Interactions of biodegradable poly([R]-3-hydroxy-10-undecenoate) with 1,2-dioleoyl-sn-glycero-3-phosphocholine lipid: a monolayer study

Polyhydroxyalkanoates (PHAs) are biodegradable, biocompatible polyesters and very attractive candidates for biomedical applications as materials for tissue engineering. They have a hydrophobic character, but some are able to spread at the air-water interface to form monomolecularly thin films (Langmuir monolayers). This is a very convenient model to analyze PHA self-assembly in two dimensions and to study their molecular interactions with other amphiphilic compounds, which is very important considering compatibility between biomaterials and cell membranes. We used the Langmuir monolayer technique and Brewster angle microscopy to study the properties of poly([R]-3-hydroxy-10-undecenoate) (PHUE) films on the free water surface in various experimental conditions. Moreover, we investigated the interactions between the polymer and one of the main biomembrane components, 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). The addition of lipid to a polymer film does not change the monolayer phase behavior; however, the interactions between these two materials are repulsive and fall in two composition-dependent regimes. In summary, this is the first systematic study of the monolayer behavior of PHUE, thus forming a solid basis for a thorough understanding of material interactions, in particular in the context of biomaterials and implants.     

Application of electrochemical impedance spectroscopy to the study of dioleoyl phosphatidylcholine monolayers on mercury

Electrochemical impedance spectroscopy has been applied to the analysis of the behavior of monolayers of dioleoyl phosphatidylcholine (DOPC) on a mercury electrode. Experiments were carried out in electrolytes KCl and NaCl (0.1 mol dm(-3)) and Mg(NO3)2 (0.05 mol dm(-3)), and the frequency dependence of the complex impedance was estimated between 65 000 and 0.1 Hz at potentials -0.4 to -1.5 V versus Ag/AgCl 3.5 mol dm(-3) KCl at uncoated and coated electrode surfaces. Experiments were also carried out in the presence of gramicidin A (gA). Between the potentials of -0.4 and -0.7 V, the DOPC monolayer behaves as an almost ideal capacitor with little frequency dispersion. At more negative potentials, the impedance data show the formation of defects (-0.7 to -0.85 V), ingression of electrolyte into the layer (capacitance peak approximately -0.935 V), reorientation of phospholipid-water structures (capacitance peak approximately -1.0 V), and initiation of phospholipid desorption (approximately -1.3 V). gA interaction with the phospholipid monolayer at -0.4 V is shown as an additional low-frequency element. A general "one capacitor model" in a RC series equivalent circuit is developed incorporating the frequency dispersion of the capacitance, distribution of the time constants of the dispersion, and a coefficient related to the interface between the solution and the coated electrode. This latter coefficient is the most robust and decreases at potentials approaching those coincident with the DOPC phase transitions.     

Electrochemical screening of anti-microbial peptide LL-37 interaction with phospholipids

LL-37 is an alpha-helical antimicrobial peptide of human origin. It is a 37 residue cathelicidin peptide. This paper explores the use of electrochemical methods to investigate the interaction of LL-37 with phospholipid and lipid A monolayers on a mercury drop electrode. Experiments were carried out in Dulbecco's phosphate buffered saline at pH approximately 7.6. The capacity-potential curves of the coated electrode in the presence and absence of LL-37 were measured using out-of-phase ac voltammetry. The frequency dependence of the complex impedance of the coated electrode in the presence and absence of LL-37 was estimated at -0.4 V versus Ag/AgCl 3.5 mol dm(-3) KCl. The monolayer permeability to ions was studied by following the reduction of Tl(I) to Tl(Hg) at the coated electrode. LL-37 shows no significant interaction with DOPC. However, LL-37 shows a small interaction with DOPG and lipid A within a DOPC monolayer where the monolayer permeability is marginally increased and the zero frequency capacitance (ZFC) is marginally decreased in both cases. LL-37 shows a significant interaction with a lipid A monolayer thereby decreasing the ZFC by 30%. The results concur with the known membrane active properties of LL-37 and establish this electrochemical approach as a key technique for screening peptides.     

