatic interactions between the species inserted in a cell membrane are not negligible, not only at nanometric but also at larger distances, suggesting another manner for regulating the membrane properties.

Dipolar interactions between domains in lipid monolayers at the air-water interface

A great variety of biologically relevant monolayers present phase coexistence characterized by domains formed by lipids in an ordered phase state dispersed in a continuous, disordered phase. From the difference in surface densities between these phases, inter-domain dipolar interactions arise. These interactions are relevant for the determination of the spacial distribution of domains as well as their dynamics. In this work, we propose a novel way of estimating the dipolar repulsion using a passive method that involves the analysis of images of the monolayer with phase coexistence. This method is based on the comparison of the pair correlation function obtained from experiments with that obtained from Brownian dynamics simulations of a model system. As an example, we determined the difference in dipolar density of a binary monolayer of DSPC/DMPC at the air-water interface from the analysis of the radial distribution of domains, and the results are compared with those obtained by surface potential determinations. A systematic analysis for the experimentally relevant parameter range is given, which may be used as a working curve for obtaining the dipolar repulsion in different systems.

The Rheological Properties of Lipid Monolayers Modulate the Incorporation of l-Ascorbic Acid Alkyl Esters

In this work, we tested the hypothesis that the incorporation of amphiphilic drugs into lipid membranes may be regulated by their rheological properties. For this purpose, two members of the l-ascorbic acid alkyl esters family (ASCn) were selected, ASC16 and ASC14, which have different rheological properties when organized at the air/water interface. They are lipophilic forms of vitamin C used in topical pharmacological preparations. The effect of the phase state of the host lipid membranes on ASCn incorporation was explored using Langmuir monolayers. Films of pure lipids with known phase states have been selected, showing liquid-expanded, liquid-condensed, and solid phases as well as pure cholesterol films in liquid-ordered state. We also tested ternary and quaternary mixed films that mimic the properties of cholesterol containing membranes and of the stratum corneum. The compressibility and shear properties of those monolayers were assessed in order to define its phase character. We found that the length of the acyl chain of the ASCn compounds induces differential changes in the rheological properties of the host membrane and subtly regulates the kinetics and extent of the penetration process. The capacity for ASCn uptake was found to depend on the phase state of the host film. The increase in surface pressure resultant after amphiphile incorporation appears to be a function of the capacity of the host membrane to incorporate such amphiphile as well as the rheological response of the film. Hence, monolayers that show a solid phase state responded with a larger surface pressure increase to the incorporation of a comparable amount of amphiphile than liquid-expanded ones. The cholesterol-containing films, including the mixture that mimics stratum corneum, allowed a very scarce ASCn uptake independently of the membrane diffusional properties. This suggests an important contribution of Cho on the maintenance of the barrier function of stratum corneum.

The Presence of Sterols Favors Sticholysin I-Membrane Association and Pore Formation Regardless of Their Ability to Form Laterally Segregated Domains

Sticholysin I (St I) is a pore-forming toxin (PFT) produced by the Caribbean Sea anemone Stichodactyla helianthus belonging to the actinoporin protein family, a unique class of eukaryotic PFT. As for actinoporins, it has been proposed that the presence of cholesterol (Chol) and the coexistence of lipid phases increase binding to the target membrane and pore-forming ability. However, little is known about the role of membrane structure and dynamics (phase state, fluidity, and the presence of lipid domains) on the activity of actinoporins or which regions of the membrane are the most favorable for protein insertion, oligomerization, and eventually pore formation. To gain insight into the role of membrane properties on the functional activity of St I, we studied its binding to monolayers and vesicles of phosphatidylcholine (PC), sphingomyelin (SM), and sterols inducing (ergosterol -Erg and cholesterol -Chol) or not (cholestenone - Cln) membrane phase segregation in liquid ordered (Lo) and liquid disordered (Ld) domains. This study revealed that St I binds and permeabilizes with higher efficiency sterol-containing membranes independently of their ability to form domains. We discuss the results in terms of the relevance of different membrane properties for the actinoporins mechanism of action, namely, molecular heterogeneity, specially potentiated in membranes with sterols inducers of phase separation (Chol or Erg) or Cln, a sterol noninducer of phase separation but with a high propensity to induce nonlamellar phase. The role of the Ld phase is pointed out as the most suitable platform for pore formation. In this regard, such regions in Chol-containing membranes seem to be the most favored due to its increased fluidity; this property promotes toxin insertion, diffusion, and oligomerization leading to pore formation.

