High-resolution STM imaging of novel poly(G)-poly(C) DNA molecules

High-resolution scanning tunneling microscopy (STM) imaging of single double-stranded poly(G)-poly(C) DNA molecules, made by a novel synthesis method, shows the molecules morphology. The STM images reveal a periodic structure of approximately 4 nm, seen as repeating "bulbs" along the molecules. These "bulbs" are associated with the molecule helix (the major grooves). "Nicks", two per 100 nm on the average, are observed along the DNA as well. The STM measurements were supported by a morphological statistics of the DNA molecule groove length and apparent height relative to the surface.     

Label-free emission assay of mercuric ions using DNA duplexes of poly(dT)

In this study, an assay to quantify the presence of mercuric ions and methyl mercury by double-stranded DNA containing a poly(dT) sequence was developed using a light switch compound, Ru(phen)(2)(dppz)(2+) (1), which is known to intercalate into double-stranded DNA. Upon treatment with mercuric ions, the metal-to-ligand charge transfer (MLCT) emission derived from the intercalation of 1 was reduced due to the formation of DNA duplexes containing T-Hg(2+)-T base pairs by the dehybridization of poly(dT)-poly(dA) duplexes at room temperature. As the concentration of Hg(2+) was increased, the emission of 1 gradually decreased. This label-free method had a detection limit of 5 nM. Other metal ions, such as K(+), Ag(+), Ca(2+), Mg(2+), Zn(2+), Mn(2+), Co(2+), Ni(2+), Cu(2+), Cd(2+), Cr(3+), Fe(3+), had no significant effect on reducing emission. This emission method can differentiate matched and mismatched poly(dT) sequences based on the dehybridization rate of dsDNA and the rate decreased in the order of T(10)C·A(11)～ T(10)A·A(11) > T(10)G·A(11) > T(11)·A(11).     

Rheological properties of poly(dimethylsiloxane)-poly(diethylsiloxane) blends

The rheological behavior of poly(dimethylsiloxane)-poly(diethylsiloxane) blends in the range 20 to 100°C, including the region of existence of poly(diethylsiloxane) in mesomorphic and amorphous states has been studied by capillary viscometry. The flow of these blends within the studied shear stress interval between 10³ and 10⁵ Pa obeys a power law. If poly(diethylsiloxane) is introduced in the mesomorphic state and serves as a matrix, the blends behave as viscoplastic bodies and feature the yield stress. The flow of these blends is accompanied by an appreciable orientation of the poly(diethylsiloxane) phase. Blends in which the mesomorphic poly(diethylsiloxane) is a disperse phase flow as abnormally viscous fluids in which poly(diethylsiloxane) plays the role of a structuring filler. The isotropization of poly(diethylsiloxane) leads to a reduction in its viscosity and, accordingly, in the viscosity of the blend. The logarithm of the effective viscosity of such blends both at the constant shear rate and constant shear stress is the linear function of their composition. The addition of poly(diethylsiloxane) to poly(dimethylsiloxane) strongly affects the degree of swelling of an extrudate at the exit of a capillary, and this parameter depends on the phase state of poly(diethylsiloxane) and its content in the blend. Upon incorporation of a small amount of a poly(dimethysiloxanel)-poly(diethylsiloxane) block copolymer (compatibilizer) into the blend, the viscosity of the blend approaches that of the predominant component. This phenomenon is apparently related to the fact that the block copolymer facilitates development of a more uniform morphology of the blend, in particular, the continuous dispersion phase. This factor, along with the specifics of the deformational behavior of poly(diethylsiloxane), also manifests itself during drawing and subsequent shrinkage of crosslinked resins prepared from the blends under study.     

An analysis of the kinetics of oxidation of ascorbate at poly(aniline)-poly(styrene sulfonate) modified microelectrodes

A detailed kinetic study is provided for the oxidation of ascorbate at poly(aniline)-poly(styrene sulfonate) coated microelectrodes. Flat films with a low degree of polymer spillover and a thickness much lower than the microelectrode radius were produced by controlled potentiodynamic electrodeposition. The currents for ascorbate oxidation are found to be independent of the polymer thickness, indicating that the reaction occurs at the outer surface of the polymer film. At low ascorbate concentrations, below around 40 mM, the currents are found to be mass transport limited. At higher ascorbate concentrations the currents became kinetically limited. The experimental data for measurements at a range of potentials are fitted to a consistent kinetic model and the results summarized in a case diagram. The results obtained for the poly(aniline)-(polystyrene sulfonate) coated microelectrode are compared to those for a poly(aniline)-poly(vinyl sulfonate) coated microelectrode and to the results of an earlier study of the reaction at poly(aniline)-poly(vinyl sulfonate) coated rotating disc electrodes. For poly(aniline)-poly(styrene sulfonate) the oxidation of ascorbate is found to proceed by one electron reaction whereas for poly(aniline)-poly(vinyl sulfonate) the reaction is found to be a two electron oxidation.     