两亲性树状聚合物*

树状聚合物及其功能化是近年来高分子科学界的研究热点之一。本文综述了不同类型的树状聚合物,分别有聚酯、聚丙三醇、聚乙烯亚胺等超支化聚合物,聚酰胺-胺、聚丙烯亚胺等树枝状聚合物。通过对树状聚合物末端大量官能团的亲水（亲油）改性可以制备两亲性树状聚合物,改性方法主要有酰胺化反应、酯化反应、麦克尔加成反应等。与通过缩聚反应所得到的上述树状聚合物不同,近年来配位聚合领域出现的通过“链行走”机理形成的树状聚乙烯,引起了高度关注,这方面着重介绍了乙烯与极性单体直接共聚合或者采用链行走与原子转移自由基聚合联用制备两亲性树状乙烯聚合物。最后对两亲性树状聚合物领域的发展前景进行了展望。     

Langmuir-Blodgett thin films of quantum dots: synthesis, surface modification, and fluorescence resonance energy transfer (FRET) studies

We describe herein studies on as-prepared hydrophobic ZnS-CdSe quantum dots (QDs) at the air-water interface. Surface pressure-area (pi-A) isotherms have been used to study the monolayer behavior. Uniform, lamellar multilayer thin films of QDs were deposited by the Langmuir-Blodgett (LB) technique. The role of two different surfactant systems commonly employed in the synthesis of these QDs (trioctylphosphine oxide-octadecylamine (TOPO-ODA) system and trioctylphosphine oxide-tetradecylphosphonic acid (TOPO-TDPA) system) on the monolayer behavior and the quality of thin films produced has been investigated. The thin films were characterized by quartz crystal microgravimetry (QCM), contact angle measurements, fluorescence spectroscopy, and transmission electron microscopy (TEM). These QD films were further modified by an amphiphilic polymer, poly(maleic anhydride-alt-1-tetradecene) (PMA). The hydrophobic interaction between the polymers and the surfactants attached to the QDs drove the self-assembly process. The carboxylic acid functional groups in the polymer were also used to immobilize avidin. We have demonstrated a proof of concept for the biosensing strategy wherein the avidin-coated QD films attracted biotinylated gold nanoparticles, resulting in fluorescence resonance energy transfer (FRET) quenching of the thin films.     

新型两亲性聚酰胺的合成及性质

为了设计天然气水合物抑制剂和了解作用机理提供依据,分析两亲性聚酰胺与水相互作用的特征与本质,本文合成了新型水溶性高分子聚柠檬酰丙二胺,在此基础上合成了一种含环己基基团的两亲性聚柠檬酰丙二胺。通过核磁共振波谱、凝胶渗透色谱和示差扫描量热法对产物的结构和性能进行了表征。研究表明,改性的聚柠檬酰胺能形成不可冰冻束缚水,而且比传统的天然气水合物抑制剂聚(N-乙烯基己内酰胺)和聚维酮形成的不可冰冻束缚水多1倍。改性后聚合物中水的比热容增加约36%。聚合物和水之间产生的疏水相互作用,可将水分子更紧密地束缚在聚合物中。改性后的聚合物的疏水性强,造成了水分子彼此紧密束缚程度提高,为不可冰冻束缚水含量的增加提供了必要的环境。     

A series of poly[N-(2-hydroxypropyl)methacrylamide] copolymers with anthracene-derived fluorophores showing aggregation-induced emission properties for bioimaging

A series of new poly[N-(2-hydroxypropyl)methacrylamide]-based amphiphilic copolymers were synthesized through a radical copolymerization of a monomeric/hydrophobic fluorophore possessing aggregation-induced emission (AIE) property with N-(2-hydroxypropyl)methacrylamide. Photophysical properties were investigated using UV-Vis absorbance and fluorescence spectrophotometry. Influences of the polymer structures with different molar ratios of the AIE fluorophores on their photophysical properties were studied. Results show that the AIE fluorophores aggregate in the cores of the micelles formed from the amphiphilic random copolymers and polymers with more hydrophobic AIE fluorophores facilitate stronger aggregations of the AIE segments to obtain higher quantum efficiencies. The polymers reported herein have good water solubility, enabling the application of hydrophobic AIE materials in biological conditions. The polymers were endocytosed by two experimental cell lines, human brain glioblastoma U87MG cells and human esophagus premalignant CP-A, with a distribution into the cytoplasm. The polymers are non-cytotoxic to the two cell lines at a polymer concentration of 1 mg/mL.     

