Backbone Stiffness Research Articles

Effect of Double-Tailed Surfactant Architecture on the Conformation, Self-Assembly, and Processing in Polypeptide−Surfactant Complexes

This work describes the solid-state conformational and structural properties of self-assembled polypeptide-surfactant complexes with double-tailed surfactants. Poly(L-lysine) was complexed with three dialkyl esters of phosphoric acid (i.e., phosphodiester surfactants), where the surfactant tail branching and length was varied to tune the supramolecular architecture in a facile way. After complexation with the branched surfactant bis(2-ethylhexyl) phosphate in an aqueous solution, the polypeptide chains adopted an alpha-helical conformation. These rod-like helices self-assembled into cylindrical phases with the amorphous alkyl tails pointing outward. In complexes with dioctyl phosphate and didodecyl phosphate, which have two linear n-octyl or n-dodecyl tails, respectively, the polypeptide formed antiparallel beta-sheets separated by alkyl layers, resulting in well-ordered lamellar self-assemblies. By heating, it was possible to trigger a partial opening of the beta-sheets and disruption of the lamellar phase. After repeated heating/cooling, all of these complexes also showed a glass transition between 37 and 50 degrees C. Organic solvent treatment and plasticization by overstoichiometric amount of surfactant led to structure modification in poly(L-lysine)-dioctyl phosphate complexes, PLL(diC8)(x) (x = 1.0-3.0). Here, the alpha-helical PLL is surrounded by the surfactants and these bottle-brush-like chains self-assemble in a hexagonal cylindrical morphology. As x is increased, the materials are clearly plasticized and the degree of ordering is improved: The stiff alpha-helical backbones in a softened surfactant matrix give rise to thermotropic liquid-crystalline phases. The complexes were examined by Fourier transform infrared spectroscopy, small- and wide-angle X-ray scattering, transmission electron microscopy, differential scanning calorimetry, polarized optical microscopy, and circular dichroism.

Dielectric properties of differently flexible polyions: a scaling approach

The dielectric relaxations associated with counterion polarization along some typical polyion lengths have been measured in an extended frequency range (from 10 kHz to 2 GHz) for four different polyelectrolyte solutions, differing for the polyion molecular weight and the backbone stiffness. Here, we deal with the so-called intermediate dielectric relaxation, falling between the polarization process concerning the whole polyion chain and the polarization process associated with the field-induced re-orientation of the water molecule dipoles. These observed intermediate relaxations have been characterized by means of two parameters, i.e., the dielectric strength Deltaepsilon and the relaxation frequency nu0, and their dependence on the polyion concentration has been described according to the scaling model of a polyelectrolyte solution. These dependencies follow the expected exponents of the scaling laws, both for the dilute and semidilute (unentangled and entangled) regimes. The different concentration regimes evidenced from dielectric relaxation measurements are in very good agreement with the ones determined by means of the zero-shear viscosity measurements. Our results confirm that the intermediate dielectric relaxation in polyelectrolyte solutions should be attributed to counterion fluctuations along some segments (e.g. the subunits of the Mandel model) of the polyion chain, independently of its overall stiffness. This counterion polarization effect is rather confined to the local structure of the polyion chain instead of the whole chain and it is largely independent of the polyion conformation.

Semistiff polymer model of unfolded proteins and its application to NMR residual dipolar couplings

We present a new statistical model of unfolded proteins in which the stiffness of polypeptide backbone is taken into account. We construct and solve a mean field equation which has the form of a diffusion equation and derive the distribution function for conformations of unfolded polypeptides. Accounting for the stiffness of the protein backbone results in a more accurate description of general properties of a polypeptide chain, such as its gyration radius. We then use the distribution function of a semistiff protein within a previously developed theoretical framework [J. Biomol. NMR 39, 1 (2007)] to determine the nuclear magnetic resonance (NMR) residual dipolar couplings (RDCs) in unfolded proteins. The calculated RDC profiles (dependence of the RDC value on the residue number) exhibit a more prominent bell-like shape and a better agreement with experimental data as compared to the previous results obtained with the random flights chain model.