Searching for line active molecules on biphasic lipid monolayers

In membranes with phase coexistence, line tension appears as an important parameter for the determination of the amount of domains, as well as their size and their shape, thus defining the membrane texture. Different molecules have been proposed as "linactants" (i.e. molecules that reduce the line tension, thereby modulating the membrane texture). In this work, we explore the efficiency of different molecules as linactants in monolayers with two coexisting phases of different thicknesses. We tested the linactant ability of a molecule with chains of different saturation degrees, another molecule with different chain lengths and a bulky molecule. In this way, we show in the same system the effect of molecules with chains of different rigidities, with an intrinsic thickness mismatch and with a bulky moiety, thereby analyzing different hypotheses of how a molecule may change the line tension in a monolayer system. Both lipids with different hydrocarbon chains did not act as linactants, while only one of the bulky molecules tested decreased the line tension in the monolayer studied. We conclude that there are no universal rules for the structure of a molecule that enable us to predict that it will behave as a linactant and thus, designing linactants appears to be a difficult task and a challenge for future studies. Furthermore, in regard to the membrane texture, there was no direct influence of the line tension in the distribution of domain sizes.

Zn(2+)-dependent surface behavior of diacylglycerol pyrophosphate and its mixtures with phosphatidic acid at different pHs

Diacylglycerol pyrophosphate (DGPP) is a minor lipid that attenuates the phosphatidic acid (PA) signal, and also DGPP itself would be a signaling lipid. Diacylglycerol pyrophosphate is an anionic phospholipid with a pyrophosphate group attached to diacylglycerol that was shown to respond to changes of pH, thus affecting the surface organization of DGPP and their interaction with PA. In this work, we have investigated how the presence of Zn(2+) modulates the surface organization of DGPP and its interaction with PA at acidic and basic pHs. Both lipids formed expanded monolayers at pHs 5 and 8. At pH 5, monolayers formed by DGPP became stiffer when Zn(2+)was added to the subphase, while the surface potential decreased. At this pH, Zn(2+) induced a phase transition from an expanded to a condensed-phase state in monolayers formed by PA. Conversely, at pH 8 the effects induced by the presence of Zn(2+) on the surface behaviors of the pure lipids were smaller. Thus, the interaction of the bivalent cation with both lipids was modulated by pH and by the ionization state of the polar head groups. Mixed monolayers of PA and DGPP showed a non-ideal behavior and were not affected by the presence of Zn(2+) at pH 8. This could be explained considering that when mixed, the lipids formed a closely packed monolayer that could not be further modified by the cation. Our results indicate that DGPP and PA exhibit expanded- and condensed-phase states depending on pH, on the proportion of each lipid in the film and on the presence of Zn(2+). This may have implications for a possible role of DGPP as a signaling lipid molecule.

Surface behavior of sphingomyelins with very long chain polyunsaturated fatty acids and effects of their conversion to ceramides

Molecular species of sphingomyelin (SM) with nonhydroxy (n) and 2-hydroxy (h) very long chain polyunsaturated fatty acids (n- and h-28:4, 30:5, and 32:5) abound in rat spermatogenic cells and spermatozoa. These SMs are located on the sperm head, where they are converted to the corresponding ceramides (Cer) after the completion of the acrosomal reaction, as induced in vitro. The aim of this study was to look into the surface properties of these unique SM species and how these properties change by the SM → Cer conversion. After isolation by HPLC, these SMs were organized in Langmuir films and studied alone, in combination with different proportions of Cer, and during their conversion to Cer by sphingomyelinase. Compression isotherms for all six SMs under study were compatible with a liquid-expanded (LE) state and showed large molecular areas. Only the longest SMs (n-32:5 and h-32:5 SM) underwent a phase transition upon cooling. Interestingly, the abundant h-28:4 Cer exhibited a highly compressible liquid-condensed (LC) phase compatible with a high conformational freedom of Cer molecules but with the characteristic low diffusional properties of the LC phase. In mixed films of h-28:4 SM/h-28:4 Cer, the components showed favorable mixing in the LE phase. The monolayer exhibited h-28:4 Cer-rich domains both in premixed films and when formed by the action of sphingomyelinase on pure h-28:4 SM films. Whereas the SMs from sperm behaved in a way similar to that of shorter acylated SMs, the corresponding Cers showed atypical rheological properties that may be relevant to the membrane structural rearrangements that take place on the sperm head after the completion of the acrosomal reaction.

