Fluctuations and the rate-limiting step of peptide-induced membrane leakage

Effects of antimicrobial peptides on membrane dynamics: A comparison of fluorescence and NMR experiments

Antimicrobial peptides (AMPs) represent a promising class of compounds to fight resistant infections. They are commonly thought to kill bacteria by perturbing the permeability of their cell membranes. However, bacterial killing requires a high coverage of the cell surface by bound peptides, at least in the case of cationic and amphipathic AMPs. Therefore, it is conceivable that peptide accumulation on the bacterial membranes might interfere with vital cellular functions also by perturbing bilayer dynamics, a hypothesis that has been termed "sand in the gearbox". Here we performed a systematic study of such possible effects, for two representative peptides (the cationic cathelicidin PMAP-23 and the peptaibol alamethicin), employing fluorescence and NMR spectroscopies. These approaches are commonly applied to characterize lipid order and dynamics, but sample different time-scales and could thus report on different membrane properties. In our case, fluorescence anisotropy measurements on liposomes labelled with probes localized at different depths in the bilayer showed that both peptides perturb membrane fluidity and order. Pyrene excimer-formation experiments showed a peptide-induced reduction in lipid lateral mobility. Finally, laurdan fluorescence indicated that peptide binding reduces water penetration below the headgroups region. Comparable effects were observed also in fluorescence experiments performed directly on live bacterial cells. By contrast, the fatty acyl chain order parameters detected by deuterium NMR spectroscopy remained virtually unaffected by addition of the peptides. The apparent discrepancy between the two techniques confirms previous sporadic observations and is discussed in terms of the different characteristic times of the two approaches. The perturbation of membrane dynamics in the ns timescale, indicated by the multiple fluorescence approaches reported here, could contribute to the antimicrobial activity of AMPs, by affecting the function of membrane proteins, which is strongly dependent on the physicochemical properties of the bilayer.

Membrane permeabilization can be crucially biased by a fusogenic lipid composition - leaky fusion caused by antimicrobial peptides in model membranes

Induced membrane permeabilization or leakage is often taken as an indication for activity of membrane-active molecules, such as antimicrobial peptides (AMPs). The exact leakage mechanism is often unknown, but important, because certain mechanisms might actually contribute to microbial killing, while others are unselective, or potentially irrelevant in an in vivo situation. Using an antimicrobial example peptide (cR₃W₃), we illustrate one of the potentially misleading leakage mechanisms: leaky fusion, where leakage is coupled to membrane fusion. Like many others, we examine peptide-induced leakage in model vesicles consisting of binary mixtures of anionic and zwitterionic phospholipids. In fact, phosphatidylglycerol and phosphatidylethanolamine (PG/PE) are supposed to reflect bacterial membranes, but exhibit a high propensity for vesicle aggregation and fusion. We describe the implications of this vesicle fusion and aggregation for the reliability of model studies. The ambiguous role of the relatively fusogenic PE-lipids becomes clear as leakage decreases significantly when aggregation and fusion are prevented by sterical shielding. Furthermore, the mechanism of leakage changes if PE is exchanged for phosphatidylcholine (PC). We thus point out that the lipid composition of model membranes can be biased towards leaky fusion. This can lead to discrepancies between model studies and activity in true microbes, because leaky fusion is likely prevented by bacterial peptidoglycan layers. In conclusion, choosing the model membrane might implicate the type of effect (here leakage mechanism) that is observed. In the worst case, as with leaky fusion of PG/PE vesicles, this is not directly relevant for the intended antimicrobial application.

Mechanism of lipid bilayer perturbation by bactericidal membrane-active small molecules

Membrane-active small molecules (MASMs) are small organic molecules designed to reproduce the fundamental physicochemical properties of natural antimicrobial peptides: their cationic charge and amphiphilic character. This class of compounds has a promising broad range of antimicrobial activity and, at the same time, solves some major limitations of the peptides, such as their high production costs and low in vivo stability. Most cationic antimicrobial peptides act by accumulating on the surface of bacterial membranes and causing the formation of defects when a threshold is reached. Due to the drastically different structures of the two classes of molecules, it is not obvious that small-molecule antimicrobials act in the same way as natural peptides, and very few data are available on this aspect. Here we combined spectroscopic studies and molecular dynamics simulations to characterize the mechanism of action of two different MASMs. Our results show that, notwithstanding their simple structure, these molecules act just like antimicrobial peptides. They bind to the membrane surface, below the head-groups, and insert their apolar moieties in the core of the bilayer. Like many natural peptides, they cause the formation of defects when they reach a high coverage of the membrane surface. In addition, they cause membrane aggregation, and this property could contribute to their antimicrobial activity.

