Pearl-necklace Structure Research Articles

A hyperelastic hydrogel with an ultralarge reversible biaxial strain.

Hyperelastic materials exhibit a nonlinear elastic response to large strains, whereas hydrogels typically possess a low elastic range due to the nonuniform cross-linking and limited chain segments among cross-links. We developed a hyperelastic hydrogel that possesses a broader elastic range by introducing a reversible pearl-necklace structure, in which beads are connected by strings. The subnanometric beads can efficiently unfold and refold under cyclic mechanical strains; thus, the hydrogel can rapidly recover after being stretched to an areal strain of more than 10,000%. Additionally, the hydrogel can quickly heal from minor mechanical damages such as needle punctures and cuts. These advancements make our ionic hydrogels ideal for multifunctional pneumatic gripper materials; they simultaneously offer an ultralarge gripping range, self-sensing capabilities, and fast healing abilities.

Hyaluronan interactions with cationic surfactants – Insights from fluorescence resonance energy transfer and anisotropy techniques

Interactions of hyaluronan with micelles formed by cationic surfactants were studied by the time-resolved measurement of fluorescence resonance energy transfer (FRET) using perylene and fluorescein as probes. Two surfactants were studied – Cetyltrimethylammonium bromide (CTAB) and Septonex. In pure micellar solutions, the same values of FRET efficiency were found for both surfactants, but values for the binding of the probe pair were lower for Septonex micelles than in the case of CTAB. This was attributed to steric effects of the carbethoxy group in the Septonex polar head. Upon the addition of hyaluronan, decreased FRET efficiency and increased binding were detected in comparison with pure surfactants. To resolve the structure of the formed aggregates, steady state and time-resolved fluorescence anisotropy was employed as an additional technique. All results indicated that cationic micelles bind to hyaluronan forming a pearl necklace structure with micelles of smaller size compared to pure surfactant. Besides theoretical interest, the studied polyelectrolyte-surfactant system may be interesting for the formulation of drug delivery systems.

Diffusion of dyes in polyelectrolyte-surfactant hydrogels.

In this work, hydrogels formed by interaction of biopolymeric electrolytes and oppositely charged surfactants are studied from the point of view of their ability to incorporate model hydrophobic dyes in their micelle-like structure. Two types of hydrogels were investigated. The first type was based on cationized dextran cross-linked by sodium dodecylsulphate. The second type was prepared by interactions of hyaluronan with carbethoxypendecinium bromide (septonex). Nile red and Atto488 were used as model dyes for the diffusion experiments. The dyes were dissolved in two different media: surfactant and physiological saline. The diffusion of dyes into hydrogel was monitored over time. Effective diffusion coefficients were determined. It was found that their values are strongly influenced by the hydrogel character, the types of dye used and the solvent. The obtained effective coefficients were higher in comparison with the values determined for the diffusion in the opposite direction (release from the hydrogel). The dyes are presented as free in physiological saline and in the form of micelles or micelle aggregates in surfactants. During diffusion into the hydrogel, they can be gradually incorporated in a “pearl necklace structure” which suppresses their mobility. In contrast, this partial immobilization of dyes can increase the concentration gradient which is a driving force of diffusion. Also, the gradual incorporation of dyes into hydrogel structures influences the values of the effective diffusion coefficients.

Tunable Pearl Necklace Structured TiO2 Spheres on Carbon Nanotubes with Enhanced Electromagnetic Wave Absorption Performance

Due to the low density and high conduction loss of carbonaceous materials, they have been considered as candidates for compositing electromagnetic wave absorbing materials. In this study, the well-designed CNT@TiO2 composites with pearl necklace structure were successfully synthesized via a facile hydrolyzation method. The absorption performances of CNT@TiO2 under different loading ratio were characterized and showed significantly different performance on the electromagnetic wave absorption. The reflection loss value reaches to –47.5 dB in the pearl necklace structured CNT@TiO2 composites obtained with a thickness of 1.6 mm at 11.2 GHz. An effective absorption bandwidth was found to reach to 4.2 GHz with a coating thickness of 1.4 mm. More, the absorption frequency range can be diverse by adjusting coating thickness. We deduce that promising electromagnetic wave absorbing property of the composites is related to the interfacial polarization induced by pearl necklace structure, improved impedance matching, conduction loss and scattering within the materials.