Microviscosity in multiple regions of complex aqueous solutions of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)

Aqueous poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO109-PPO41-PEO109) copolymers are nonionic surfactants that self-organize to form aggregate structures with increasing temperature or concentration. We have studied two concentrations over a range of temperatures so that the copolymers are in one of three microphases: unimers, micelles, or hydrogels formed from body centered cubic aggregates of micelles. Three different coumarin dyes were chosen based on their hydrophobicity so that different aggregate regions could be probed independently-water insoluble coumarin 153 (C153), hydrophobic coumarin 102 (C102), and the hydrophilic sodium carboxylate form of coumarin 343 (C343-). Fluorescence anisotropy experiments provide detailed information on the local microviscosity. C153 experiences a fourfold increase in reorientation time and hence microviscosity with increasing temperature through the microphase transition from unimers to micelles. C102 also shows an increase in microviscosity with temperature but smaller in magnitude and with the microphase transition shifted to higher temperature relative to C153. C343- shows only a slight sensitivity to the microphase transition. For any of the three coumarin probes, fluorescence anisotropies do not show any correlation with the microphase transition to form cubic hydrogels.     

Conformations of poly{G}-poly{C} pi stacks with high hole mobility

Charge transfer properties of DNA depend strongly on the pi stack conformation. In the present paper, we identify conformations of homogeneous poly-{G}-poly-{C} stacks that should exhibit high charge mobility. Two different computational approaches were applied. First, we calculated the electronic coupling squared, V(2), between adjacent base pairs for all 1 ps snapshots extracted from 15 ns molecular dynamics trajectory of the duplex G(15). The average value of the coupling squared is found to be 0.0065 eV(2). Then we analyze the base-pair and step parameters of the configurations in which V(2) is at least an order of magnitude larger than . To obtain more consistent data, approximately 65,000 configurations of the (G:C)(2) stack were built using systematic screening of the step parameters shift, slide, and twist. We show that undertwisted structures (twist<20 degrees) are of special interest, because the pi stack conformations with strong electronic couplings are found for a wide range of slide and shift. Although effective hole transfer can also occur in configurations with twist=30 degrees and 35 degrees, large mutual displacements of neighboring base pairs are required for that. Overtwisted conformation (twist> or =38 degrees) seems to be of limited interest in the context of effective hole transfer. The results may be helpful in the search for DNA based elements for nanoelectronics.     

Temperature-dependent aggregation and disaggregation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer in aqueous solution

Aggregation and disaggregation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers, Pluronics P103 and P104, in aqueous solutions during a heating and cooling cycle were investigated by dynamic laser scattering (DLS) and 1H NMR spectroscopy. Temperature hysteresis was observed by DLS when cooling the copolymer aqueous solutions because larger aggregates existed at temperatures lower than critical micellization temperature (CMT), but no temperature differences were observed by NMR. This phenomenon was explained as the forming of water-swollen micelles at temperatures lower than CMT during the cooling process.     

Synthesis and properties of poly(butylene terephthalate)-poly(ethylene oxide)-poly(dimethylsiloxane) block copolymers

Poly(butylene terephthalate)-poly(ethylene oxide)-poly(dimethyl siloxane)-poly(ethylene oxide) block copolymers, (PBT-PEO-PDMS-PEO)_m, are synthesized by polycondensation (PC) of dimethylterephtalate (DMT), 1,4-butanediol (BDO) and PEO-PDMS-PEO. The soft block has been incorporated from 10 to 70 wt-%; the total molecular weight (MW) of the block-copolymers amounts to 16000 - 20000 g/mol. One major problem of polyether-PBT thermoplastic elastomers is their poor thermo-oxidative stability. Due to the excellent heat stability of PDMS, the resistance of this new thermoplastic elastomer against thermo-oxidative degradation has been increased 80 %! From differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) in the PEO-PDMS-PEO based COPEs, three phases can be distinguished. Besides the crystalline PBT phase, an amorphous mixed phase of PBT and PEO and an almost pure PDMS phase have been found. Due to the high concentration of the mixed PBT-PEO phase, the low temperature modulus and the glass transition temperature, T_g, are not dominated by the pure PDMS phase (T_g = −114°C). Depending on the amount of PBT and PEO present, the main glass transition lies in the range of −50°C to 50°C.     