Leaky membrane fusion: an ambivalent effect induced by antimicrobial polycations

Both antimicrobial peptides and their synthetic mimics are potential alternatives to classical antibiotics. They can induce several membrane perturbations including permeabilization. Especially in model studies, aggregation of vesicles by such polycations is often reported. Here, we show that unintended vesicle aggregation or indeed fusion can cause apparent leakage in model studies that is not possible in most microbes, thus potentially leading to misinterpretations. The interactions of a highly charged and highly selective membrane-active polycation with negatively charged phosphatidylethanolamine/phosphatidylglycerol (PE/PG) vesicles are studied by a combination of biophysical methods. At low polycation concentrations, apparent vesicle aggregation was found to involve exchange of lipids. Upon neutralization of the negatively charged vesicles by the polycation, full fusion and leakage occurred and leaky fusion is suspected. To elucidate the interplay of leakage and fusion, we prevented membrane contacts by decorating the vesicles with PEG-chains. This inhibited fusion and also leakage activity. Leaky fusion is further corroborated by increased leakage with increasing likeliness of vesicle-vesicle contacts. Because of its similar appearance to other leakage mechanisms, leaky fusion is difficult to identify and might be overlooked and more common amongst polycationic membrane-active compounds. Regarding biological activity, leaky fusion needs to be carefully distinguished from other membrane permeabilization mechanisms, as it may be less relevant to bacteria, but potentially relevant for fungi. Furthermore, leaky fusion is an interesting effect that could help in endosomal escape for drug delivery. A comprehensive step-by-step protocol for membrane permeabilization/vesicle leakage using calcein fluorescence lifetime is provided in the ESI.

Tracking the Endosomal Escape: A Closer Look at Calcein and Related Reporters

Crossing the cellular membrane and delivering active pharmaceuticals or biologicals into the cytosol of cells is an essential step in the development of nanomedicines. One of the most important intracellular processes regarding the cellular uptake of biologicals is the endolysosomal pathway. Sophisticated nanocarriers have been developed overcoming a major hurdle, the endosomal entrapment, and delivering their cargo to the required site of action. In parallel, in vitro assays have been established analyzing the performance of these nanocarriers. Among them, the release of the membrane-impermeable dye calcein has become a popular and straightforward method. It is accessible for most researchers worldwide, allows for rapid conclusions about the release potential, and enables the study of release mechanisms. This review is intended to provide an overview and guidance for scientists applying the calcein release assay. It comprises a survey of several applications in the study of endosomal escape, considerations of potential pitfalls, challenges and limitations of the assay, and a brief summary of complementary methods. Based on this review, we hope to encourage further research groups to take advantage of the calcein release assay for their own purposes and help to create a database for more efficient cross-correlations between nanocarriers. This article is protected by copyright. All rights reserved.

Estimation of pore dimensions in lipid membranes induced by peptides and other biomolecules: A review

The cytoplasmic membrane is one of the most frequent cell targets of antimicrobial peptides (AMPs) and other biomolecules. Understanding the mechanism of action of AMPs at the molecular level is of utmost importance for designing of new membrane-specific molecules. In particular, the formation of pores, the structure and size of these pores are of great interest and require nanoscale resolution approaches, therefore, biophysical strategies are essential to achieve an understanding of these processes at this scale. In the case of membrane active peptides, pore formation or general membrane disruption is usually the last step before cell death, and so, pore size is generally directly associated to pore structure and stability and loss of cellular homeostasis, implicated in overall peptide activity. Up to date, there has not been a critical review discussing the methods that can be used specifically for estimating the pore dimensions induced by membrane active peptides. In this review we discuss the scope, relevance and popularity of the different biophysical techniques such as liposome leakage experiments, advanced microscopy, neutron or X-ray scattering, electrophysiological techniques and molecular dynamics studies, all of them useful for determining pore structure and dimension.

Time-resolved SANS reveals pore-forming peptides cause rapid lipid reorganization

Time-resolved SANS showed alamethicin and melittin promote DMPC lipid vesicle mixing and perturb DMPC kinetics in similar ways.

Hidden complexity in membrane permeabilization behavior of antimicrobial polycations

There are diverse membrane permeabilization behaviors of antimicrobial polycations in zwitterionic or charged vesicles; different mechanisms may occur over time.

Controlling the Formation of Peptide Films: Fully Developed Helical Peptides are Required to Obtain a Homogenous Coating over a Large Area

The influence of conformational dynamics on the self-assembly process of a conformationally constrained analogue of the natural antimicrobial peptide Trichogin GA IV was analysed by spectroscopic methods, microscopy imaging at nanometre resolution, and molecular dynamics simulations. The formation of peptide films at the air/water interface and their deposition on a graphite or a mica substrate were investigated. A combination of experimental evidence with molecular dynamics simulation was used to demonstrate that only the fully developed helical structure of the analogue promotes formation of ordered aggregates that nucleate the growth of micrometric rods, which give rise to homogenous coating over wide regions of the hydrophilic mica. This work proves the influence of helix flexibility on peptide self-organization and orientation on surfaces, key steps in the design of bioinspired organic/inorganic hybrid materials.