Ultrathin conjugated polymer network films of carbazole functionalized poly(p-phenylenes) via electropolymerization

Ultrathin films of a cross-linked and chemically distinct conjugated poly(p-phenylene) network via electropolymerization are described. The amphiphilic network precursor was synthesized by incorporating the alkoxy carbazole group (-O(CH2)5Cb) to a poly(p-phenylene) (C6PPPOH) backbone. In order to investigate the combined thin film electrochemical and photophysical properties of poly(p-phenylene)s and polycarbazole conjugated polymers, C6PPPC5Cb was deposited on substrates using the Langmuir Blodgett Kuhn (LBK) method. The monolayer isotherm of the polymer, C6PPPC5Cb, showed a liquid expanded region slightly different from the parent polymer C6PPPOH. Multilayers (up to 30 layers) were transferred to different substrates such as quartz, gold coated LaSFN9 and ITO substrates for analysis. For conversion to a conjugated polymer network (CPN) film, the electroactive carbazole group was electropolymerized using cyclic voltammetry (CV) resulting in polycarbazole linking units. The differences in the film properties and corresponding changes in the electrochemical behavior indicate the importance of film thickness and electron/ion transport process in cross linked network films. From the electrochemical studies, the scan rate was found to have a considerable effect on electropolymerization with higher oxidation and reduction peak values found for the rigid network polymer compared to the uncrosslinked polymers.     

Hofmeister effects of anions on self-assembled thermogels

Thermogels are temperature-responsive soft biomaterials with numerous biomedical applications. They possess high water content and can spontaneously gelate by forming non-covalent physical crosslinks between their constituent amphiphilic polymers when warmed. However, despite the ubiquity of salts in biological fluids and buffer media, the influence of salts on thermogelling polymers and the overall physical properties of the resulting hydrogels are poorly understood. Herein, we elucidate the effects of common inorganic salts on the gelation and micellization properties of a thermogelling polymer containing poly(ethylene glycol), poly(propylene glycol), and poly(caprolactone) components. The identity of the salts' anions and their concentrations was found to exhibit significant effects on the thermogel properties, in some cases being able to decrease the sol-to-gel phase transition by up to 10 °C. We demonstrate that these notable influences are likely brought about by the changes in solvation of both the polymer's hydrophobic and hydrophilic segments, as well as by direct interactions of poorly hydrated anions with the hydrophobic polymer segments. Our findings show that the effects of salts on amphiphilic thermogelling polymers are non-negligible and hence need to be taken into account for engineering and optimization of thermogel properties for different biomedical applications.     

Effects of block copolymer's architecture on its association with lipid membranes: experiments and simulations

Triblock copolymers of the form poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) have been shown to effectively interact with and restore activity of damaged cell membranes. To better understand the interaction between these polymers and cell membranes, we have modeled the outer leaflet of a cell membrane with a lipid monolayer spread at the air-water interface and injected poloxamers of varying architectures into the subphase beneath the monolayer. Subsequent interactions of the polymer with the monolayer upon compression were monitored with concurrent Langmuir isotherm and fluorescence microscopy measurements. Monte Carlo simulations were run in parallel using a coarse-grained model to capture interactions between lipids and poloxamers. Changing the ratio of the PEO to PPO block lengths (NPEO:NPPO) affects the equilibrium spreading pressure of the polymer. Poloxamers with a relatively longer central hydrophobic block are less soluble, resulting in more polymer adsorbed to the interface and therefore a higher equilibrium spreading pressure. Simulation results show that changing the poloxamer structure effectively affects its solubility. This is also reflected in the degree of lipid corralling as poloxamers with a higher chemical potential (and resulting higher equilibrium spreading pressure) cause the neighboring lipid domains to be more ordered. Upon lateral compression of the monolayers, the polymer is expelled from the film beyond a certain squeeze-out pressure. A poloxamer with a higher NPEO:NPPO ratio (with either NPEO or NPPO held constant in each series) has a lower squeeze-out pressure. Likewise when the total size of the polymer is varied with a constant hydrophilic:hydrophobic ratio, smaller poloxamers are squeezed out at a lower pressure. Our simulation results capture the trends of our experimental observations, both indicating how the interactions between lipids and poloxamers can be tuned by the polymer architecture.     