Polytriphenylene Dendrimers: A Unique Design for Blue‐Light‐Emitting Materials

Into the blue: A unique family of polytriphenylene dendrimers (see picture) that emit blue light with high photoluminescence quantum yield were synthesized. The triphenylene units are twisted out-of-plane, and the highly stiff dendritic backbones prevent intermolecular fluorescence quenching.

Dissecting Entropic Coiling and Poor Solvent Effects in Protein Collapse

The early events in protein collapse and folding are guided by the protein's elasticity. The contributions of entropic coiling and poor solvent effects like hydrophobic forces to the elastic response of proteins are currently unknown. Using molecular simulations of stretched ubiquitin in comparison with models of proteins as entropic chains, we find a surprisingly high stiffness of the protein backbone, reflected by a persistence length of 1.2 nm, which is significantly reduced by hydrophobic forces acting between protein side chains to an apparent persistence length of 0.3-0.6 nm frequently observed in single-molecule stretching experiments. Thus, the poor solvent conditions of a protein in water lead to a protein compaction much beyond the coiling of an entropic chain and thereby allow a protein to appear softer than when using good solvents.

Isolation and viscometric characterization of hydrocolloids from mulberry ( Morus alba L.) leaves

Hydrocolloid in mulberry leaves was extracted with water or sodium bicarbonate, and characterized rheologically in a highly dilute to a semi-dilute regime. Mulberry leaf hydrocolloid was composed of mainly carbohydrate with high levels of uronic acid. The hydrodynamic behavior of mulberry leaf hydrocolloid in deionized water demonstrated a typical polyelectrolyte behavior in the dilute domain, as indicated by a nonlinear Huggins' plot. Intrinsic viscosity value for alkaline-extracted (Alk) and water-extracted (Hw) hydrocolloid was around 7.25 dL/g and 3.61 dL/g, respectively. The specific viscosity versus concentration plot for both Alk and Hw exhibited a power-law dependence on concentration, with an onset of molecular entanglement occurring at a concentration of around 0.6 g/dL and 1.8 g/dL, respectively. Coil-overlap parameter for Alk and Hw was in the range of 4.35–6.61. The relative chain stiffness parameter ( B) for Alk and Hw was in the range of 0.004–0.013, indicating a stiff backbone.

Crystallinity and Cross-Linking in Porous Polymers Synthesized from Long Side Chain Monomers through Emulsion Templating

A polyHIPE is a cross-linked, porous polymer synthesized through polymerization in the external phase of a high internal phase emulsion (HIPE). Crystalline polyHIPE polymers were synthesized through copolymerization of stearyl acrylate and divinylbenzene (DVB). Polymerizations initiated at the water/organic interface were compared to equivalent bulk polymers, to equivalent polymerizations initiated in the organic phase, and to model copolymers containing an equivalent non-cross-linking comonomer (styrene). The higher of the two main melting endotherms was related to a more ordered crystalline structure formed by stearyl acrylate-rich molecular segments that are preferentially synthesized by initiation at the interface. Cross-linking restricts molecular mobility, tying the polymer backbones together and reducing the peak temperatures and crystallinities. The decrease in modulus with increasing DVB content for the more crystalline polyHIPE reflects the dominant effect of crystallinity. The increase in modulus with increasing DVB content for the less crystalline polyHIPE reflects the dominant effects of cross-linking and polymer backbone stiffness.

First-Principles Free-Energy Analysis of Helix Stability: The Origin of the Low Entropy in π Helices

The temperature dependence of the stability of infinite poly-L-alanine alpha, pi, and 310 helices with respect to the fully extended structure (FES) is calculated using density functional theory and the harmonic approximation. We find that the vibrational entropy strongly reduces the stability of the helical conformations with respect to the FES. By mapping the ab initio data on an approximate mechanical model, we show that this effect is exclusively due to the formation of hydrogen bonds, whereas changes in the backbone stiffness are practically negligible. We furthermore observe that the temperature dependence is largest for the pi helix and smallest for the 310 helix and demonstrate that these trends are a generic behavior related to the geometric peculiarities of the respective helical conformations and independent of the specific amino acid sequence.