Self-assembly of poly(ferrocenyldimethylsilane)-poly(benzyl ether) linear-dendritic organometallic polymer

A kind of linear-dendritic organometallic polymer poly(ferrocenyldimethylsilane)-poly(benzyl ether) (PFS-PBE) was synthesized and the self-assembly behaviors of the polymer was studied, which has illuminated the formation of multiple morphologies and, in particular, adjusting the concentration of polymers and adding poly(benzyl ether) into self-assembled solution can form different size and morphology. Furthermore, it was found that the aging time of samples could affect self-assembled morphology. The micelle formation mechanisms were also demonstrated.     

Immobilisation of enzymes on poly(aniline)-poly(anion) composite films. Preparation of bioanodes for biofuel cell applications.

Immobilisation of enzymes is important for applications such as biosensors or biofuel cells. A poly(histidine) tag had been introduced on the C terminus of a lactate dehydrogenase enzyme. This mutant enzyme was then immobilised onto poly(aniline) (PANi)-poly(anion) composite films, PANi-poly(vinylsulfonate) (PVS) or PANi-poly(acrylate) (PAA). The NADH produced by the immobilised enzyme in the presence of beta-nicotinamide adenine dinucleotide (NAD(+)) and lactate is oxidised at the poly(aniline)-coated electrode at 0.05 to 0.1 V vs. saturated calomel electrode (SCE) at 35 degrees C.     

Self-diffusion study of poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) block copolymers in aqueous solution

The self-diffusion of poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) block copolymers dissolved in deuterated water was investigated by means of pulsed field gradient NMR (PFG-NMR). The polymer forms micelles in the solution and, with increasing temperature, clouding and phase demixing occurs. The self-diffusion coefficient indicates the association of the polymer molecules in the vicinity of the cloud point because of its maximum with increasing temperature. Above the cloud point, two kinds of diffusing species are observed due to phase separation. The faster diffusing species is attributed to the polymer-poor phase. The self-diffusion coefficient of the polymer-rich phase species decreases with increasing temperature above the cloud point due to further association and dehydration. The correlation length of the diffusing associates, calculated from the self-diffusion coefficient and the viscosity by means of the Stokes-Einstein equation is nearly independent of temperature and concentration up to 30 wt-% polymer concentration. The correlation length is about 1.4 nm. It shows a slight maximum at the cloud point.     

Effect of acid on the aggregation of poly(ethylene xide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers

The acid effect on the aggregation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers EO(20)PO(70)EO(20) has been investigated by transmission electron microscopy (TEM), particle size analyzer (PSA), Fourier transformed infrared, and fluorescence spectroscopy. The critical micellization temperature for Pluronic P123 in different HCl aqueous solutions increases with the increase of acid concentration. Additionally, the hydrolysis degradation of PEO blocks is observed in strong acid concentrations at higher temperatures. When the acid concentration is low, TEM and PSA show the increase of the micelle mean diameter and the decrease of the micelle polydispersity at room temperature, which demonstrate the extension of EO corona and tendency of uniform micelle size because of the charge repulsion. When under strong acid conditions, the aggregation of micelles through the protonated water bridges was observed.     

Immobilisation of lactate dehydrogenase on poly(aniline)-poly(acrylate) and poly(aniline)-poly(vinyl sulphonate) films for use in a lactate biosensor

The immobilisation of enzymes on an electrode surface, in such a manner that they retain both substrate specificity and high levels of catalytic activity, is of great importance in bioelectrochemistry. This includes areas such as the development of enzyme-catalysed fuel cell electrodes, biosensors and other biotechnological applications. We have investigated the catalytic activity of hexahistidine tagged variants of lactate dehydrogenase (EC 1.1.1.27) from the thermophile Bacillus stearothermophilus both in solution and when immobilised on poly(aniline)-poly(acrylate) (PANi-PAA) or poly(aniline)-poly(vinyl sulphonate) (PANi-PVS) composite films. Both the C- and N-terminally tagged enzymes are readily immobilised on the modified electrode and catalyse the conversion of lactate and NAD⁺ to pyruvate and NADH. The NADH that is generated can be readily oxidised at the PANi-modified electrode surface.In solution, the activity of the C-tagged enzyme (LDH-CHis) was some 30% less that of the wild-type under comparable conditions, whereas the N-tagged enzyme was found to possess essentially the same activity as the wild-type. However, when the enzymes were immobilised on PANi-PAA and PANi-PVS the C-tagged enzyme films showed a higher NADH-dependent current than the wild-type LDH whilst the N-tagged enzyme had the highest of the three. In addition, the C-tagged enzyme film appeared more stable than the wild-type LDH-PANi film. A novel immobilisation chemistry of the enzyme is proposed to account for these observations.     