Transport of a model diffusion probe in polyelectrolyte-surfactant hydrogels

Interactions of biopolymeric electrolytes with oppositely charged surfactants can result in micelle-like nanocontainers capable to dissolve hydrophobic substances. These micelle-like nanostructures can solve problems with hydrophobic nature of pharmaceutically active substances and be potentially usable for medical applications. In this study, two types of hydrogels were prepared: 1) based on hyaluronan and cationic surfactant septonex (carbethoxypendecinium bromide) and 2) based on diethylaminoethyl dextran and sodium dodecylsulphate. Nile red as a dye diffusion probe was incorporated into hydrogel in their preparation and then its release into surfactant solution and physiological saline was monitored. The release of NR from hydrogels was characterized by the effective diffusion coefficients of NR and structural parameters of hydrogels. The diffusion of NR in hydrogels was dependent on the molecular weight of polyelectrolyte and the ratio between charges of polyelectrolyte and surfactant. In the case of lower ratios, a bigger part of NR was probably incorporated in “pearl necklace structure” which suppressed its mobility. The increase of concentration of surfactant was connected with higher content of NR in free micelles or micelle-like aggregates and the increase in the release of NR. The formation of bigger micelles based on NR and SDS (in comparison with micelles based on NR and septonex surfactant) resulted in their lower diffusivity.

Single-Chain Conformational Characteristics of Comb-Like Polyelectrolytes: Molecular Dynamics Simulation Study

The single-chain conformation of a model comb polyelectrolyte, in which the electric charges are located on the last monomer of side chains, has been studied using molecular dynamics simulations. The model chain has a hydrophobic backbone and hydrophilic side chains. The influence of parameters such as the length of backbone and side chains, the grafting density of side chains, flexibility of the chain and different strengths of backbone hydrophobicity have been investigated. At high grafting density of side chains, a transition from turn to pearl-necklace and finally to globular conformation is observed by increasing the hydrophobicity. Radius of gyration (Rg) of the backbone depends linearly on the number of backbone monomers at low values of the backbone hydrophobicity, but at high values of the backbone hydrophobicity, because of the appearance of turn-like segments between successive globular sections in the pearl-necklace structure, the scaling exponent of Rg versus N is larger. Also, the chain flexibility affects the backbone conformation significantly. The increase of the persistence length leads to the appearance of the folded and then the extended conformations. In addition to the statistical properties of the chains, the investigation of the relaxation of the chains conformation shows that, the backbone takes almost its final conformation in the early time steps of the simulation for all conditions. It means that the relaxation time of the chain is relatively short (fast relaxation).

Effect of water content on properties of one-step catalyzed silica aerogels via ambient pressure drying

Tetraethylorthosilane was hydrolyzed and condensed in methanol as solvent, and catalytic aqueous ammonium fluoride and different amounts of water as co-solvent. The prepared gels were aged and solvent exchanged by n-hexane. The surfaces of the gels were modified using trimethylchlorosilane. Finally ambient pressure drying was performed. The surface area of the samples was determined by BET analysis and the pore size distributions were measured using the BJH method. FT-IR spectroscopy was used to confirm surface silylation. Compressive properties were investigated before drying. By increasing the amount of water, gelation time decreased significantly and compression modulus rose by increasing water content. Specific compression modulus (modulus/density) of the samples exhibited decrease with larger water content. The BET and BJH results showed that at first by adding more water, the specific surface area of silica aerogels decreased, this declination is more significant for the sample with 3 molar parts of water and after that, the surface area again increased slightly. The FE-SEM micrographs showed the pearl necklace structure for prepared samples.

Morphological transformation of vesicles into tubular structures by adding polyampholytes or dendritic glycopolymers

For the first time tubulating properties of spherical dendritic glycopolymers and linear alternating polyampholytes against non-uniform negatively charged giant vesicles are proven by light microscopy and cryo-scanning electron microscopy study. Real time observation of the morphological transformation from giant vesicles to tubular structures, simulating morphogenesis in living cells, is given by using the cationic and H-bond active dendritic glycopolymer accompanied by reducing the size of the giant vesicles and the evidence of vesicle–vesicle interaction which was only postulated in a previous study. Similar morphogenesis of non-uniform giant vesicles into tubular network structure can be observed by using a polyampholyte in the stretched conformation at pH 9. Pearl necklace and tubular network structure formation are also observed by applying anionic vesicles of significant smaller dimensions with average size dimensions of 35nm, after adding the polyampholyte at pH 9. However, the fitting accuracy between the functional groups along the backbone chain of the polyampholyte on one side and the vesicle surface on the other side is of high importance for the transformation process by using polyampholytes. The resulting tubular and network structures offer new fields of application as microfluidic transport channels or template phases for the shape controlled formation of nanoparticles.