Organo-soluble Polyimides and Their Applications to Photosensitive Cover Layer Materials in Flexible Printed Circuit Boards

Two types of photosensitive polyimides (PSPIs) were developed as novel cover layer materials for FPC applications in this work. One of them, PSPI-1 was highly soluble in common organic solvents such as NMP in spite of its stiff backbone structure. The PSPI-1 film formed by simple solution-casting exhibited a low CTE of 19-24 ppm/K in addition to a high Tg at 292°C, high thermal stability, low water absorption (0.3%), comparatively good transparency, and sufficient film flexibility. On the other hand, another system, PSPI-2 displayed an extremely low modulus. Both PSPI systems completely inhibited curling of the PSPI/copper laminates. These PSPI systems also possessed fine positive-tone patterning ability, reliable insulation property, and the highest level of non-flammability(UL-94: V-0 or VTM-0). The results revealed that the present PSPI systems are promising candidates as novel cover layer materials.

Cyclohexyl-Substituted Polyglycolides with High Glass Transition Temperatures

The substituted glycolides rac-dicyclohexylglycolide (rac-3,6-dicyclohexyl-1,4-dioxane-2,5-dione), meso-dicyclohexylglycolide, R,R-dicyclohexylglycolide, and rac-methylcyclohexylglycolide (rac-3-cyclohexyl-6-methyl-1,4-dioxane-2,5-dione) have been synthesized, and both solution and bulk polymerizations of these monomers are reported. The polymerization kinetics of these new monomers were studied and compared to data for rac-diisopropylglycolide (rac-3,6-diisopropyl-1,4-dioxane-2,5-dione) and rac-lactide. The solution polymerization rates followed the order: rac-dicyclohexylglycolide < rac-diisopropylglycolide < rac-methylcyclohexylglycolide < rac-lactide. The glass transition temperature of poly(rac-dicyclohexylglycolide) is 98 °C, consistent with a stiff polyglycolide backbone. meso-Dicyclohexylglycolide and R,R-dicyclohexylglycolide were synthesized and polymerized to study the effect of stereochemistry on the polymer properties. The glass transition temperature of meso-dicyclohexylglycolide was 96 °C,...

Sculptured Layer‐by‐Layer Films

The precise assembly of nanostructured materials into two (2D) and three-dimensional (3D) periodic microscopic arrays is achieved by employing various assembling technologies such as microparticle self-organization, photolithography, holographic lithography, selective chemical etching, ink printing, laser-based polymerization, selective responsive grafting, and inversion of bilayers. Soft lithography, such as microcontact printing has been widely applied to tackle this challenge in a few steps, with a submicrometer resolution, and at low cost. It has been demonstrated that the most popular fabrication of well-defined nanostructured materials with layer-by-layer (LbL) assembly can be combined with photolithography and microprinting to fabricate complex 2D and 3D structures with modulated distribution of different components. Examples of the micropatterned assembly of nanoparticles, microchannels, antireflective coatings, and Raman arrays have already reported. A variety of functional materials have been utilized in the LbL construction to make these structures suitable for prospective applications as antiwetting coatings, sensitive films for solar cells, fuel cells, ultra-strong nanomaterials, microcapsules, and membranes for controlled drug release. On the other hand, microstructural arrays based upon the principles of either diffraction or refraction with in-printed microscopic modulations are extensively used as optical components such as grating, beam splitters, microlenses, displays, and mirrors. A vast majority of LbL structures have been fabricated on solid planar or curved (microparticle) supports and represent essentially 2D planar structures with vertical (along the surface normal) modulations of chemical composition. Micropatterning of LbL films has been recently reviewed by Hammond. Few examples of free-standing LbL structures have been reported either in the form of uniform, micropatterned, or curved shell structures. Therefore, although potential for the fabrication of interesting modulated structures, which provides for optical effects, is inherently present in the LbL technology, few attempts have been made to investigate the feasibility of the fabrication of modulated LbL structures for light control. For example, in a recent study, Rubner et al. have demonstrated that a properly matched LbL coating can serve for constructive–destructive light interferences with efficient antireflective ability. In a related study, Rubner and Cohen have demonstrated that a proper modulation of the reflective index within LBL films might lead to a pronounced structural color effect with selective reflection of visible light controlled by the reflective index modulation rather than the presence of conjugated molecules with proper electronic structures. Although these initial studies showed some potential in LbL technology to generate nanostructured materials with the ability to control visible light diffraction/reflection, they have been limited to one-dimensional modulation of the refractive properties. In this communication, an example of 3D LbL grating structures with the ability to diffract light because of the modulation of local LbL film shape with microscopic periodicity and nanometer-scale vertical modulations is reported. In these freely suspended 3D LbL films, the effective modulation of the refractive properties is caused by the topological variation of the local film shape, thus representing a purely structural color effect. A simple and economical spin-assisted LbL assembly of conjugated polyelectrolytes on a sacrificial microimprinted modulated substrate was employed here to generate a robust, free-standing sculptured LbL structure with an effective thickness of 60 nm and a 160 nm peak-to-peak vertical modulation on a square lattice with 2.5 lm lateral periodicity (Figure 1). These films demonstrate efficient optical grating properties and bright structural colors in a reflective mode controlled by the in-plane spacing and the angle of incidence. The conjugated polyelectrolyte, poly(2,5-methoxypropyloxy sulfonate phenylene vinylene) (MPS-PPV) applied in the LbL assembly synthesized here was critical in providing a mechanically strong structure as has been tested in our previous work to fabricate mechanically robust planar ultrathin LbL films. MPS-PPV is a well known water-soluble conjugated polymer with a highly charged, stiff backbone and strong fluorescence properties. Planar LbL films of MPS-PPV and poly(allylamine hydrochloride) (PAH) demonstrate excellent mechanical properties combined with high fluorescence as has been demonstrated in our previous work. C O M M U N IC A IO N