Stiffness of lipid monolayers with phase coexistence

The surface dilational modulus--or compressibility modulus--has been previously studied for monolayers composed of pure materials, where a jump in this modulus was related with the onset of percolation as a result of the establishment of a connected structure at the molecular level. In this work, we focused on monolayers composed of two components of low lateral miscibility. Our aim was to investigate the compressibility of mixed monolayers at pressures and compositions in the two-phase region of the phase diagram, in order to analyze the effect of the mechanical properties of each phase on the stiffness of the composite. In nine different systems with distinct molecular dipoles and charges, the stiffness of each phase and the texture at the plane of the monolayer were studied. In this way, we were able to analyze the general compressibility of two-phase lipid monolayers, regardless of the properties of their constituent parts. The results are discussed in the light of the following two hypotheses: first, the stiffness of the composite could be dominated by the stiffness of each phase as a weighted sum according to the percentage of each phase area, regardless of the distribution of the phases in the plane of the monolayer. Alternatively, the stiffness of the composite could be dominated by the mechanical properties of the continuous phase. Our results were better explained by this latter proposal, as in all the analyzed mixtures it was found that the mechanical properties of the percolating phase were the determining factors. The value of the compression modulus was closer to the value of the connected phase than to that of the dispersed phase, indicating that the bidimensional composites displayed mechanical properties that were related to the properties of each phases in a rather complex manner.

Molecular determinants for the line tension of coexisting liquid phases in monolayers

The line tension (λ) in biphasic membranes has been determined in monolayers and bilayers using a variety of techniques. In this work we present a novel approach to the determination of λ in monolayers with liquid/liquid phase coexistence, overcoming several of the drawbacks of current techniques. Using our method, we determined the line tension of liquid/liquid phases in binary mixtures of different lipids and a molecule similar to cholesterol but less oxidizable. We analyzed the effect of the hydrocarbon chain length and the polar head-group of the non-sterol lipid and found the latter to exert much more influence than the former. The presence of PE led to high λ values, PG to low values and PS and PC to intermediate values. The line tension showed a strong correlation with the critical packing parameter of the phospholipid. The spontaneous curvature displayed by the phases constituted by a particular lipid appears to be an important parameter for determining the line tension in mixed films.

Surface phase behavior and domain topography of ascorbyl palmitate monolayers

Ascorbyl palmitate (ASC(16)) is a molecule of potential pharmacological interest due to its antioxidant properties and amphiphilic nature. The surface behavior of ASC(16) was studied using Langmuir monolayers and Brewster angle microscopy. This molecule formed stable monolayers at room temperature that showed phase transition from a liquid-expanded to liquid-condensed or crystalline phase, depending on the subphase conditions. Using a theoretical approach, we were able to explain the behavior of the ASC(16) film at different bulk pH values and salt conditions based on the surface pH and the dissociation fraction of the film. Both condensed phases corresponded to highly packed conditions with the crystalline phase occurring at a low charge density, showing molecular tilting and preferential growth at characteristic angles, while the liquid-condensed phase formed in highly charged surfaces revealed small flowerlike domains probably as a consequence of internal dipole repulsion. A smaller perpendicular dipole moment was observed for the crystalline than the liquid-condensed phase which may explain the domain features. In conclusion, ASC(16) showed a complex surface behavior that was highly sensitive to subphase conditions.