Interactions between Individual Charged Dendronized Polymers and Surfaces

Polymethacrylate-based amino-terminated dendronized polymers up to generation 5 were adsorbed on mica, hydrophobized mica, and gold and studied with the atomic force microscope (AFM). These studies were carried out in a mildly acidic electrolyte solution where these polymers are fully ionized. The polymers were imaged in intermitted contact mode in solution, and desorption forces of individual polymer chains were investigated by single molecule force spectroscopy. The adsorbed chains have a uniform envelope up to generation 4, but the generation 5 polymer has a pronounced pearl-necklace structure. Force spectroscopy clearly reflects this difference. While lower generation polymers show peeling events with a single force plateau or pulling event with a single spike, those of generation 5 are characterized by two different force plateaus as well as multiple pulling events. These features are probably related to the unwrapping of pearls in the polymer backbone. We suspect that the pearls result from the high hydrophobicity of the polymer backbone.

Conformation of polyelectrolytes using cellular automata techniques

A cellular automata model of polyelectrolytes is studied using NetLogo, a multi-agent modelling environment based on the Java virtual machine. NetLogo allows easy construction of cellular automata models, is free to use and has a detailed documentation and examples library. In order to illustrate the working principles of NetLogo, we study polyelectrolyte conformations in a liquid salt solution using the so called cluster entropy S_C as an order parameter. Our results predict the correct evolution of polyelectrolyte conformations, from collapsed clustered structures, at low charge/high salt concentration conditions, to elongated chains, at high charge/low salt concentration, as had been reported in the literature. In both of these situations the cluster entropy is approximately zero S_C ≈ 0. In intermediate charge and salt concentration conditions the well-known pearl necklace structure is observed, in this case the values of S_{C} reach a maximum. The use of NetLogo modelling environment along the cluster entropy concept, permits quick and objective descriptions of the spatial properties of polyelectrolytes.

One‐Pot Radical Polymerization of One Inimer to Form One‐Dimensional Polymeric Nanomaterials

A one-pot polymerization strategy is put forward for producing 1D polymeric nanomaterials directly from a single inimer. An inimer bearing an NMP initiating site is polymerized in the presence of a RAFT CTA to form a pearl-necklace structure constituted of hyperbranched polymer "pearls". The obtained 1D nanorods show high regularity evidenced by the long-range order in small angle XRD patterns and the formation of liquid crystalline phases.

Macromolecular HPMA-Based Nanoparticles with Cholesterol for Solid-Tumor Targeting: Detailed Study of the Inner Structure of a Highly Efficient Drug Delivery System

We report a rigorous investigation into the detailed structure of nanoparticles already shown to be successful drug delivery nanocarriers. The basic structure of the drug conjugates consists of an N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer bearing the anticancer drug doxorubicin (Dox) bound via a pH-sensitive hydrazone bond and a defined amount of cholesterol moieties that vary in hydrophobicity. The results show that size, anisotropy, and aggregation number N(aggr) of the nanoparticles grows with increasing cholesterol content. From ab initio calculations, we conclude that the most probable structure of HPMA copolymer-cholesterol nanoparticles is a pearl necklace structure, where ellipsoidal pearls mainly composed of cholesterol are covered by a HPMA shell; pearls are connected by bridges composed of hydrophilic HPMA copolymer chains. Using a combination of techniques, we unambiguously show that the Dox moieties are not impregnated inside a cholesterol core but are instead uniformly distributed across the whole nanoparticle, including the hydrophilic HPMA shell surface.

Nitrogen-doped carbon materials produced from hydrothermally treated tannin

This manuscript presents a simple one-pot synthesis for producing nitrogen-doped carbon materials (NCMs) with nitrogen content up to 6.3wt.%. Carbon gels were produced by direct pyrolysis without the expensive step of supercritical drying in CO2. NCMs were synthesized by hydrothermal carbonization (HTC) of aminated tannin. Amination of tannin was carried out in aqueous ammonia solution according to two different methods: (1) amination at room temperature followed by evaporation and HTC of the resultant powder in pure water at 180°C for 24h, or (2) direct HTC in the same conditions of the tannin–ammonia solution. In both cases, further pyrolysis was carried out at 900°C under nitrogen flow. We obtained very different NCMs depending on the method. Method (1) led to microporous carbon gels having a surface area of 500m2g−1 without need of supercritical drying, and a nitrogen content of 2.9wt.%, whereas method (2) led to carbons microspheres arranged in pearl-necklace structure, having a surface area around 60m2g−1 and a higher nitrogen content of 6.3wt.%. In both cases, X-ray photoelectron spectroscopy showed mainly pyrrolic and pyridinic nitrogen functionalities.