Thiol–norbornene materials: Approaches to develop high Tg thiol–ene polymers

AbstractThe ability to prepare high Tg low shrinkage thiol–ene materials is attractive for applications such as coatings and dental restoratives. However, thiol and nonacrylated vinyl materials typically consist of a flexible backbone, limiting the utility of these polymers. Hence, it is of importance to synthesize and investigate thiol and vinyl materials of varying backbone chemistry and stiffness. Here, we investigate the effect of backbone chemistry and functionality of norbornene resins on polymerization kinetics and glass transition temperature (Tg) for several thiol–norbornene materials. Results indicate that Tgs as high as 94 °C are achievable in thiol–norbornene resins of appropriately controlled chemistry. Furthermore, both the backbone chemistry and the norbornene moiety are important factors in the development of high Tg materials. In particular, as much as a 70 °C increase in Tg was observed in a norbornene–thiol specimen when compared with a sample prepared using allyl ether monomer of analogous backbone chemistry. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5686–5696, 2007

Lateral Mobility of Polyelectrolyte Chains in Multilayers

In this work, the lateral mobility of polyelectrolyte multilayers was investigated by means of the fluorescence recovery after photobleaching (FRAP) technique, with special attention to the effect of relevant parameters during and after preparation. Different polyelectrolytes with respect to charge density, stiffness, and hydrophilicity were compared. From the experimental results emerged that the density of charged sites along the polymer is the most important parameter controlling the formation of polymer complexes. At higher charge density, more complexes are formed, and the diffusion coefficient decreases. It was observed that the intrinsic backbone stiffness reduces the interpenetration of polyelectrolyte layers and the formation of complexes promoting the lateral mobility. In addition, the lateral mobility increases with increasing ionic strength and with decreasing hydration shell of the added anion in the polyelectrolyte solution. The effect of heating or annealing in electrolyte solution after preparation was also investigated along with the embedding of the probing layer at controlled distances to the multilayer surface.

Possibilities of utilizing the differences of fish tissues stiffness in the mechanization of cyprinid deheading

This article discusses the types of deheading cuts used in fish processing. The cutting strength of deheading was also analyzed. Based on the potential stiffness difference of fish tissues, it was revealed that by steering the relief angle of the blade to the cut surface it is possible to make an efficient deheading V-cut with one knife instead of the two circular knives that have been used to date. The results of the theoretical analyses were confirmed by a study which was preformed on different tissue samples – the stiffness of backbone is 994 times bigger than the stiffness of muscle tissue measured in comparable conditions. This allows to construct a simple V-cutting deheading machine with one knife.