Electron transfer studies on cholesterol LB films assembled on thiophenol and 2-naphthalenethiol self-assembled monolayers

We have formed the cholesterol monolayer and multilayer LB films on the self-assembled monolayers of 2-naphthalenethiol (2-NT) and thiophenol (TP) and studied the electrochemical barrier properties of these composite films using cyclic voltammetry and electrochemical impedance spectroscopy. We have also characterized the cholesterol monolayer film using grazing angle FTIR, scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Cholesterol has a long hydrophobic steroid chain, which makes it a suitable candidate to assemble on the hydrophobic surfaces. We find that the highly hydrophobic surface formed by the self-assembled monolayers (SAM) of 2-NT and TP act as effective platforms for the fabrication of cholesterol monolayer and multilayer films. The STM studies show that the cholesterol monolayer films on 2-NT form striped patterns with a separation of 1.0 nm between them. The area per cholesterol molecule is observed to be 0.64 nm2 with a tilt angle of about 28.96 degrees from the surface normal. The electrochemical studies show a large increase in charge transfer resistance and lowering of interfacial capacitance due to the formation of the LB film of cholesterol. We have compared the behavior of this system with that of cholesterol monolayer and multilayers formed on the self-assembled monolayer of thiophenol.     

Amphiphilic graft copolymers - preparation and interfacial properties

Amphiphilic graft copolymers containing poly(ethylene oxide) (PEO) grafts have been prepared by various methods, for example, by coupling of reactive hydrophobic backbone polymers with end-functionalised PEO, by macromonomer copolymerisation, and by anionic graft polymerisation of EO onto polymer backbones carrying functional groups as initiator precursors. The graft copolymers are amphiphilic and were shown to accumulate at surfaces and interfaces in solution and in the solid state. Amphiphilic starch derivatives were prepared by reaction of amylose and starch with aliphatic α-epoxides.     

The effect of polymer architecture on the nano self-assemblies based on novel comb-shaped amphiphilic poly(allylamine)

Twelve novel poly(allylamine) (PAA)-based, comb-shaped amphiphilic polymers have been developed. Hydrophobic groups of cetyl, palmitoyl and cholesteryl were randomly grafted to PAA and quaternisation was carried out on some modified polymers. Polymers were characterised using ¹H NMR, elemental analysis and differential scanning calorimetry. All polymers formed nano self-assemblies in the aqueous solution with a positive zeta potential and were able to encapsulate a hydrophobic agent, methyl orange, in the core. The critical aggregation concentration (CAC) and the microviscosity were found to be dependent on the polymer hydrophobicity. Being the most hydrophobic polymer, cholesteryl-grafted PAA had the lowest CAC (0.02 mg mL⁻¹) and the highest microviscosity. They appeared to form dense nanoparticles and were transformed into novel nanostructures in the presence of free cholesterol. Palmitoyl-grafted polymers formed nanoparticles while cetyl-grafted polymers formed polymeric micelles. The flexibility of cetyl chains possibly resulted in the formation of multicore polymeric micelles.     

Hybrid organic/inorganic films of conducting polymers modified with phthalocyanines. II. EIS studies and film characterization

Conducting polymers were modified with Cu-phthalocyanine or Co-phthalocyanine embedded in a sol–gel matrix. The resulting films were characterized using electrochemical impedance spectroscopy, Fourier transform infrared spectroscopy and scanning electron microscopy. Electrochemical impedance spectroscopy data showed that the application of the sol–gel layer to the conductive polymer caused a noticeable increase in the impedance of the film across the frequency ranges studied. The hydrophobic character of the film was greatly influenced by the sol–gel and caused an increase in its capacitance. A modified ‘Randles’ equivalent cell was used to correlate the electrochemical parameters of the films. Elemental analysis and infrared data confirmed the presence of the phthalocyanine moieties in the film and the empirical formula of the film was estimated. The surface morphology of the sol–gel-modified conducting polymer was distinctly amorphous compared to the poly(3-methyl thiophene).     