Quantified Membrane Permeabilization Indicates the Lipid Selectivity of Membrane-Active Antimicrobials

Most antimicrobial peptides (AMPs) and their synthetic mimics (SMAMPs) are thought to act by permeabilizing cell membranes. For antimicrobial therapy, selectivity for pathogens over mammalian cells is a key requirement. Understanding membrane selectivity is thus essential for designing AMPs and SMAMPs to complement classical antibiotics in the future. This study focuses on membrane permeabilization induced by SMAMPs and their selectivity for membranes with different lipid compositions. We measure release and fluorescence lifetime of a self-quenching dye in lipid vesicles. Apart from the dose-response, we quantify the strength of individual leakage events, and, employing cumulative kinetics, categorize permeabilization behavior. We propose that differing selectivities in a series of SMAMPs arise from a combination of the effect of the antimicrobial agent and the susceptibility of the membrane (with a given lipid composition) for certain types of leakage behavior. The unselective and hemolytic SMAMP is found to act mainly by the asymmetry stress mechanism, mediated by hydrophobic insertion of SMAMPs into lipid layers. The more selective SMAMPs induced leakage events occurring stochastically over several hours. Lipid intrinsic properties might additionally amplify the efficiency of leakage events. Leakage behavior changes with both the design of the SMAMP and the lipid composition of the membrane. Understanding how leakage behavior contributes to the selectivity and activity of antimicrobial agents will aid the design and screening of antimicrobials. An understanding of the underlying processes facilitates the comparison of membrane permeabilization across in vitro and in vivo assays.

Aggregation determines the selectivity of membrane-active anticancer and antimicrobial peptides: The case of killerFLIP

Host defense peptides selectively kill bacterial and cancer cells (including those that are drug-resistant) by perturbing the permeability of their membranes, without being significantly toxic to the host. Coulombic interactions between these cationic and amphipathic peptides and the negatively charged membranes of pathogenic cells contribute to the selective toxicity. However, a positive charge is not sufficient for selectivity, which can be achieved only by a finely tuned balance of electrostatic and hydrophobic driving forces. A common property of amphipathic peptides is the formation of aggregated structures in solution, but the role of this phenomenon in peptide activity and selectivity has received limited attention. Our data on the anticancer peptide killerFLIP demonstrate that aggregation strongly increases peptide selectivity, by reducing the effective peptide hydrophobicity and thus the affinity towards membranes composed of neutral lipids (like the outer layer of healthy eukaryotic cell membranes). Aggregation is therefore a useful tool to modulate the selectivity of membrane active peptides and peptidomimetics.

Trichogin GA IV Alignment and Oligomerization in Phospholipid Bilayers

Trichogin GA IV is a short peptaibol with antimicrobial activity. This uncharged, but amphipathic, sequence is aligned at the membrane interface and undergoes a transition to an aggregated state that inserts more deeply into the membrane, an assembly that predominates at a peptide-to-lipid ratio (P/L) of 1:20. In this work, the natural trichogin sequence was prepared and reconstituted into oriented lipid bilayers. The ¹⁵ N NMR chemical shift is indicative of a well-defined alignment of the peptide parallel to the membrane surface at P/Ls of 1:120 and 1:20. When the P/L is increased to 1:8, an additional peptide topology is observed that is indicative of a heterogeneous orientation, with helix alignments ranging from around the magic angle to perfectly in-plane. The topological preference of the trichogin helix for an orientation parallel to the membrane surface was confirmed by attenuated total reflection FTIR spectroscopy. Furthermore, ¹⁹ F CODEX experiments were performed on a trichogin sequence with ¹⁹ F-Phe at position 10. The CODEX decay is in agreement with a tetrameric complex, in which the ¹⁹ F sites are about 9-9.5 Å apart. Thus, a model emerges in which the monomeric peptide aligns along the membrane surface. When the peptide concentration increases, first dimeric and then tetrameric assemblies form, made up from helices oriented predominantly parallel to the membrane surface. The formation of these aggregates correlates with the release of vesicle contents including relatively large molecules.

Molecular Sponge: pH-Driven Reversible Squeezing of Stimuli-Sensitive Peptide Monolayers

The cyclic change of structure, thickness, and density, with pH switching from acidic (pH = 3) to basic (pH = 11) condition, has been revealed for chemisorbed monolayers of the peptide Lipo-Aib-Lys-Leu-Aib-Lys-Lys-Leu-Aib-Lys-Ile-Lol, a trichogin GA IV-analogue carrying Lys residues instead of Gly ones at positions 2, 5, 6, and 9, while a homologous peptide not containing Lys residues does not show any response to pH changes. Experimental and theoretical results, obtained by means of quartz crystal microbalance with dissipation monitoring, surface plasmon resonance, nanoplasmonic sensing technique, Fourier transform infrared-reflection attenuated spectroscopy and dynamic force spectroscopy, and molecular dynamics simulations provide detailed information on the overall monolayer structure changes with pH, including the analysis of the intra- and interchain peptide dynamics, the structure of the peptide layer/water/solid interface, as well as the position and role of solvation and nonsolvation water. The observed stimuli-responsive behavior of L1 peptide monolayers is accounted in terms of the occurrence of a pH-induced wetting/dewetting process, due to the pH-induced switching of the hydrophilic character of charged lysine groups to hydrophobic one of the same uncharged groups, along the peptide chain. This behavior in turn promotes the collective change of the aggregation state of the peptide chains. The present results may pave the way to critically reexamine the mechanism of stimuli-responsive systems.

