Understanding the interaction of block copolymers with DMPC lipid bilayer using coarse-grained molecular dynamics simulations

Polymeric Microreactors with pH-Controlled Spatial Localization of Cascade Reactions

Lipid and polymer vesicles provide versatile means of creating systems that mimic the architecture of cells. However, these constructs cannot mimic the adaptive compartmentalization observed in cells, where the assembly and disassembly of subcompartments are dynamically modulated by environmental cues. Here, we describe a fully polymeric microreactor with a coacervate-in-vesicle architecture that exhibits an adaptive response to pH. The system was fabricated by microfluidic generation of semipermeable biomimetic polymer vesicles within 1 min using oleyl alcohol as the oil phase. The polymersomes allowed for the diffusion of protons and substrates acting as external signals. Using this method, we were able to construct adaptive microreactors containing internal polyelectrolyte-based catalytic organelles capable of sequestering and localizing enzymes and reaction products in a dynamic process driven by an external stimulus. This approach provides a platform for the rapid and efficient construction of robust adaptive microreactors that can be used in catalysis, biosensing, and cell mimicry.

Interaction of Macromolecular Chain with Phospholipid Membranes in Solutions: A Dissipative Particle Dynamics Simulation Study

The interaction between macromolecular chains and phospholipid membranes in aqueous solution was investigated using dissipative particle dynamics simulations. Two cases were considered, one in which the macromolecular chains were pulled along parallel to the membrane surfaces and another in which they were pulled vertical to the membrane surfaces. Several parameters, including the radius of gyration, shape factor, particle number, and order parameter, were used to investigate the interaction mechanisms during the dynamics processes by adjusting the pulling force strength of the chains. In both cases, the results showed that the macromolecular chains undergo conformational transitions from a coiled to a rod-like structure. Furthermore, the simulations revealed that the membranes can be damaged and repaired during the dynamic processes. The role of the pulling forces and the adsorption interactions between the chains and membranes differed in the parallel and perpendicular pulling cases. These findings contribute to our understanding of the interaction mechanisms between macromolecules and membranes, and they may have potential applications in biology and medicine.

HRMAS-NMR and simulation study of the self-assembly of surfactants on carbon nanotubes

Polyethoxylated surfactants, such as those of the Tween and Pluronic series, are commonly used to disperse carbon nanotubes (CNTs) and other nanoparticles. However, the current understanding of the nature of interactions between these surfactants and CNTs is limited. The nature of the interactions between surfactants (Tween-80 [T80] and Pluronic F68 [PF68]) and CNTs was investigated using high-resolution magic angle spinning nuclear magnetic resonance (HRMAS-NMR) and coarse-grained molecular dynamics (MD) simulations. HRMAS-NMR revealed that T80 molecules interact with single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) via the oleyl chain, whereas PF68 molecules interact with the surface of SWCNTs and MWCNTs via the polypropylene oxide residues. The polyethylene oxide chains were oriented towards the external aqueous environment. The HRMAS-NMR results were supported by MD simulations, and the latter provided further insights into the nature of the interactions.

Different effects of two Poloxamers (L61 and F68) on the conductance of bilayer lipid membranes

The integral conductance of planar lipid bilayer membranes in the presence of two Poloxamers (Pluronics) L61 and F68 with the same lengths of hydrophobic poly(propylene oxide) blocks and the different lengths of hydrophilic poly(ethylene oxide) blocks increases with an increase in the concentration of both Pluronics; however, the shape of the conductance-concentration curves is super linear for L61 and sublinear for F68. In the presence of both Pluronics, rare discrete current jumps are observed against the background of continuous current. At high concentrations, the I-V curves of membranes with both L61 and F68 became nonlinear at sufficiently low voltages but differed significantly. At voltages greater than 50 mV, the conductance of membranes with L61 increased sharply and quantized jumps were observed toward higher conductance, which could be interpreted as the appearance of additional pores. On the contrary, the conductance of membranes with F68 decreased and quantized jumps to lower conductance were observed, which could be interpreted as blocking of already existing pores. We attributed the differences in the conductance-concentration and I-V curves of these two Pluronics to their different effects on the dynamics of membrane hydration and, accordingly, on the probability of formation of conducting pores.

Direct and Reverse Pluronic Micelles: Design and Characterization of Promising Drug Delivery Nanosystems

Pluronics are a family of amphiphilic block copolymers broadly explored in the pharmaceutical field. Under certain conditions, Pluronics self-assemble in different structures including nanosized direct and reverse micelles. This review provides an overview about the main parameters affecting the micellization process of Pluronics, such as polymer length, fragments distribution within the chain, solvents, additives and loading of cargo. Furthermore, it offers a guide about the most common techniques used to characterize the structure and properties of the micelles. Finally, it presents up-to-date approaches to improve the stability and drug loading of Pluronic micelles. Special attention is paid to reverse Pluronics and reverse micelles, currently underexplored in the literature. Pluronic micelles present a bright future as drug delivery agents. A smart design and thorough characterization will improve the transfer to clinical applications.