End functionalization of hyperbranched poly(siloxysilane): Novel crosslinking agents and hyperbranched–linear star block copolymers

Functionalized hyperbranched poly(siloxysilane)s have been prepared by hydrosilylation reactions involving the multiple silicon hydride (SiH) groups of the polymer to introduce other reactive groups such as epoxy, amine, and hydroxyl groups. The possible use of these modified polymers as novel crosslinking agents is discussed. The same hydrosilylation reaction is used to attach preformed linear poly(isobutylene) (PIB) or poly(ethylene oxide) (PEO) onto the hyperbranched polymer to afford unusual hyperbranched–linear star block copolymers. The PIB‐derived copolymer is shown to be very hydrophobic, whereas its PEO‐derived counterpart is amphiphilic. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2970–2978, 2000     

Poly(ethylene oxide)s hydrophobically modified. Adsorption and spreading at the air-water interface

A comparative study of spread and adsorbed monolayer of poly(ethylene oxide)s of different molecular weight hydrophobically modified with alkyl isocyanates of different length chain is reported. The modification of the polymer was carried out according to reported procedures. The polymers obtained were studied at the air-water interface by Langmuir isotherms for spread monolayers and by Gibbs isotherms for the adsorption process. Isotherms obtained are interpreted in terms of the hydrophobic and hydrophilic balance of the polymers. Limiting area per repeating unit (A(0)) and collapse pressure (pi(c)) from spread monolayers were obtained. Spread monolayers of the hydrophobically modified polymers show larger collapse pressure values than unmodified polymer monolayers. In the adsorption process the excess surface concentration Gamma(infinity), area per repeat unit sigma, and efficiency of the adsorption were determined. The values of the area occupied per repeat unit in adsorbed monolayer (sigma) were larger than those of the spread monolayer. The efficiency of the adsorption of poly(ethylene oxide)s increases with the hydrophobic modification and with the alkyl chain length.     

An electrochemical study into the interaction between complement-derived peptides and DOPC mono- and bilayers

Electrochemical methods employing the hanging mercury drop electrode were used to study the interaction between variants of the complement-derived antimicrobial peptide CNY21 (CNYITELRRQH ARASHLGLAR) and dioleoyl phosphatidylcholine (DOPC) monolayers. Capacitance potential and impedance measurements showed that the CNY21 analogues investigated interact with DOPC monolayers coating the mercury drop. Increasing the peptide hydrophobicity by substituting the two histidine residues with leucine resulted in a deeper peptide penetration into the hydrophobic region of the DOPC monolayer, indicated by an increase in the dielectric constant of the lipid monolayer (Deltaepsilon = 2.0 after 15 min interaction). Increasing the peptide net charge from +3 to +5 by replacing the histidines by lysines, on the other hand, arrests the peptide in the lipid head group region. Reduction of electroactive ions (Tl+, Pb2+, Cd2+, and Eu3+) at the monolayer-coated electrode was employed to further characterize the types of defects induced by the peptides. All peptides studied permeabilize the monolayer to Tl+ to an appreciable extent, but this effect is more pronounced for the more hydrophobic peptide (CNY21L), which also allows penetration of larger ions and ions of higher valency. The results for the various ions indicate that charge repulsion rather than ion size is the determining factor for cation penetration through peptide-induced defects in the DOPC monolayer. The effects obtained for monolayers were compared to results obtained with bilayers from liposome leakage and circular dichroism studies for unilamellar DOPC vesicles, and in situ ellipsometry for supported DOPC bilayers. Trends in peptide-induced liposome leakage were similar to peptide effects on electrochemical impedance and permeability of electroactive ions for the monolayer system, demonstrating that formation of transmembrane pores alone does not constitute the mechanism of action for the peptides investigated. Instead, our results point to the importance of local packing defects in the lipid membrane in close proximity to the adsorbed peptide molecules.     