The cyclic peptide labaditin does not alter the outer membrane integrity of Salmonella enterica serovar Typhimurium

Antimicrobial peptides are a promising class of new antibiotics with the ability to kill bacteria by disrupting their cell membrane, which is especially difficult for Gram-negative bacteria whose cell wall contains an outer layer of lipopolysaccharides (LPS). Here we show that the cyclic decapeptide Labaditin (Lo), with proven activity against the Gram-positive Staphylococcus aureus and Streptococcus mutans, is not able to kill the Gram-negative Salmonella enterica serovar Typhimurium (S.e.s. Typhimurium). We found that Lo induced significant changes in the surface pressure isotherms of Langmuir monolayers representing the Salmonella enterica serovar Typhimurium inner membrane (S.e.s. Typhimurium IM), and caused leakage in large unilamellar vesicles made with this IM lipid composition. On the basis of these results one should expect bactericidal activity against S.e.s. Typhimurium. However, Lo could not interact with a monolayer of LPS, causing no significant changes in either the surface pressure isotherms or in the polarization-modulated infrared reflection absorption spectra (PM-IRRAS). Therefore, the failure of Lo to kill S.e.s. Typhimurium is associated with the lack of interaction with LPS from the outer bacteria membrane. Our approach with distinct monolayer compositions and combined techniques to investigate molecular-level interactions is useful for drug design to fight antibiotic-resistant bacteria.

The singular behavior of a β-type semi-synthetic two branched polypeptide: three-dimensional structure and mode of action

Dendrimeric peptides make a versatile group of bioactive peptidomimetics and a potential new class of antimicrobial agents to tackle the pressing threat of multi-drug resistant pathogens. These are branched supramolecular assemblies where multiple copies of the bioactive unit are linked to a central core. Beyond their antimicrobial activity, dendrimeric peptides could also be designed to functionalize the surface of nanoparticles or materials for other medical uses. Despite these properties, however, little is known about the structure-function relationship of such compounds, which is key to unveil the fundamental physico-chemical parameters and design analogues with desired attributes. To close this gap, we focused on a semi-synthetic, two-branched peptide, SB056, endowed with remarkable activity against both Gram-positive and Gram-negative bacteria and limited cytotoxicity. SB056 can be considered the smallest prototypical dendrimeric peptide, with the core restricted to a single lysine residue and only two copies of the same highly cationic 10-mer polypeptide; an octanamide tail is present at the C-terminus. Combining NMR and Molecular Dynamics simulations, we have determined the 3D structure of two analogues. Fluorescence spectroscopy was applied to investigate the water-bilayer partition in the presence of vesicles of variable charge. Vesicle leakage assays were also performed and the experimental data were analyzed by applying an iterative Monte Carlo scheme to estimate the minimum number of bound peptides needed to achieve the release. We unveiled a singular beta hairpin-type structure determined by the peptide chains only, with the octanamide tail available for further functionalization to add new potential properties without affecting the structure.

Biomembrane Permeabilization: Statistics of Individual Leakage Events Harmonize the Interpretation of Vesicle Leakage

The mode of action of membrane-active molecules, such as antimicrobial, anticancer, cell penetrating, and fusion peptides and their synthetic mimics, transfection agents, drug permeation enhancers, and biological signaling molecules (e.g., quorum sensing), involves either the general or local destabilization of the target membrane or the formation of defined, rather stable pores. Some effects aim at killing the cell, while others need to be limited in space and time to avoid serious damage. Biological tests reveal translocation of compounds and cell death but do not provide a detailed, mechanistic, and quantitative understanding of the modes of action and their molecular basis. Model membrane studies of membrane leakage have been used for decades to tackle this issue, but their interpretation in terms of biology has remained challenging and often quite limited. Here we compare two recent, powerful protocols to study model membrane leakage: the microscopic detection of dye influx into giant liposomes and time-correlated single photon counting experiments to characterize dye efflux from large unilamellar vesicles. A statistical treatment of both data sets does not only harmonize apparent discrepancies but also makes us aware of principal issues that have been confusing the interpretation of model membrane leakage data so far. Moreover, our study reveals a fundamental difference between nano- and microscale systems that needs to be taken into account when conclusions about microscale objects, such as cells, are drawn from nanoscale models.