Formation of Pearl-Necklace Monomorphic G-Quadruplexes in the Human CEB25 Minisatellite

CEB25 is a human minisatellite locus, composed of slightly polymorphic 52-nucleotide (nt) tandem repeats. Genetically, most if not all individuals of the human population are heterozygous, carrying alleles ranging from 0.5 to 20 kb, maintained by mendelian inheritance but also subject to germline instability. To provide insights on the biological role of CEB25, we interrogated its structural features. We report the NMR structure of the G-quadruplex formed by the conserved 26-nt G-rich fragment of the CEB25 motif. In K(+) solution, this sequence forms a propeller-type parallel-stranded G-quadruplex involving a 9-nt central double-chain-reversal loop. This long loop is anchored to the 5'-end of the sequence by an A·T Watson-Crick base pair and a potential G·A noncanonical base pair. These base pairs contribute to the stability of the overall G-quadruplexstructure, as measured by an increase of about 17 kcal/mol in enthalpy or 6 °C in melting temperature. Further, we demonstrate that such a monomorphic structure is formed within longer sequence contexts folding into a pearl-necklace structure.

Genome organization and epigenetic marks in mouse germinal vesicle oocytes

During the final step of oogenesis, the oocyte nucleus is subject to large-scale modifications that correlate with transcriptional silencing. While oocytes with dense chromatin around the nucleolus are silent (SN, surrounded nucleolus), oocytes with uncondensed chromatin (NSN, non-surrounded nucleolus) are transcriptionally active. It is believed that epigenetic mechanisms that participate in gene expression regulation could play a role in this event. In this context, we examined the behaviour of heterochromatin and related histone modifications during the NSN to SN transition by immunostaining. Using fluorescent in situ hybridization on three dimensional-preserved nuclei (3D-FISH), we also studied the distribution of centromeric, pericentromeric and ribosomal (rDNA) sequences in relation to the nucleolus (also called the nucleolus-like body, NLB). We observed that in NSN-type oocytes, pericentromeric heterochromatin is aggregated within chromocenters. In SN-type oocytes, pericentromeric heterochromatin and centromeres form a discontinuous ring around the NLB. rDNA sequences, which initially present a pearl necklace structure, gather together in seven highly condensed foci at the NLB periphery. H3K9me3 and H4K20me3 heterochromatin marks clearly label chromocenters, whereas H3K4me3 and H4K5ac are totally excluded from heterochromatin regions, even in the very compact SN-nuclei. Remarkably, H3K27me3 displays an intermediate behavior. It appears that GV oocyte nuclei exhibit a specific epigenetic landscape. Histone modifications, related to both active and repressive chromatin structures, seem to follow the large-scale chromatin movements that occur during the NSN to SN transition. We also demonstrate that, while heterochromatin regions re-localize around the NLB, rDNA sequences adopt a highly compact structure in SN-type oocytes.

Minimal-memory realization of pearl-necklace encoders of general quantum convolutional codes

Quantum convolutional codes, like their classical counterparts, promise to offer higher error correction performance than block codes of equivalent encoding complexity, and are expected to find important applications in reliable quantum communication where a continuous stream of qubits is transmitted. Grassl and Roetteler devised an algorithm to encode a quantum convolutional code with a "pearl-necklace encoder." Despite their theoretical significance as a neat way of representing quantum convolutional codes, they are not well-suited to practical realization. In fact, there is no straightforward way to implement any given pearl-necklace structure. This paper closes the gap between theoretical representation and practical implementation. In our previous work, we presented an efficient algorithm for finding a minimal-memory realization of a pearl-necklace encoder for Calderbank-Shor-Steane (CSS) convolutional codes. This work extends our previous work and presents an algorithm for turning a pearl-necklace encoder for a general (non-CSS) quantum convolutional code into a realizable quantum convolutional encoder. We show that a minimal-memory realization depends on the commutativity relations between the gate strings in the pearl-necklace encoder. We find a realization by means of a weighted graph which details the non-commutative paths through the pearl-necklace. The weight of the longest path in this graph is equal to the minimal amount of memory needed to implement the encoder. The algorithm has a polynomial-time complexity in the number of gate strings in the pearl-necklace encoder.