Electrochemical impedance spectroscopy studies of the Ag(111)/<Emphasis Type="Italic">n</Emphasis>-decanethiol monolayer/NaNO<Subscript>3</Subscript> interface

Surface properties of a modified single-crystal silver (111) face are studied in aqueous 1 M NaNO₃ solutions with different acidity in the presence of a monolayer n-decanethiol (DT) film by the methods of electrochemical impedance spectroscopy (EIS) and cyclic voltammetry that employs the meniscus contact technique. It is shown experimentally that in the potential range from 0 to −0.5 V, a DT-modified silver electrode in NaNO₃ solutions with pH 6 behaves as an ideal capacitance in a wide frequency range (from 0.01 Hz to 100 kHz). It is found that with the increase in the solution acidity up to pH 4 and 2, the region of stable capacitance characteristics narrows down. The concomitant changes in the adsorbed monolayer structure lead to deviations of impedance characteristics measured using a low-frequency ac signal from those of the ideal capacitance. A potential shift to values more negative than −0.5 V leads to the destruction of the monolayer. Based on the analysis of experimental EIS characteristics, system’s deviation from the ideal behavior that may indicate violation of the layer continuity is assessed and the relaxation frequencies of the DT/Ag(111) interface in solutions of different acidity are calculated. Estimates of capacitance, resistance, and thickness of the adsorbed monolayer are obtained as a function of the solution pH.     

Controlling the self-assembly structure of magnetic nanoparticles and amphiphilic block-copolymers: from micelles to vesicles

We report how to control the self-assembly of magnetic nanoparticles and a prototypical amphiphilic block-copolymer composed of poly(acrylic acid) and polystyrene (PAA-b-PS). Three distinct structures were obtained by controlling the solvent-nanoparticle and polymer-nanoparticle interactions: (1) polymersomes densely packed with nanoparticles (magneto-polymersomes), (2) core-shell type polymer assemblies where nanoparticles are radially arranged at the interface between the polymer core and the shell (magneto-core shell), and (3) polymer micelles where nanoparticles are homogeneously incorporated (magneto-micelles). Importantly, we show that the incorporation of nanoparticles drastically affects the self-assembly structure of block-copolymers by modifying the relative volume ratio between the hydrophobic block and the hydrophilic block. As a consequence, the self-assembly of micelle-forming block-copolymers typically produces magneto-polymersomes instead of magneto-micelles. On the other hand, vesicle-forming polymers tend to form magneto-micelles due to the solubilization of nanoparticles in polymer assemblies. The nanoparticle-polymer interaction also controls the nanoparticle arrangement in the polymer matrix. In N,N-dimethylformamide (DMF) where PS is not well-solvated, nanoparticles segregate from PS and form unique radial assemblies. In tetrahydrofuran (THF), which is a good solvent for both nanoparticles and PS, nanoparticles are homogeneously distributed in the polymer matrix. Furthermore, we demonstrated that the morphology of nanoparticle-encapsulating polymer assemblies significantly affects their magnetic relaxation properties, emphasizing the importance of the self-assembly structure and nanoparticle arrangement as well as the size of the assemblies.     

Zwitterionic Polymer Design that Inhibits Aggregation and Facilitates Insulin Refolding: Mechanistic Insights and Importance of Hydrophobicity

The synthesis of a zwitterionic polymer, poly‐sulfobetaine (poly‐SPB), which shows remarkable efficiency in the suppression of insulin aggregation is described. Hydrophobic modification of the polymer results in almost complete inhibition at very low polymer concentrations. Further studies reveal that these polymers facilitate the complete retention of insulin's secondary structure, which is otherwise lost after incubation. Refolding studies show that addition of polymers to preaggregated insulin sample leads to the refolding of denatured insulin, indicating their potential to facilitate the refolding of the denatured proteins. In addition, 2D NMR studies show that the presence of hydrophobic poly‐SPB alters the hydrophobic environment of insulin, which may suppress hydrophobic interactions that lead to aggregation. These results indicate the enormous potential of these polymers for suppressing insulin aggregation and provide significant insight into the complex mechanism of insulin protection by zwitterionic polymers.