Molecular Dynamics Simulations of Comb-Branched Poly(epoxide ether)-Based Polymer Electrolytes

Molecular dynamics (MD) simulations using many-body polarizable force field were performed on comb-branched poly(epoxide ether) (PEPE) polymer electrolytes doped with lithium bistrifluoromethanesulfonamide (LiTFSI) salt as a function of temperatures from 333 to 423 K at ether oxygen (EO) to lithium ratio of 20. MD simulations predicted electrolyte conductivity in good agreement with experiments. The fraction of solvent-separated ions and lithium cation environment for PEPE/LiTFSI were similar to those found for the linear poly(ethylene oxide) (PEO)/LiTFSI electrolyte. The Li+ cations had the highest probability to be coordinated by EOs near the PEPE polymer backbone and the lowest probability being coordinated by EO's at the end of side chains. Segmental dynamics of the backbone was slower by 2 orders of magnitude compared to the dynamics of side-chain ends. The Li+ self-diffusion coefficient was approximately an order of magnitude lower than the TFSI- anion self-diffusion coefficient. Visualization of the lithium motion revealed that the most mobile Li+ cations moved by hopping from a side chain to another without being complexed by the backbone. The influence of the backbone−Li+ interactions and the backbone stiffness on ion transport was investigated in MD simulations performed on the PEPE/LiTFSI-like electrolytes with the same PEPE architecture but a very stiff backbone that does not complex lithium cations. The ion transport in these model electrolytes was compared to that of the original PEPE/LiTFSI electrolyte.

Friction of Octadecyltrichlorosilane Monolayer Self-Assembled on Silicon Wafer in 0% Relative Humidity

Self-assembled octadecyltrichlorosilane monolayers on silicon wafer were slid against a smooth steel ball at a 350 to 750 MPa contact pressure and in 0.02 to 1 cm/s velocity ranges in a nominal 0% relative humidity environment. The friction was found to increase monotonically over sliding time stretching to 20 h. Within a framework that the friction is related to the generation of strains and defects at the molecular scale we explore the dissipation process in some detail by varying the normal load, velocity, and a priori defect population in the monolayer. The results indicate that the rate of friction change is significantly influenced by the relative magnitude of the time allowed to the molecules to relax in intermittent contact and the characteristic relaxation time that reflects the collective response of a large number of molecules in contact at a given time. We propose a simple model to account for the observations, where the friction at the commencement of sliding is governed by backbone stiffness. This process for all practical purposes is independent of sliding time. The kinetic friction on the other hand is related to time-dependent defect generation and accumulation at the terminal end and is found to consist of reversible and irreversible components. The latter provides the monolayer with a memory of its previous loading history.

Characteristic Features of Chitosan/Glycerol Blends Dynamics

AbstractSummary: The phase relaxation behavior of chitosan and its blends with glycerol has been studied by dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetry (DSC) techniques. The analysis of DMTA spectra of chitosan leads to recognition of two molecular relaxations in chitosan and its blends. The analysis of enthalpic and entropic contributions suggest that both processes involve a large degree of cooperativity. At high temperature, the α‐relaxation has been assigned to its glass transition temperature Tg on the basis of the stiffness of the chitosan backbone, the high activation energy, and the plasticization with glycerol. Tg may be originated by torsional oscillations between two glucopyranose rings across a β‐1,4‐glycosidic oxygen and a cooperative hydrogen bond reordering. The other is a low temperature β‐relaxation that, in chitosan, has been associated to motions of lateral pendant groups in the glucopyranose rings and in the blends to the movements of a glycerol hydrogel bonded to the lateral groups of chitosan. Its composition dependence has been modeled by means of the Schneider‐Riegel‐Penzel equation in which the interactions among functional groups when a glass‐forming liquid is adsorbed to a stiff, highly functionalized macromolecule such as chitosan are taken into account.A three‐dimensional plot representing the tan δ for a series of chitosan and chitosan/glycerol blends as a function of temperature and composition.magnified imageA three‐dimensional plot representing the tan δ for a series of chitosan and chitosan/glycerol blends as a function of temperature and composition.