Interaction of triblock copolymers (Pluronic®) with DMPC vesicles: a photophysical and computational study

Pluronic/lipid mix promises stealth liposomes with long circulation time and long-term stability for pharmaceutical applications. However, the influence of Pluronics on several aspects of lipid membranes has not been fully elucidated. Herein it was described the effect of Pluronics on the structured water, alkyl chain conformation, and kinetic stability of dimyristoylphosphatidylcholine (DMPC) liposomes using interfacial and deeper fluorescent probes along with computational molecular modeling data. Interfacial water changed as a function of Pluronics' hydrophobicity with polypropylene oxide (PPO) anchoring the copolymers in the lipid bilayer. Pluronics with more than 30-40 PO units had facilitated penetration at the bilayer while shorter PPO favored a more interfacial interaction. Low Pluronic concentrations provided long-term stability of vesicles by steric effects of polyethylene oxide (PEO), but high amounts destabilized the vesicles as a sum of water-bridge cleavage at the polar head group and the reduced alkyl-alkyl interactions among the lipids. The high kinetic stability of Pluronic/DMPC vesicles is a proof-of-concept of its advantages and applicability in nanotechnology over conventional liposome-based pharmaceutical products for future biomedical applications.

Insights into the Phase Diagram of Pluronic L64 Using Coarse-Grained Molecular Dynamics Simulations

We investigate the concentration-dependent phase diagram of pluronic L64 in aqueous media at 300 and 320 K using coarse-grained (CG) molecular dynamics (MD) simulations. The CG model is derived by adapting the Martini model for nonbonded interactions along with the Boltzmann inversion (BI) of bonded interactions from all-atom (AA) simulations. Our derived CG model successfully captures the experimentally observed micellar-, hexagonal-, lamellar-, and polymer-rich solution phase. The end-to-end distance reveals the conformational change from an open-chain structure in the micellar phase to a folded-chain structure in the lamellar phase, increasing the orientational order. An increase in temperature leads to expulsion of water molecules from the L64 moiety, suggesting an increase in L64 hydrophobicity. Thermodynamic analysis using the two-phase thermodynamics (2PT) method suggests the entropy of the system decreases with increasing L64 concentration and the decrease in free energy (F) with temperature is mainly driven by the entropic factor (-TS). Further, the increase in aggregation number at lower concentrations and self-assembly at very high concentrations is energetically driven, whereas the change from the micellar phase to the lamellar phase with increasing L64 concentration is entropically driven. Our model provides molecular insights into L64 phases which can be further explored to design functionality-based suprastructures for drug delivery and tissue engineering applications.

Temperature-Transferable Coarse-Grained Model for Poly(propylene oxide) to Study Thermo-Responsive Behavior of Triblock Copolymers

Thermo-responsive behavior of ethylene oxide (EO)-propylene oxide (PO) copolymers makes them suitable for many potential applications. Reproducing the origins of the tunable properties of EO-PO copolymers using coarse-grained (CG) models such as the MARTINI force field is critically important for building a better understanding of their behavior. In the present work, we have investigated the effects of coarse-graining on the water-polymer interaction across a temperature range. We compared the performance of different all-atom force fields to find the most appropriate one for the purpose of PO block parameterization in the MARTINI platform. We parameterized a CG temperature-dependent PO model based on the reproduction of the atomistic free energy of transfer of propylene oxide trimer from octane to water over a range of temperatures (20-60 °C) and compared the atomistic bond and angle distributions. Then, we used the model to study the effects of EO/PO ratio, molecular weight, and concentration on the thermo-responsive behavior of EO-PO copolymers in water. The results show an excellent agreement with experiments in different areas. Our temperature-dependent model reproduces (1) micellar phase above critical micelle temperature (CMT) and unimer phase below CMT for different Pluronics (a class of EO-PO triblock copolymers) spanning many EO/PO ratios and molecular weights; (2) spherical-to-rodlike micellar shape transition for Pluronics with 60 wt % of PO content or more; (3) diffusion coefficients for Pluronics with high PO content (P104 Pluronic with a PO mass of 3500 g mol^-1) across a broad range of temperatures; and (4) micelle core size and micelle diameter similar to experimental results. Overall, our model improves the temperature sensitivity of EO-PO copolymers of existing models significantly, particularly for copolymers that are dominated by PO agents.

Chimeric liposomes decorated with P407: an alternative biomaterial for producing stealth nano-therapeutics

The aim of the present study is the development and evaluation of the physicochemical properties of chimeric hydrogenated soya phosphatidylcholine (HSPC) and egg phosphatidylcholine (EggPC) liposomes with incorporated triblock copolymer Poloxamer P407 (P407). The physicochemical assay was held in water HPLC-grade and Foetal Bovine Serum (FBS), in order to determine whether these systems can be used as drug or antigen delivery nanosystems. Dynamic and electrophoretic light scattering (DLS/ELS) techniques were used for the measurement of the hydrodynamic diameter, the polydispersity index, and the ζ-potential of the prepared nanosystems. The incorporation of the P407 resulted in a size reduction of all systems. A decrease in the hydrodynamic diameter and polydispersity index were also found as a result of increasing the storage temperature from 4 °C to 25 °C, attributed to P407. The experiments that were carried out in FBS, showed that the addition of P407 improved systems stealth properties. Concluding, we propose P407 as a promising alternative to PEG in the development of lipid nanoparticles with optimized bio- and shelf-stability.