Electrostatic field effects on membrane domain segregation and on lateral diffusion

Natural membranes are organized structures of neutral and charged molecules bearing dipole moments which generate local non-homogeneous electric fields. When subjected to such fields, the molecules experience net forces that can modify the lipid and protein organization, thus modulating cell activities and influencing (or even dominating) the biological functions. The energetics of electrostatic interactions in membranes is a long-range effect which can vary over distance within r^-1 to r^-3. In the case of a dipole interacting with a plane of dipoles, e.g. a protein interacting with a lipid domain, the interaction is stronger than two punctual dipoles and depends on the size of the domain. In this article, we review several contributions on how electrostatic interactions in the membrane plane can modulate the phase behavior, surface topography and mechanical properties in monolayers and bilayers.

Phase diagram of mixed monolayers of stearic acid and dimyristoylphosphatidylcholine. Effect of the acid ionization

The aim of this work is to study the phase diagram of mixed monolayers composed of dimyristoylphosphatidylcholine (DMPC) and stearic acid (SA) at different ionic strength and bulk pH of the aqueous subphase. In this way, the effect of ionization of SA on the interaction and thus on phase separation with the DMPC matrix can be analyzed. To this purpose, we first determined the ionization state of pure SA monolayers as a function of the bulk subphase pH. The SA monolayers are nearly fully ionized at pH 10 and essentially neutral at pH 4 and the mixture of DMPC and SA was studied at those two pHs. We found that the DMPC-enriched phase admits more SA if the SA monolayer is in a liquid-expanded state, which is highly related to the acid ionization state, and thus to the bulk pH and ionic strength. At pH 4 the molecules hardly mix while at pH 10 the mixed monolayer with DMPC can admit between 30 and 100% of SA (depending on the lateral pressure) before phase separation is established. The addition of calcium ions to the subphase has a condensing effect on SA monolayers at all pHs and the solubility of SA in the DMPC matrix does not depend on the bulk pH in these conditions. The observed phase diagrams are independent on the manner in which the state of the mixed film is reached and may thus be considered states of apparent equilibrium.

Redox-active tyrosine residue in the microcin J25 molecule

Microcin J25 (MccJ25) is a 21 amino acid lasso-peptide antibiotic produced by Escherichia coli and composed of an 8-residues ring and a terminal 'tail' passing through the ring. We have previously reported two cellular targets for this antibiotic, bacterial RNA polymerase and the membrane respiratory chain, and shown that Tyr9 is essential for the effect on the membrane respiratory chain which leads to superoxide overproduction. In the present paper we investigated the redox behavior of MccJ25 and the mutant MccJ25 (Y9F). Cyclic voltammetry measurements showed irreversible oxidation of both Tyr9 and Tyr20 in MccJ25, but infrared spectroscopy studies demonstrated that only Tyr9 could be deprotonated upon chemical oxidation in solution. Formation of a long-lived tyrosyl radical in the native MccJ25 oxidized by H₂O₂ was demonstrated by Electron Paramagnetic Resonance Spectroscopy; this radical was not detected when the reaction was carried out with the MccJ25 (Y9F) mutant. These results show that the essential Tyr9, but not Tyr20, can be easily oxidized and form a tyrosyl radical.

Rheological properties of a two phase lipid monolayer at the air/water interface: effect of the composition of the mixture

Many biologically relevant monolayers show coexistence of discrete domains of a long-range ordered condensed phase dispersed in a continuous, disordered, liquid-expanded phase. In this work, we determined the viscous and elastic components of the compressibility modulus and the shear viscosity of monolayers exhibiting phase coexistence with the aim at elucidating the contribution of each phase to the observed monolayer mechanical properties. To this purpose, mixed monolayers with different proportions of distearoylphosphatidylcholine (DSPC) and dimyristoylphosphatidylcholine (DMPC) were prepared and their rheological properties were analyzed. The relationship between the phase diagram of the mixture at 10 mN m(-1) and the rheological properties was studied. We found that the monolayer shear viscosity is highly dependent on the presence of domains and on the domain density. In turn, the monolayer compressibility is only influenced by the presence of domains for high domain densities. For monolayers that look homogeneous on the micrometer scale (DSPC amount lower that 23 mol %), all the analyzed rheological properties remain similar to those observed for pure DMPC monolayers, indicating that in this proportion range the DSPC molecules contribute as DMPC to the surface rheology in spite of having hydrocarbon chains four carbons longer.