Exchange of Gramicidin between Lipid Bilayers: Implications for the Mechanism of Channel Formation

The canonical mechanism of gramicidin (gA) channel formation is transmembrane dimerization of nonconducting subunits that reside in opposite bilayer leaflets. The channels do not open and close; they appear and disappear due to subunit association and dissociation. Many different types of experiments support this monomer ↔ dimer mechanism. Recently, however, this mechanism was challenged, based on experiments with lipid vesicle-incorporated gA under conditions where vesicle fusion could be controlled. In these experiments, sustained channel activity was observed long after fusion had been terminated, which led to the proposal that gA single-channel current transitions result from closed-open transitions in long-lived bilayer-spanning dimers. This proposal is at odds with 40 years of experiments, but involves the key assumption that gA monomers do not exchange between bilayers. We tested the possibility of peptide exchange between bilayers using three different types of experiments. First, we demonstrated the exchange of gA between 1,2-dierucoyl-sn-glycero-3-phosphocholine (DC_22:1PC) or 1,2-dioleoyl-sn-glycero-3-phosphocholine (DC_18:1PC) lipid vesicles using a fluorescence assay for gA channel activity. Second, we added gA-free DC_22:1PC vesicles to both sides of planar DC_18:1PC bilayers preincubated with gA, which reduced channel activity up to 10-fold. Third, we added gA-containing DC_22:1PC vesicles to one or both sides of DC_18:1PC planar bilayers, which produced much higher channel activity when the gA-containing vesicles were added to both sides of the bilayer, as compared to one side only. All three types of experiments show that gA subunits can exchange between lipid bilayers. The exchange of subunits between bilayers thus is firmly established, which becomes a crucial consideration with respect to the mechanism of channel formation.

Selective Membrane Disruption Mechanism of an Antibacterial γ-AApeptide Defined by EPR Spectroscopy

γ-AApeptides are a new class of antibacterial peptidomimetics that are not prone to antibiotic resistance and are highly resistant to protease degradation. It is not clear how γ-AApeptides interact with bacterial membranes and alter lipid assembly, but such information is essential to understanding their antimicrobial activities and guiding future design of more potent and specific antimicrobial agents. Using electron paramagnetic resonance techniques, we characterized the membrane interaction and destabilizing mechanism of a lipo-cyclic-γ-AApeptide (AA1), which has broad-spectrum antibacterial activities. The analyses revealed that AA1 binding increases the membrane permeability of POPC/POPG liposomes, which mimic negatively charged bacterial membranes. AA1 binding also inhibits membrane fluidity and reduces solvent accessibility around the lipid headgroup region. Moreover, AA1 interacts strongly with POPC/POPG liposomes, inducing significant lipid lateral-ordering and membrane thinning. In contrast, minimal membrane property changes were observed upon AA1 binding for liposomes mimicking mammalian cell membranes, which consist of neutral lipids and cholesterol. Our findings suggest that AA1 interacts and disrupts bacterial membranes through a carpet-like mechanism. The results showed that the intrinsic features of γ-AApeptides are important for their ability to disrupt bacterial membranes selectively, the implications of which extend to developing new antibacterial biomaterials.

Innovative chemical gels meet enzymes: A smart combination for cleaning paper artworks

HYPOTHESIS

Due to their highly retentive properties, innovative recently developed, semi-interpenetrated hydrogels made up of poly(vinyl pyrrolidone) (PVP) chains embedded in a poly(2-hydroxyethyl methacrylate) (p(HEMA)) network should be efficiently used as cleaning material for fragile and degraded paper artworks. In restoration practice, indeed the wet cleaning of these artworks is usually performed by immersion of paper in water, a procedure which may lead to several drawbacks, including paper fibers swelling and dissolution of water-soluble original components.

EXPERIMENTS

This class of gels were yet presented in literature, but their interactions with paper materials and ability to be spiked with active enzymes (as cleaning agents), have not been analyzed. To establish the suitability of these hydrogels as paper cleaning materials, first, a rheological and microstructural characterization of the gels was performed. Moreover, diffusion of macromolecules inside gels was studied using fluorescence microscopy, to check if these innovative hydrogels can be used as carriers for hydrolytic enzymes. Indeed, pastes and glues are usually found in old paper artworks, and their removal is a very delicate operation that requires a selective action, which is granted by specific hydrolytic enzymes. At the same time, spectroscopic analyses on paper samples under investigation before and after cleaning treatment has been performed, thus assessing the capabilty of these gels as cleaning materials.

FINDINGS

With the aim of demonstrating the versatility of these hydrogels, several case studies, i.e., the removal of grime and water-soluble cellulose degradation byproducts, the removal of animal glue and the removal of starch paste from real samples, are presented. Results obtained with these gels have been compared to those obtained by using another gel used for paper artworks cleaning, i.e., Gellan gel.

Membrane negative curvature induced by a hybrid peptide from pediocin PA-1 and plantaricin 149 as revealed by atomistic molecular dynamics simulations