Surfactant-free production of biomimetic giant unilamellar vesicles using PDMS-based microfluidics

Microfluidic production of giant lipid vesicles presents a paradigm-shift in the development of artificial cells. While production is high-throughput and the lipid vesicles are mono-disperse compared to bulk methods, current technologies rely heavily on the addition of additives such as surfactants, glycerol and even ethanol. Here we present a microfluidic method for producing biomimetic surfactant-free and additive-free giant unilamellar vesicles. The versatile design allows for the production of vesicle sizes ranging anywhere from ~10 to 130 µm with either neutral or charged lipids, and in physiological buffer conditions. Purity, functionality, and stability of the membranes are validated by lipid diffusion, protein incorporation, and leakage assays. Usability as artificial cells is demonstrated by increasing their complexity, i.e., by encapsulating plasmids, smaller liposomes, mammalian cells, and microspheres. This robust method capable of creating truly biomimetic artificial cells in high-throughput will prove valuable for bottom-up synthetic biology and the understanding of membrane function.

Polymeric Drug Delivery System Based on Pluronics for Cancer Treatment

Pluronic polymers (pluronics) are a unique class of synthetic triblock copolymers containing hydrophobic polypropylene oxide (PPO) and hydrophilic polyethylene oxide (PEO) arranged in the PEO-PPO-PEO manner. Due to their excellent biocompatibility and amphiphilic properties, pluronics are an ideal and promising biological material, which is widely used in drug delivery, disease diagnosis, and treatment, among other applications. Through self-assembly or in combination with other materials, pluronics can form nano carriers with different morphologies, representing a kind of multifunctional pharmaceutical excipients. In recent years, the utilization of pluronic-based multi-functional drug carriers in tumor treatment has become widespread, and various responsive drug carriers are designed according to the characteristics of the tumor microenvironment, resulting in major progress in tumor therapy. This review introduces the specific role of pluronic-based polymer drug delivery systems in tumor therapy, focusing on their physical and chemical properties as well as the design aspects of pluronic polymers. Finally, using newer literature reports, this review provides insights into the future potential and challenges posed by different pluronic-based polymer drug delivery systems in tumor therapy.

The Martini Model in Materials Science

The Martini model, a coarse-grained force field initially developed with biomolecular simulations in mind, has found an increasing number of applications in the field of soft materials science. The model's underlying building block principle does not pose restrictions on its application beyond biomolecular systems. Here, the main applications to date of the Martini model in materials science are highlighted, and a perspective for the future developments in this field is given, particularly in light of recent developments such as the new version of the model, Martini 3.

A thermodynamic study of F108 and F127 block copolymer interactions with liposomes at physiological temperature

The interactions of egg yolk phosphatidylcholine liposomes with F108 and F127 triblock copolymers, in the monomer state, were analyzed by isothermal titration calorimetry (ITC) at 37 °C. According to the results, the critical micelle concentration was determined to be 0.4 and 0.04 wt.% for F108 and F127, respectively, by surface tension at 37 °C. According to the results, liposomes/poloxamers were not favoured energetically, since endothermic interactions were observed. However, positive changes in entropy promoted a spontaneous process. F127 had a greater partition coefficient (51.97 ± 1.77 × 10⁴), stronger affinity, than F108 (8.19 ± 0.37 × 10⁴) towards the vesicle lipid bilayer due to its larger hydrophobic block. After the ITC experiments, an increased vesicle size (within about 1-3 nm average) by dynamic light scattering and the formation of bilayer discs by electron microscopy (EM) was observed at low copolymer concentrations (0.57 mol% of F108 and 1.01 mol% of F127). The EM and ITC results confirmed the intimate association of the copolymers with the membrane instead of being simply absorbed onto the bilayer surface. Our results indicate that the temperature of the system (37 °C), the copolymer concentration and hydrophobic chain length are important factors for the interaction of poloxamers with lipid bilayers and the stability of liposomes.

Improving Stability of Tear Film Lipid Layer via Concerted Action of Two Drug Molecules: A Biophysical View

The tear film at the ocular surface is covered by a thin layer of lipids. This oily phase stabilizes the film by decreasing its surface tension and improving its viscoelastic properties. Clinically, destabilization and rupture of the tear film are related to dry eye disease and are accompanied by changes in the quality and quantity of tear film lipids. In dry eye, eye drops containing oil-in-water emulsions are used for the supplementation of lipids and surface-active components to the tear film. We explore in detail the biophysical aspects of interactions of specific surface-active compounds, cetalkonium chloride and poloxamer 188, which are present in oil-in-water emulsions, with tear lipids. The aim is to better understand the macroscopically observed eye drops-tear film interactions by rationalizing them at the molecular level. To this end, we employ a multi-scale approach combining experiments on human meibomian lipid extracts, measurements using synthetic lipid films, and in silico molecular dynamics simulations. By combining these methods, we demonstrate that the studied compounds specifically interact with the tear lipid film enhancing its structure, surfactant properties, and elasticity. The observed effects are cooperative and can be further modulated by material packing at the tear-air interface.

The coarse-grained models of poly(ethylene oxide) and poly(propylene oxide) homopolymers and poloxamers in big multipole water (BMW) and MARTINI frameworks

Polyethylene oxide (PEO) and poly(propylene oxide) (PPO), especially their tri-block copolymers PEO-PPO-PEO (poloxamers), have a broad range of applications in biotechnology and medical science. Understanding their specific interactions with biomembranes is the key to unveil the unique features of poloxamers either as membrane-healing or membrane pore-forming agents. Based on the coarse-graining convention of the MARTINI force field and the big multipole water (BMW) model, which has a three charged site topology and can reproduce the correct dipole moment of four-water clusters, we generated coarse-grained (CG) models with analytical and numerical potentials for PEO and PPO homopolymers and poloxamers in dilute solution. The effective bonded interaction potentials between CG beads were determined from the probability distributions of bond lengths, angles and dihedrals that are determined from atomistic simulations. The nonbonded interaction parameters were fine-tuned to reproduce the conformational properties of atomistic PEO and PPO homopolymers and poloxamers via extensive CG simulations of PEO and PPO homopolymers and poloxamers in a BMW water environment. The reported CG models provide a promising framework for a comprehensive understanding of the microstructural, conformational, and dynamic properties of poloxamers and their delicate interactions with other species in an explicit water environment.