Pearl-necklace structures of molecular brushes with rigid backbone under poor solvent conditions: A simulation study

Bottle-brush polymers, where flexible side chains containing N=20 to 50 effective monomers are grafted to a rigid backbone, are studied by molecular dynamics simulations, varying the grafting density σ and the solvent quality. Whereas for poor solvents and large enough σ the molecular brush is a cylindrical object, homogeneous in axial direction, for intermediate values of σ an axially inhomogeneous structure of "pearl-necklace" type is formed. The "pearls," however, have a strongly nonspherical ellipsoidal shape, due to the fact that several side chains cluster together in one pearl, qualitatively consistent with predictions of Sheiko et al. [Eur. Phys. J. E 13, 125 (2004)] We analyze the structure of these pearls and study both the transition to the axially uniform cylinder at high σ and to the trivial pearl-necklace structure at small σ, where each pearl contains a single collapsed chain only.

PH Sensitive Polymer Nanoparticles: Effect of Hydrophobicity on Self-Assembly

The influence of hydrophobicity on formation, stability, and size of pH-responsive methacryloylated oligopeptide-based polymer nanoparticles has been studied by dynamic light scattering (DLS), transmission electron microscopy (Cryo-TEM), and NMR. Different polyanions/surfactant systems have been studied at constant polymer concentration and within a broad range of surfactant concentrations. The two newly synthesized pH-sensitive hydrophobic polyanions, poly(N(ω)-methacryloyl glycyl-L-leucine) and poly(N(ω)-methacryloyl glycyl-L-phenylalanyl-L-leucinyl-glycine), and three nonionic surfactants (Brij97, Brij98, and Brij700) have been investigated. The surfactants were different in the length of hydrophilic poly(ethylene oxide) (PEO) chain. In surfactant-free solution at basic pH, the polyanions form hydrophobic domains. In the presence of a surfactant, our results prove the complex formation at high pH between the nonionic surfactant and the polyelectrolyte; a pearl-necklace structure is formed. At low pH below critical pH (pH(tr)), reversible nanoscale structures occur in solutions for all systems. The detailed mechanism of the formation of pH-sensitive nanoparticles from polymer-surfactant complex with varying pH is established. Our results suggest that the polymer hydrophobicity is of primary importance in pretransitional behavior of the complex. Once preliminary nanoparticle nuclei are formed, the hydrophobicity of the polymer plays a minor role on further behavior of formed nanostructures. The subsequent transformation of nanoparticles is determined by the surfactant hydrophilicity, the length of hydrophilic tail that prevents further aggregation due to steric repulsions.

Amphiphilic Graft Copolymers in Selective Solvents: Molecular Dynamics Simulations and Scaling Theory

Intramolecular structures in amphiphilic graft copolymers with hydrophilic side chains and hydrophobic backbone have been studied by molecular dynamics simulations. In accordance with earlier theoretical predictions [Borisov, O.; Zhulina, E. Macromolecules 2005, 38, 2506−2514], we have found that balance of repulsive and attractive intramolecular interactions may result in the pearl-necklace-type conformations. The globular “pearls” are formed by collapsed segments of the main chain comprising multiple spacers; these pearls are stabilized against aggregation by repulsive interactions of the hydrophilic grafts. The size of the pearls is controlled by the intramolecular hydrophilic−hydrophobic balance, whereas the total number of pearls in the graft copolymer depends on the length of the main chain. For graft copolymer with relatively long spacers we have observed intramolecular conformational transition from pearl-necklace structure to unimolecular cylindrical micelle upon a decrease in the solvent quality for the main chain. The results of simulations, in particular behavior of polymers of finite chain length, are rationalized on the basis of revised and extended scaling theory.

On the Effective Charge of Hydrophobic Polyelectrolytes

In this paper, we analyze the behavior of hydrophobic polyelectrolytes. It has been proposed that this system adopts a pearl necklace structure reminiscent of the Rayleigh instability of a charged droplet. Using a Poisson-Boltzmann approach, we calculate the counterion distribution around a given pearl, assuming the latter to be penetrable for the counterions. This allows us to calculate the effective electric charge of the pearl as a function of the chemical charge. Our predictions are in good agreement with the recent experimental measurements of the effective charge by Essafi et al. (Essafi, W.; Lafuma, F.; Baigl, D.; Williams, C. E. Europhys. Lett. 2005, 71, 938.). Our results allow us to understand the large deviation from the Manning law observed in these experiments.