Antimicrobial peptides (AMPs) are cationic peptides that kill bacteria with a broad spectrum of action, low toxicity to mammalian cells and exceptionally low rates of bacterial resistance. These features have led to considerable efforts in developing AMPs as an alternative antibacterial therapy. In vitro studies have shown that AMPs interfere with membrane bilayer integrity via several possible mechanisms, which are not entirely understood. We have performed the synthesis, membrane lysis measurements, and biophysical characterization of a novel hybrid peptide. These measurements show that PA-Pln149 does not form nanopores, but instead promotes membrane rupture. It causes fast rupture of the bacterial model membrane (POPG-rich) at concentrations 100-fold lower than that required for the disruption of mammalian model membranes (POPC-rich). Atomistic molecular dynamics (MD) simulations were performed for single and multiple copies of PA-Pln149 in the presence of mixed and pure POPC/POPG bilayers to investigate the concentration-dependent membrane disruption by the hybrid peptide. These simulations reproduced the experimental trend and provided a potential mechanism of action for PA-Pln149. It shows that the PA-Pln149 does not form nanopores, but instead promotes membrane destabilization through peptide aggregation and induction of membrane negative curvature with the collapse of the lamellar arrangement. The sequence of events depicted for PA-Pln149 may offer insights into the mechanism of action of AMPs previously shown to induce negative deformation of membrane curvature and often associated with peptide translocation via non-bilayer intermediate structures.

IN VITRO RELEASE KINETICS MODEL FITTING OF LIPOSOMES: AN INSIGHT

Liposomes are emerging cargoes for bioactive delivery owing to their widely accepted biocompatible and biodegradable nature. It is always a challenge to control the release of payload for effective delivery to the site of interest. Over the couple of decennia, mathematical modeling of release process is a need of time whether the drug remains in the circulation or reaches at the target site. For establishing a better in vitro - in vivo correlation, release kinetics models viz. Peppas, Higuchi, Weibull, Zero Order and First order including mechanistic models like All-or-None, Toroidal, and Biomembrane models etc. are continuously exploited to predict drug release profile. Most of these models rely on the diffusion equations based on the composition of liposomes and conditions of release. Here, we summarized the crucial reports exploring these models and associated interventions to know the underlying physicochemical release phenomenon. Such mathematical model fitting can be a promising approach to deduce release/delivery process to help in designing the safe and efficacious ("Smart") liposomes.

Interaction Study of Phospholipid Membranes with an N-Glucosylated β-Turn Peptide Structure Detecting Autoantibodies Biomarkers of Multiple Sclerosis

The interaction of lipid environments with the type I’ β-turn peptide structure called CSF114 and its N-glucosylated form CSF114(Glc), previously developed as a synthetic antigenic probe recognizing specific autoantibodies in a subpopulation of multiple sclerosis patients’ serum, was investigated by fluorescence spectroscopy and electrochemical experiments using large unilamellar vesicles, mercury supported lipid self-assembled monolayers (SAMs) and tethered bilayer lipid membranes (tBLMs). The synthetic antigenic probe N-glucosylated peptide CSF114(Glc) and its unglucosylated form interact with the polar heads of lipid SAMs of dioleoylphosphatidylcholine at nonzero transmembrane potentials, probably establishing a dual electrostatic interaction of the trimethylammonium  and phosphate groups of the phosphatidylcholine polar head with the Glu⁵ and His⁹ residues on the opposite ends of the CSF114(Glc) β-turn encompassing residues 6-9. His⁹ protonation at pH 7 eliminates this dual interaction. CSF114(Glc) is adsorbed on top of SAMs of mixtures of dioleoylphosphatidylcholine with sphingomyelin, an important component of myelin, whose proteins are hypothesized to undergo an aberrant N-glucosylation triggering the autoimmune response. Incorporation of the type I’ β-turn peptide structure CSF114 into lipid SAMs by potential scans of electrochemical impedance spectroscopy induces defects causing a slight permeabilization toward cadmium ions. The N-glucopeptide CSF114(Glc) does not affect  tBLMs to a detectable extent.

Electrophysiology investigation of Trichogin GA IV activity in planar lipid membranes reveals ion channels of well-defined size

Trichogin GA IV, an antimicrobial peptaibol, exerts its function by augmenting membrane permeability, but the molecular aspects of its pore-forming mechanism are still debated. Several lines of evidence indicate a 'barrel-stave' channel structure, similar to that of alamethicin, but the length of a trichogin helix is too short to span a normal bilayer. Herein, we present electrophysiology measurements in planar bilayers, showing that trichogin does form channels of a well-defined size (R=4.2⋅10(9)  Ω; corresponding at least to a trimeric aggregate) that span the membrane and allow ion diffusion, but do not exhibit voltage-dependent rectification, unlike those of alamethicin.

Cleaning of paper artworks: development of an efficient gel-based material able to remove starch paste

The removal of old glue from paper artworks is of paramount importance for the preservation of its integrity during the restoration process. Wet cleaning is one of the traditional methods, although it may cause damages on artworks. In this work, an advantageous alternative method, based on the use of a rigid hydrogel, for a simple and localized removal of starch paste from paper supports is presented. The use of an appropriate hydrogel allows to overcome many of the problems faced by restorers minimizing damages, through a controlled release of water to the artwork, and a simple and not invasive application and removal. At the same time, the specific and targeted enzyme activity leads to a significant reduction in the application time of the cleaning procedure. In this context, experiments were carried out applying Gellan hydrogel carrying α-amylase enzyme on several paper samples soiled with starch paste. To assess the cleaning efficacy of the proposed hydrogel, we have used a multidisciplinary approach, by means of spectroscopic techniques, scanning electron microscopy, chromatographic analysis, and pH investigations.