Molecular Simulations of PEGylated Biomolecules, Liposomes, and Nanoparticles for Drug Delivery Applications

Since the first polyethylene glycol (PEG)ylated protein was approved by the FDA in 1990, PEGylation has been successfully applied to develop drug delivery systems through experiments, but these experimental results are not always easy to interpret at the atomic level because of the limited resolution of experimental techniques. To determine the optimal size, structure, and density of PEG for drug delivery, the structure and dynamics of PEGylated drug carriers need to be understood close to the atomic scale, as can be done using molecular dynamics simulations, assuming that these simulations can be validated by successful comparisons to experiments. Starting with the development of all-atom and coarse-grained PEG models in 1990s, PEGylated drug carriers have been widely simulated. In particular, recent advances in computer performance and simulation methodologies have allowed for molecular simulations of large complexes of PEGylated drug carriers interacting with other molecules such as anticancer drugs, plasma proteins, membranes, and receptors, which makes it possible to interpret experimental observations at a nearly atomistic resolution, as well as help in the rational design of drug delivery systems for applications in nanomedicine. Here, simulation studies on the following PEGylated drug topics will be reviewed: proteins and peptides, liposomes, and nanoparticles such as dendrimers and carbon nanotubes.

Selective photodynamic effects on cervical cancer cells provided by P123 Pluronic®-based nanoparticles modulating hypericin delivery

At present, cervical cancer is the fourth leading cause of cancer among women worldwide with no effective treatment options. In this study we aimed to evaluate the efficacy of hypericin (HYP) encapsulated on Pluronic® P123 (HYP/P123) photodynamic therapy (PDT) in a comprehensive panel of human cervical cancer-derived cell lines, including HeLa (HPV 18-positive), SiHa (HPV 16-positive), CaSki (HPV 16 and 18-positive), and C33A (HPV-negative), compared to a nontumorigenic human epithelial cell line (HaCaT). Were investigated: (i) cell cytotoxicity and phototoxicity, cellular uptake and subcellular distribution; (ii) cell death pathway and cellular oxidative stress; (iii) migration and invasion. Our results showed that HYP/P123 micelles had effective and selective time- and dose-dependent phototoxic effects on cervical cancer cells but not in HaCaT. Moreover, HYP/P123 micelles accumulated in endoplasmic reticulum, mitochondria and lysosomes, resulting in photodynamic cell death mainly by necrosis. HYP/P123 induced cellular oxidative stress mainly via type II mechanism of PDT and inhibited cancer cell migration and invasion mainly via MMP-2 inhibition. Taken together, our results indicate a potentially useful role of HYP/P123 micelles as a platform for HYP delivery to more specifically and effectively treat cervical cancers through PDT, suggesting they are worthy for in vivo preclinical evaluations.

Structural behavior of amphiphilic polyion complexes interacting with saturated lipid membranes investigated by coarse-grained molecular dynamic simulations

Neutral polyelectrolyte complexes (PECs) made from an amphiphilic multiblock copolymer of type (A_nB_n)_m and an oppositely charged polyion and interacting with a dipalmitoylphosphatidylcholine (DPPC) lipid membrane.

Influence of Cholesterol and Bilayer Curvature on the Interaction of PPO-PEO Block Copolymers with Liposomes

Interactions of nonionic poly(ethylene oxide)- b-poly(propylene oxide) (PEO-PPO) block copolymers, known as Pluronics or poloxamers, with cell membranes have been widely studied for a host of biomedical applications. Herein, we report how cholesterol within phosphatidylcholine (POPC) lipid bilayer liposomes and bilayer curvature affects the binding of several PPO-PEO-PPO triblocks with varying PPO content and a tPPO-PEO diblock, where t refers to a tert-butyl end group. Pulsed-field-gradient NMR was employed to quantify the extent of copolymer associated with liposomes prepared with cholesterol concentrations ranging from 0 to 30 mol % relative to the total content of POPC and cholesterol and vesicle extrusion radii of 25, 50, or 100 nm. The fraction of polymer bound to the liposomes was extracted from NMR data on the basis of the very different mobilities of the bound and free polymers in aqueous solution. Cholesterol concentration was manipulated by varying the molar percentage of this sterol in the POPC bilayer preparation. The membrane curvature was varied by adjusting the liposome size through a conventional pore extrusion technique. Although the PPO content significantly influences the overall amount of block copolymer adsorbed to the liposome, we found that polymer binding decreases with increasing cholesterol concentration in a universal fashion, with the fraction of bound polymer dropping 10-fold between 0 and 30 mol % cholesterol relative to the total content of POPC and cholesterol. Increasing the bilayer curvature (decreasing the radius of the liposome) in the absence of cholesterol increases polymer binding between 2- and 4-fold over the range of liposome sizes studied. These results demonstrate that cholesterol plays a dominant role, and bilayer curvature has a less significant impact as the curvature decreases, on polymer-membrane association.