Semiflexible amphiphilic polymers: Cylindrical-shaped, collagenlike, and toroidal structures

A coarse-grained model is used to study the conformational properties of semiflexible polymers with amphiphilic monomer units containing both hydrophilic and hydrophobic interaction sites. The hydrophobically driven conformational transitions are studied using molecular dynamics simulations for the chains of varying stiffness, as characterized by intrinsic Kuhn segment lengths that vary over a decade. It is shown that the energy of hydrophobic attraction required for the realization of the coil-to-globule transition increases with increasing chain stiffness. For rather stiff backbone, the coil-to-globule transition corresponds to a first order phase transition. We find that depending on the chain stiffness, a variety of thermodynamically stable anisometric chain morphologies are possible in a solvent selectively poor for hydrophobic sites of amphiphilic monomer units. For flexible chains, the amphiphilic polymer forms a cylindrical globule having blob structure with nearly spherical blobs. With increasing stiffness, the number of blobs composing the globule decreases and the shape of blobs transforms into elongated cylinder. Further increase in stiffness leads to compaction of macromolecules into a collagenlike structure when the chain folds itself several times and different strands wind round each other. In this state, the collagenlike structures coexist with toroidal globules, both conformations having approximately equal energies.

Supramolecular Template‐Directed Synthesis of Perfect Phenelenediimino‐Bridged Ladderlike Polyphenylsiloxanes

A template synthesis of the soluble, high molecular weight (Mw), and perfect p-phenylenediimino (p-PDA)-bridged ladderlike polyphenylsiloxane (PLPS) is reported. First, N,N'-bis(phenyldichlorosilyl)-p-PDA monomers were self-assembled into a perfect ladder superstructure (LS) by concerted interaction of aromatic pi-pi stacking and hydrogen bonding. Then the LS underwent a novel stoichiometric hydrolysis/dehydrochlorination-condensation reaction instead of the usual hydrolysis/dehydration-condensation reaction, leading to the PLPS. The perfect ladder structures of both the PLPS and, in particular, the unstable supramolecular LS were confirmed as follows. 1) There are two Bragg peaks in solid and/or solution X-ray diffraction (XRD) spectra representing the ladder width (w) and ladder thickness (t); these peaks were consistent with those calculated by molecular simulation. 2) Both the PLPS and LS have extremely sharp absorption peaks with small half-peak widths (Delta(1/2) < 0.3 ppm) in 29Si NMR spectra, suggesting the presence of the perfect ladder structure for PLPS and LS. 3) Moreover, no noticeable absorption peaks for the Si-OH and NH2 groups were observed in FT-IR and 29Si NMR spectra, indicating that the Si-N bond of the [triple bond]Si-NH-C6H4-NH-Si[triple bond] ladder rung of PLPS and LS is not cleaved. 4) Both PLPS and LS show similar emission peaks (in fluorescence spectroscopy) attributed to the excimer formed by face-to face pi-pi stacking of phenyl groups along the ladder chain rather than a branched direction. 5) Differential scanning calorimetry (DSC) measurements indicate a high glass temperature (Tg = 176.4 degrees C) for PLPS. As circumstantial evidence, this result further indicates very high stiffness of PLPSs ladder backbone as compared with flexible single-chain polyphenylmethylsiloxane with a low Tg = -69.4 degrees C.

Biomimetic Facially Amphiphilic Antibacterial Oligomers with Conformationally Stiff Backbones

A foldamer has been designed with a conformationally stiff backbone that is facially amphiphilic. The oligomer has excellent antimicrobial activity and was found to be 18 times more active toward bacterial cells than human red blood cells. The oligomer is built from arylamide bonds around a central 4,6-dicarboxy pyrimidine ring. The conformation was studied by X-ray crystallography and solution NMR spectroscopy. Density-functional (DFT) calculations were performed to guide the design. These calculations accurately predicted the overall conformation as well as NMR chemical shifts. Antibacterial activity was demonstrated against E. coli, a gram-negative strain, and B. subtilis, a gram-positive strain. The minimal inhibitory concentration is 0.8 microg/ml.