Novel fluorescent security marker. Part II: application of novel 6-alkoxy-2-amino-3,5-pyridinedicarbonitrile nanoparticles in safety paper

Application of newly synthesized fluorescent nanoparticles of 6-alkoxy-2-amino-3,5-pyridinedicarbonitriles as security markers for enhancing the safety property of bagasse-based paper sheets (as valuable documents) was studied.”.

α-Synuclein oligomers induced by docosahexaenoic acid affect membrane integrity

A key feature of Parkinson disease is the aggregation of α-synuclein and its intracellular deposition in fibrillar form. Increasing evidence suggests that the pathogenicity of α-synuclein is correlated with the activity of oligomers formed in the early stages of its aggregation process. Oligomers toxicity seems to be associated with both their ability to bind and affect the integrity of lipid membranes. Previously, we demonstrated that α-synuclein forms oligomeric species in the presence of docosahexaenoic acid and that these species are toxic to cells. Here we studied how interaction of these oligomers with membranes results in cell toxicity, using cellular membrane-mimetic and cell model systems. We found that α-synuclein oligomers are able to interact with large and small unilamellar negatively charged vesicles acquiring an increased amount of α-helical structure, which induces small molecules release. We explored the possibility that oligomers effects on membranes could be due to pore formation, to a detergent-like effect or to fibril growth on the membrane. Our biophysical and cellular findings are consistent with a model where α-synuclein oligomers are embedded into the lipid bilayer causing transient alteration of membrane permeability.

Long-time-scale interaction dynamics between a model antimicrobial peptide and giant unilamellar vesicles

The interaction dynamics between a lytic peptide and a biomembrane was studied using time-lapse fluorescence lifetime imaging microscopy. The model membrane was 1,2-dipalmitoyl-sn-glycero-3-phosphochloine giant unilamellar vesicles (GUVs), and the peptide was the K14 derivative of melittin, to which the polarity-sensitive fluorescent probe AlexaFluor 430 was grafted. The interaction of the peptide with the GUVs resulted in a progressive quenching of the fluorescence lifetime over a period of minutes. From previous photophysics characterization of the peptide, we were able to deconvolve the contribution of three distinct peptide states to the lifetime trajectory and use this data to develop a kinetics model for the interaction process. It was found that the peptide-membrane interaction was well described by a two-step mechanism: peptide monomer adsorption followed by membrane surface migration, assembly, and insertion to form membrane pores. There was an equilibrium exchange between pore and surface monomers at all lipid/peptide (L/P) concentration ratios, suggesting that the fully inserted phase was reached, even at low peptide concentrations. In contrast to previous studies, there was no evidence of critical behavior; irrespective of L/P ratio, lytic pores were the dominant peptide state at equilibrium and were formed even at very low peptide concentrations. We suggest that this behavior is seen in GUVs because their low curvature means low Laplace pressure. Membrane elasticity is therefore relatively ineffective at damping the thermal fluctuations of lipid molecules that lead to random molecular-level lipid protrusions and membrane undulations. The transient local membrane deformations that result from these thermal fluctuations create the conditions necessary for facile peptide insertion.

riDOM, a cell penetrating peptide. Interaction with phospholipid bilayers

Melittin is an amphipathic peptide which has received much attention as a model peptide for peptide-membrane interactions. It is however not suited as a transfection agent due to its cytolytic and toxicological effects. Retro-inverso-melittin, when covalently linked to the lipid 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (riDOM), eliminates these shortcomings. The interaction of riDOM with phospholipid membranes was investigated with circular dichroism (CD) spectroscopy, dynamic light scattering, ζ-potential measurements, and high-sensitivity isothermal titration calorimetry. riDOM forms cationic nanoparticles with a diameter of ~13nm which are well soluble in water and bind with high affinity to DNA and lipid membranes. When dissolved in bilayer membranes, riDOM nanoparticles dissociate and form transient pores. riDOM-induced membrane leakiness is however much reduced compared to that of authentic melittin. The secondary structure of the ri-melittin is not changed when riDOM is transferred from water to the membrane and displays a large fraction of β-structure. The (31)P NMR spectrum of the nanoparticle is however transformed into a typical bilayer spectrum. The Gibbs free energy of riDOM binding to bilayer membranes is -8.0 to -10.0kcal/mol which corresponds to the partition energy of just one fatty acyl chain. Half of the hydrophobic surface of the riDOM lipid extension with its 2 oleic acyl chains is therefore involved in a lipid-peptide interaction. This packing arrangement guarantees a good solubility of riDOM both in the aqueous and in the membrane phase. The membrane binding enthalpy is small and riDOM binding is thus entropy-driven.