PEO-b-PPO star-shaped polymers enhance the structural stability of electrostatically coupled liposome/polyelectrolyte complexes

We propose a strategy to counteract the salt-driven disassembly of multiliposomal complexes made by electrostatic co-assembly of anionic small unilamellar liposomes and cationic star-shaped polyelectrolytes (made of quaternized poly(dimethylaminoethyl methacrylate) (qPDMAEMA100)3.1). The combined action of (qPDMAEMA100)3.1 and a nonionic star-shaped polymer (PEO12-b-PPO45)4, which comprises diblock copolymer arms uniting a poly(ethylene oxide) PEO inner block and a poly(propylene oxide) PPO terminal block, leads to a stabilization of these complexes against disintegration in saline solutions. Hereby, the anchoring of the PPO terminal blocks to the lipid bilayer and the bridging between several liposomes are at the origin of the promoted structural stability. Two-focus fluorescence correlation spectroscopy verifies the formation of multiliposomal complexes with (PEO12-b-PPO45)4. The polyelectrolyte and the amphiphilic polymer work synergistically, as the joint action still assures some membrane integrity, which is not seen for the mere (PEO12-b-PPO45)4-liposome interaction alone.

Selective Photodynamic Effects on Breast Cancer Cells Provided by p123 Pluronic®- Based Nanoparticles Modulating Hypericin Delivery

BACKGROUND

Breast cancer is the most relevant type of cancer and the second cause of cancer- related deaths among women in general. Currently, there is no effective treatment for breast cancer although advances in its initial diagnosis and treatment are available. Therefore, the value of novel anti-tumor therapeutic modalities remains an immediate unmet need in clinical practice. Following our previous work regarding the properties of the Pluronics with different photosensitizers (PS) for photodynamic therapy (PDT), in this study we aimed to evaluate the efficacy of supersaturated hypericin (HYP) encapsulated on Pluronic® P123 (HYP/P123) against breast cancer cells (MCF-7) and non-tumorigenic breast cells (MCF-10A).

METHODS

Cell internalization and subcellular distribution of HYP/P123 was confirmed by fluorescence microscopy. The phototoxicity and citototoxicity of HYP/P123 was assessed by trypan blue exclusion assay in the presence and absence of light. Long-term cytotoxicity was performed by clonogenic assay. Cell migration was determined by the wound-healing assay. Apoptosis and necrosis assays were performed by annexin VFITC/ propidium Iodide (PI) by fluorescence microscopy.

RESULTS

Our results showed that HYP/P123 micelles had high stability and high rates of binding to cells, which resulted in the selective internalization in MCF-7, indicating their potential to permeate the membrane of these cells. Moreover, HYP/P123 micelles accumulated in mitochondria and endoplasmic reticulum organelles, resulting in the photodynamic cell death by necrosis. Additionally, HYP/P123 micelles showed effective and selective time- and dose dependent phototoxic effects on MCF-7 cells but little damage to MCF-10A cells. HYP/P123 micelles inhibited the generation of cellular colonies, indicating a possible capability to prevent the recurrence of breast cancer. We also demonstrated that HYP/P123 micelles inhibit the migration of tumor cells, possibly by decreasing their ability to form metastases.

CONCLUSION

Taken together, the results presented here indicate a potentially useful role of HYP/P123 micelles as a platform for HYP delivery to more specifically and effectively treat human breast cancers through photodynamic therapy, suggesting they are worthy for in vivo preclinical evaluations.

Muscle membrane integrity in Duchenne muscular dystrophy: recent advances in copolymer-based muscle membrane stabilizers

The scientific premise, design, and structure-function analysis of chemical-based muscle membrane stabilizing block copolymers are reviewed here for applications in striated muscle membrane injury. Synthetic block copolymers have a rich history and wide array of applications from industry to biology. Potential for discovery is enabled by a large chemical space for block copolymers, including modifications in block copolymer mass, composition, and molecular architecture. Collectively, this presents an impressive chemical landscape to leverage distinct structure-function outcomes. Of particular relevance to biology and medicine, stabilization of damaged phospholipid membranes using amphiphilic block copolymers, classified as poloxamers or pluronics, has been the subject of increasing scientific inquiry. This review focuses on implementing block copolymers to protect fragile muscle membranes against mechanical stress. The review highlights interventions in Duchenne muscular dystrophy, a fatal disease of progressive muscle deterioration owing to marked instability of the striated muscle membrane. Biophysical and chemical engineering advances are presented that delineate and expand upon current understanding of copolymer-lipid membrane interactions and the mechanism of stabilization. The studies presented here serve to underscore the utility of copolymer discovery leading toward the therapeutic application of block copolymers in Duchenne muscular dystrophy and potentially other biomedical applications in which membrane integrity is compromised.

What causes the anomalous aggregation in pluronic aqueous solutions?

Pluronic (PL) block copolymers have been widely used as delivery carriers, molecular templates for porous media, and process additives for affecting rheological behavior. Unlike most surfactant systems, where unimer transforms into micelle with increased surfactant concentration, anomalous large PL aggregates below the critical micelle concentration (CMC) were found throughout four types of PL (F108, F127, F88 and P84). We characterized their structures using dynamic light scattering and small-angle X-ray/neutron scattering. Molecular dynamics simulations suggest that the PPO segments, though weakly hydrophobic interaction (insufficient to form micelles), promote the formation of large aggregates. Addition of acid or base (e.g. citric acid, acetic acid, HCl and NaOH) in F108 solution significantly suppresses the aggregate formation for up to 20 days due to the repulsion force from the attached H3O+ molecules on the EO segment in both PEO and PL and the reduction of CMC through the salting out effect, respectively.

Molecular Mechanism of Lipid Nanodisk Formation by Styrene-Maleic Acid Copolymers

Experimental characterization of membrane proteins often requires solubilization. A recent approach is to use styrene-maleic acid (SMA) copolymers to isolate membrane proteins in nanometer-sized membrane disks, or so-called SMA lipid particles (SMALPs). The approach has the advantage of allowing direct extraction of proteins, keeping their native lipid environment. Despite the growing popularity of using SMALPs, the molecular mechanism behind the process remains poorly understood. Here, we unravel the molecular details of the nanodisk formation by using coarse-grained molecular dynamics simulations. We show how SMA copolymers bind to the lipid bilayer interface, driven by the hydrophobic effect. Due to the concerted action of multiple adsorbed copolymers, large membrane defects appear, including small, water-filled pores. The copolymers can stabilize the rim of these pores, leading to pore growth and membrane disruption. Although complete solubilization is not seen on the timescale of our simulations, self-assembly experiments show that small nanodisks are the thermodynamically preferred end state. Our findings shed light on the mechanism of SMALP formation and on their molecular structure. This can be an important step toward the design of optimized extraction tools for membrane protein research.

Effects of Various Pharmaceutical Excipients on the Intestinal Transport and Absorption of Sulfasalazine, a Typical Substrate of Breast Cancer Resistance Protein Transporter

Breast cancer resistance protein (BCRP) transporter is an efflux transporter that utilizes energy from adenosine triphosphate hydrolysis to push its substrates, regardless of the concentration gradient. Its presence on the apical membrane of the intestinal mucosa is a major obstacle for the intestinal absorption of its substrates. In this study, we examined the effects of various pharmaceutical excipients on the intestinal transport and absorption of sulfasalazine, a BCRP substrate. Four excipients, including 0.05% and 0.075% BL-9EX, 0.01% and 0.05% Brij 97, 0.075% Labrasol, and 0.05% and 0.1% Tween 20 decreased the secretory transport of sulfasalazine in an in vitro diffusion chamber. Further investigation in an in situ closed loop experiment in rats showed that 0.05% and 0.1% BL-9EX and 0.1% Brij 97 effectively enhanced the intestinal absorption of sulfasalazine while maintaining minimal toxicity to the intestinal mucosa. However, 0.1% Brij 97 also increased the intestinal absorption of 5(6)-carboxyfluorescein, a paracellular marker compound. These findings suggest that BL-9EX might effectively inhibit the BCRP-mediated efflux of sulfasalazine in vivo, indicating that BL-9EX could improve the intestinal absorption of sulfasalazine and other BCRP substrates.

Surface Plasmon Resonance Study of the Binding of PEO-PPO-PEO Triblock Copolymer and PEO Homopolymer to Supported Lipid Bilayers

Poloxamer 188 (P188), a poly(ethylene oxide)- b-poly(propylene oxide)- b-poly(ethylene oxide) triblock copolymer, protects cell membranes against various external stresses, whereas poly(ethylene oxide) (PEO; 8600 g/mol) homopolymer lacks protection efficacy. As part of a comprehensive effort to elucidate the protection mechanism, we used surface plasmon resonance (SPR) to obtain direct evidence of binding of the polymers onto supported lipid bilayers. Binding kinetics and coverage of P188 and PEO were examined and compared. Most notably, PEO exhibited membrane association comparable to that of P188, evidenced by comparable association rate constants and coverage. This result highlights the need for additional mechanistic understanding beyond simple membrane association to explain the differential efficacy of P188 in therapeutic applications.

Diblock copolymer bilayers as model for polymersomes: A coarse grain approach

This paper presents a new model for polymersomes developed using a poly(ethylene oxide)-poly(butadiene) diblock copolymer bilayer. The model is based on a coarse-grained approach using the MARTINI force field. Since no MARTINI parameters exist for poly(butadiene), we have refined these parameters using quantum mechanical calculations and molecular dynamics simulations. The model has been validated using extensive molecular dynamics simulations in systems with several hundred polymer units and reaching up to 6 μs. These simulations show that the copolymer coarse grain model self-assemble into bilayers and that NPT and NP_NγT ensemble runs reproduce key structural and mechanical experimental properties for different copolymer length chains with a similar hydrophilic weight fraction.

How the Incorporation of Pluronic Block Copolymers Modulates the Response of Lipid Membranes to Mechanical Stress

We employ atomistic molecular dynamics simulations to investigate the effect that the incorporation of the nonionic amphiphilic copolymer known as Pluronic L64 has on the mechanical stability of a DPPC membrane. The simulations reveal that the incorporation of the polymer chains leads to membranes that can sustain increasing mechanical stresses. Analysis of mechanical, structural, and dynamic properties of the membrane shows that the polymer chains interact strongly with the lipids in the vicinity, restraining their mobility and imparting better mechanical stability to the membrane. The hybrid membranes under tension remain thicker, more ordered, and stiffer in comparison to their lipid analogues. Trans-bilayer lipid movements (flip-flop) are observed and appear to be triggered by the presence of the polymer chains. A careful analysis of the pore formation under high tensions reveals two distinctive mechanisms that depend on the distribution of the hydrophilic polymer blocks in the bilayer. Finally, the rate of growth of the formed membrane defects is slowed down in the presence of polymers. These findings show that Pluronic block copolymers could be exploited for the formation of optimized hybrid nanodevices with controlled elastic and dynamic properties.