Nucleation in mesoscopic systems under transient conditions: peptide-induced pore formation in vesicles

Attachment of lytic peptides to the lipid membrane of virions or bacteria is often accompanied by their aggregation and pore formation, resulting eventually in membrane rupture and pathogen neutralization. The membrane rupture may occur gradually via formation of many pores or abruptly after the formation of the first pore. In academic studies, this process is observed during interaction of peptides with lipid vesicles. We present an analytical model and the corresponding Monte Carlo simulations focused on the pore formation in such situations. Specifically, we calculate the time of the first nucleation-limited pore-formation event and show the distribution of this time in the regime when the fluctuations of the number of peptides attached to a vesicle are appreciable. The results obtained are used to clarify the mechanism of the pore formation and membrane destabilization observed recently during interaction of highly active α-helical peptide with sub-100-nm lipid vesicles that mimic enveloped viruses with nanoscale membrane curvature. The model proposed and the analysis presented are generic and may be applicable to other meso- and nanosystems.

Membrane thickness and the mechanism of action of the short peptaibol trichogin GA IV

Trichogin GA IV (GAIV) is an antimicrobial peptide of the peptaibol family, like the extensively studied alamethicin (Alm). GAIV acts by perturbing membrane permeability. Previous data have shown that pore formation is related to GAIV aggregation and insertion in the hydrophobic core of the membrane. This behavior is similar to that of Alm and in agreement with a barrel-stave mechanism, in which transmembrane oriented peptides aggregate to form a channel. However, while the 19-amino acid long Alm has a length comparable to the membrane thickness, GAIV comprises only 10 amino acids, and its helix is about half the normal bilayer thickness. Here, we report the results of neutron reflectivity measurements, showing that GAIV inserts in the hydrophobic region of the membrane, causing a significant thinning of the bilayer. Molecular dynamics simulations of GAIV/membrane systems were also performed. For these studies we developed a novel approach for constructing the initial configuration, by embedding the short peptide in the hydrophobic core of the bilayer. These calculations indicated that in the transmembrane orientation GAIV interacts strongly with the polar phospholipid headgroups, drawing them towards its N- and C-termini, inducing membrane thinning and becoming able to span the bilayer. Finally, vesicle leakage experiments demonstrated that GAIV activity is significantly higher with thinner membranes, becoming similar to that of Alm when the bilayer thickness is comparable to its size. Overall, these data indicate that a barrel-stave mechanism of pore formation might be possible for GAIV and for similarly short peptaibols despite their relatively small size.

Trichogin GA IV: a versatile template for the synthesis of novel peptaibiotics

Trichogin GA IV, isolated from the fungus Trichoderma longibrachiatum, is the prototype of lipopeptaibols, the sub-class of short-length peptaibiotics exhibiting membrane-modifying properties. This peptaibol is predominantly folded in a mixed 3(10)-/α- helical conformation with a clear, albeit modest, amphiphilic character, which is likely to be responsible for its capability to perturb bacterial membranes and to induce cell death. In previous papers, we reported on the interesting biological properties of trichogin GA IV, namely its good activity against Gram positive bacteria, in particular methicillin-resistant S. aureus strains, its stability towards proteolytic degradation, and its low hemolytic activity. Aiming at broadening the antimicrobial activity spectrum by increasing the peptide helical amphiphilicity, in this work we synthesized, by solution and solid-phase methodologies, purified and fully characterized a set of trichogin GA IV analogs in which the four Gly residues at positions 2, 5, 6, 9, lying in the poorly hydrophilic face of the helical structure, are substituted by one (position 2, 5, 6 or 9), two (positions 5 and 6), three (positions 2, 5, and 9), and four (positions 2, 5, 6, and 9) Lys residues. The conformational preferences of the Lys-containing analogs were assessed by FT-IR absorption, CD and 2D-NMR techniques in aqueous, organic, and membrane-mimetic environments. Interestingly, it turns out that the presence of charged residues induces a transition of the helical conformation adopted by the peptaibols (from 3(10)- to α-helix) as a function of pH in a reversible process. The role played in the analogs by the markedly increased amphiphilicity was further tested by fluorescence leakage experiments in model membranes, protease resistance, antibacterial and antifungal activities, cytotoxicity, and hemolysis. Taken together, our biological results provide evidence that some of the least substituted among these analogs are good candidates for the development of new membrane-active antimicrobial agents.

Fluorescence spectroscopy and molecular dynamics simulations in studies on the mechanism of membrane destabilization by antimicrobial peptides

Since their initial discovery, 30 years ago, antimicrobial peptides (AMPs) have been intensely investigated as a possible solution to the increasing problem of drug-resistant bacteria. The interaction of antimicrobial peptides with the cellular membrane of bacteria is the key step of their mechanism of action. Fluorescence spectroscopy can provide several structural details on peptide-membrane systems, such as partition free energy, aggregation state, peptide position and orientation in the bilayer, and the effects of the peptides on the membrane order. However, these "low-resolution" structural data are hardly sufficient to define the structural requirements for the pore formation process. Molecular dynamics simulations, on the other hand, provide atomic-level information on the structure and dynamics of the peptide-membrane system, but they need to be validated experimentally. In this review we summarize the information that can be obtained by both approaches, highlighting their versatility and complementarity, suggesting that their synergistic application could lead to a new level of insight into the mechanism of membrane destabilization by AMPs.