Quantifying Binding of Ethylene Oxide-Propylene Oxide Block Copolymers with Lipid Bilayers

Block copolymers composed of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) have been widely used in cell membrane stabilization and permeabilization. To explore the mechanism of interaction between PPO-PEO block copolymers and lipid membranes, we have investigated how polymer structure influences the polymer-lipid bilayer association by varying the overall molecular weight, the hydrophobic and hydrophilic block lengths, and the end-group structure systematically, using 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) unilamellar liposomes as model membranes. Pulsed-field-gradient NMR (PFG-NMR) was employed to probe polymer diffusion in the absence and presence of liposomes. The echo decay curves of free polymers in the absence of liposomes are single exponentials, indicative of simple translational diffusion, while in the presence of liposomes, the decays are biexponential, with the slower decay corresponding to polymers bound to liposomes. The binding percentage of polymer to the liposome was quantified by fitting the echo decay curves to a biexponential model. The NMR experiments show that increasing the total molecular weight and hydrophobicity of the polymer can significantly enhance the polymer-lipid bilayer association, as the binding percentage and liposome surface coverage both increase. We hypothesize that the hydrophobic PPO block inserts into the lipid bilayer due to the fact that little molecular exchange between bound and free polymers occurs on the time scale of the diffusion experiments. Additionally, as polymer concentration increases, the liposome surface coverage increases and approaches a limit. These results demonstrate that PFG-NMR is a simple yet powerful method to quantify interactions between polymers and lipid bilayers.

PEO-PPO Diblock Copolymers Protect Myoblasts from Hypo-Osmotic Stress In Vitro Dependent on Copolymer Size, Composition, and Architecture

Poloxamer 188, a triblock copolymer of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), protects cellular membranes from various stresses. Though numerous block copolymer variants exist, evaluation of alternative architecture, composition, and size has been minimal. Herein, cultured murine myoblasts are exposed to the stresses of hypotonic shock and isotonic recovery, and membrane integrity was evaluated by quantifying release of lactate dehydrogenase. Comparative evaluation of a systematic set of PEO-PPO diblock and PEO-PPO-PEO triblock copolymers demonstrates that the diblock architecture can be protective in vitro. Short PPO blocks hinder protection with >9 PPO units needed for protection at 150 μM and >16 units needed at 14 μM. Addition of a tert-butyl end group enhances protection at reduced concentration. When the end group and PPO length are fixed, increasing the PEO length improves protection. This systematic evaluation establishes a new in vitro screening tool for evaluating membrane-sealing amphiphiles and provides mechanistic insight to guide future copolymer design for membrane stabilization in vivo.

Effects of Membrane Defects and Polymer Hydrophobicity on Networking Kinetics of Vesicles

The kinetics of clustering unilamellar vesicles induced by inverse Pluronics [poly(propylene oxide)_m-poly(ethylene oxide)_n-poly(propylene oxide)_m, PO_m-EO_n-PO_m] was investigated via experiments and molecular dynamic simulations. Two important factors for controlling the networking kinetics are the membrane defects, presumably located at the interfacial region between two lipid domains induced by acyl chain mismatch, and the polymer hydrophobicity. As expected, the clustering rate increases significantly with increasing bilayer defects on the membrane where the insertion of PPO is likely to take place because of the reduced energy barrier for the insertion of PO. The hydrophobic interaction between the PO blocks and membranes with the defects region dictates the "anchoring" kinetics, which is controlled by the association-dissociation of PO with the lipid membrane. As a result, the dependence of clustering rate on polymer concentration is strongly influenced by the hydrophobicity of the PO blocks. Nevertheless, longer PO blocks show stronger association with the membrane, resulting in faster consumption of the "active" sites made of these defect regions (causing mostly "invalid" insertions) with increasing polymer concentration, hence inhibiting the formation of large networking clusters, while shorter PO blocks undergo more frequent association with/dissociation from the defects, allowing continuous formation of larger clusters with increasing polymer concentration. This study provides important insights into how the organization and dynamics of a biomembrane influence its interaction with foreign amphiphilic molecules.

Understanding the Interaction of Pluronics L61 and L64 with a DOPC Lipid Bilayer: An Atomistic Molecular Dynamics Study

We investigate the interactions of Pluronics L61 and L64 with a dioleylphosphatidylcholine (DOPC) lipid bilayer by atomistic molecular dynamics simulations using the all-atom OPLS force field. Our results show that the initial configuration of the polymer with respect to the bilayer determines its final conformation within the bilayer. When the polymer is initially placed at the lipid/water interface, we observe partial insertion of the polymer in a U-shaped conformation. On the other hand, when the polymer is centered at the bilayer, it stabilizes to a transmembrane state, which facilitates water transport across the bilayer. We show that membrane thickness decreases while its fluidity increases in the presence of Pluronics. When the polymer concentration inside the bilayer is high, pore formation is initiated with L64. Our results show good agreement with existing experimental data and reveal that the hydrophilic/lipophilic balance of the polymer plays a critical role in the interaction mechanisms as well as in the dynamics of Pluronics with and within the bilayer.

Perfluorinated Moieties Increase the Interaction of Amphiphilic Block Copolymers with Lipid Monolayers

The interaction of amphiphilic and triphilic block copolymers with lipid monolayers has been studied. Amphiphilic triblock copolymer PGMA20-PPO34-PGMA20 (GP) is composed of a hydrophobic poly(propylene oxide) (PPO) middle block that is flanked by two hydrophilic poly(glycerol monomethacrylate) (PGMA) side blocks. The attachment of a perfluoro-n-nonyl residue (F9) to either end of GP yields a triphilic polymer with the sequence F9-PGMA20-PPO34-PGMA20-F9 (F-GP). The F9 chains are fluorophilic, i.e., they have a tendency to demix in hydrophilic as well as in lipophilic environments. We investigated (i) the adsorption of both polymers to differently composed lipid monolayers and (ii) the compression behavior of mixed polymer/lipid monolayers. The lipid monolayers are composed of phospholipids with PC or PE headgroups and acyl chains of different length and saturation. Both polymers interact with lipid monolayers by inserting their hydrophobic moieties (PPO, F9). The interaction is markedly enhanced in the presence of F9 chains, which act as membrane anchors. GP inserts into lipid monolayers up to a surface pressure of 30 mN/m, whereas F-GP inserts into monolayers at up to 45 mN/m, suggesting that F-GP also inserts into lipid bilayer membranes. The adsorption of both polymers to lipid monolayers with short acyl chains is favored. Upon compression, a two-step squeeze-out of F-GP occurs, with PPO blocks being released into the aqueous subphase at 28 mN/m and the F9 chains being squeezed out at 48 mN/m. GP is squeezed out in one step at 28 mN/m because of the lack of F9 anchor groups. The liquid expanded (LE) to liquid condensed (LC) phase transition of DPPC and DMPE is maintained in the presence of the polymers, indicating that the polymers can be accommodated in LE- and LC-phase monolayers. These results show how fluorinated moieties can be included in the rational design of membrane-binding polymers.

Behavior of P85 and P188 Poloxamer Molecules: Computer Simulations Using United-Atom Force-Field

To study the interaction between poloxamer molecules and lipid bilayers using molecular dynamics simulation technique with the united-atom resolution, we augmented the GROMOS force-field to include poloxamers. We validated the force-field by calculating the radii of gyration of two poloxamers, P85 and P188, solvated in water and by considering the poloxamer density distributions at the air/water interface. The emphasis of our simulations was on the study of the interaction between poloxamers and lipid bilayer. At the water/lipid bilayer interface, we observed that both poloxamers studied, P85 and P188, behaved like surfactants: the hydrophilic blocks of poloxamers became adsorbed at the polar interface, while their hydrophobic block penetrated the interface into the aliphatic tail region of the lipid bilayer. We also observed that when P85 and P188 poloxamers interacted with damaged membranes that contained pores, the hydrophobic blocks of copolymers penetrated into the membrane in the vicinity of the pore and compressed the membrane. Due to this compression, water molecules were evacuated from the pore.

Millimeter-area, free standing, phospholipid bilayers

Minimal model biomembrane studies have the potential to unlock the fundamental mechanisms of cellular function that govern the processes upon which life relies. However, existing methods to fabricate free-standing model membranes currently have significant limitations. Bilayer sizes are often tens of micrometers, decoupling curvature or substrate effects, orthogonal control over tension, and solvent exchange combined with microscopy techniques is not possible, which restricts the studies that can be performed. Here, we describe a versatile platform to generate free standing, planar, phospholipid bilayers with millimeter scale areas. The technique relies on an adapted thin-film balance apparatus allowing for the dynamic control of the nucleation and growth of a planar black lipid membrane in the center of an orifice surrounded by microfluidic channels. Success is demonstrated using several different lipid types, including mixtures that show the same temperature dependent phase separation as existing protocols, moreover, membranes are highly stable. Two advantages unique to the proposed method are the dynamic control of the membrane tension and the possibility to make extremely large area membranes. We demonstrate this by showing how a block polymer, F68, used in drug delivery increases the membrane compliance. Together, the results demonstrate a new paradigm for studying the mechanics, structure, and function of model membranes.

Properties of Poloxamer Molecules and Poloxamer Micelles Dissolved in Water and Next to Lipid Bilayers: Results from Computer Simulations

To study the properties of poloxamer molecules P85 and P188 and micelles containing these poloxamers in bulk water and also next to lipid bilayers, we performed coarse-grained molecular dynamics computer simulations. We used MARTINI force-field and adjusted Lennard-Jones nonbonded interaction strength parameters for poloxamer beads to take into account the presence of polarizable water. Simulations of systems containing poloxamer molecules or micelles solvated in bulk water showed that structural properties, such as radii of gyration of the molecules and micelles, agree with the ones inferred from experiments. We observed that P85 micelle is almost spherical in shape, whereas the P188 micelle is distorted from being spherical. Simulations containing systems with the water-lipid bilayer interface showed that hydrophilic blocks of poloxamers interact with lipid headgroups of the bilayer and remain at the interface, whereas hydrophobic blocks prefer to insert into the central hydrophobic region of the bilayer. Simulations containing poloxamer micelles next to lipid bilayer showed no permeation of these micelles into the bilayer. To study the "healing" properties of P188 poloxamer, we performed simulations on a system containing a P188 micelle next to "damaged" lipid bilayer containing a pore. We observed that hydrophobic chains of poloxamers got inserted into the bilayer through the pore region, ultimately closing the pore.

Concentration-induced structural transition of block polymer self-assemblies on a nanoparticle surface: computer simulation

Film structure of asymmetric triblock copolymers assembled on different degrees of hydrophobic NP surfaces was controlled by concentration.