Small-angle neutron scattering and theoretical investigation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) stabilized oil-in-water microemulsions

The aim of this study is to determine the effects of oil solutes and alcohol cosolvents on the structure of oil-in-water microemulsions stabilized by poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers. The systems investigated involved the solubilization of 1,3,5-trimethylbenzene or 1,2-dichlorobenzene by P123 (EO(20)-PO(70)-EO(20)) pluronic surfactant micelles in water and water + ethanol solvents. The structures of these swollen micelles were determined by small-angle neutron scattering (SANS). A thermodynamic model was employed to interpret the characterization data. The results of the thermodynamic model for micellization agreed well with the SANS data from samples of micelles swollen by both oils. The model predicted the size of the micelles within 5% accuracy using only one fitting parameter, the micelle polydispersity. Ethanol had significantly different effects on the polymer micelles that contained solubilized oil compared to pure polymer micelles. For pure polymer micelles, the addition of ethanol increased the solubility of the polymer and, therefore, decreased the total volume fraction of micelles, while for polymer-oil aggregates, ethanol tended to have a positive effect on the volume fraction of micelles. SANS results showed that the greatest divergence from pure aqueous solvent results occurred at oil concentrations above the microemulsion stability limit.     

Phase behavior of a mixture of poly(isoprene)-poly(oxyethylene) diblock copolymer and poly(oxyethylene) surfactant in water

The phase behavior of a mixture of poly(isoprene)-poly(oxyethylene) diblock copolymer (PI-PEO or C250EO70) and poly(oxyethylene) surfactant (C12EO3, C12EO5, C12EO6, C12EO7, and C12EO9) in water was investigated by phase study, small-angle X-ray scattering, and dynamic light scattering (DLS). The copolymer is not soluble in surfactant micellar cubic (I1), hexagonal (H1), and lamellar (Lalpha) liquid crystals, whereas an isotropic copolymer fluid phase coexists with these liquid crystals. Although the PI-PEO is relatively lipophilic, it increases the cloud temperatures of C12EO3-9 aqueous solutions at a relatively high PI-PEO content in the mixture. Most probably, in the copolymer-rich region, PI-PEO and C12EOn form a spherical composite micelle in which surfactant molecules are located at the interface and the PI chains form an oil pool inside. In the C12EO5/ and C12EO6/PI-PEO systems, one kind of micelles is produced in the wide range of mixing fraction, although macroscopic phase separation was observed within a few days after the sample preparation. On the other hand, small surfactant micelles coexist with copolymer giant micelles in C12EO7/ and C12EO9/PI-PEO aqueous solutions in the surfactant-rich region. The micellar shape and size are calculated using simple geometrical relations and compared with DLS data. Consequently, a large PI-PEO molecule is not soluble in surfactant bilayers (Lalpha phase), infinitely long rod micelles (H1 phase), and spherical micelles (I1 phase or hydrophilic spherical micelles) as a result of the packing constraint of the large PI chain. However, the copolymer is soluble in surfactant rod micelles (C12EO5 and C12EO6) because a rod-sphere transition of the surfactant micelles takes place and the long PI chains are incorporated inside the large spherical micelles.     

Structural properties of thermoresponsive poly(N-isopropylacrylamide)-poly(ethyleneglycol) microgels

We present investigations of the structural properties of thermoresponsive poly(N-isopropylacrylamide) (PNiPAM) microgels dispersed in an aqueous solvent. In this particular work poly(ethyleneglycol) (PEG) units flanked with acrylate groups are employed as cross-linkers, providing an architecture designed to resist protein fouling. Dynamic light scattering (DLS), static light scattering (SLS), and small angle neutron scattering (SANS) are employed to study the microgels as a function of temperature over the range 10 °C ≤ T ≤ 40 °C. DLS and SLS measurements are simultaneously performed and, respectively, allow determination of the particle hydrodynamic radius, R(h), and radius of gyration, R(g), at each temperature. The thermal variation of these magnitudes reveals the microgel deswelling at the PNiPAM lower critical solution temperature (LCST). However, the hydrodynamic radius displays a second transition to larger radii at temperatures T ≤ 20 °C. This feature is atypical in standard PNiPAM microgels and suggests a structural reconfiguration within the polymer network at those temperatures. To better understand this behavior we perform neutron scattering measurements at different temperatures. In striking contrast to the scattering profile of soft sphere microgels, the SANS profiles for T ≤ LCST of our PNiPAM-PEG suspensions indicate that the particles exhibit structural properties characteristic of star polymer configurations. The star polymer radius of gyration and correlation length gradually decrease with increasing temperature despite maintenance of the star polymer configuration. At temperatures above the LCST, the scattered SANS intensity is typical of soft sphere systems.     

Multidimensional chromatographic techniques for hydrophilic copolymers II. Analysis of poly(ethylene glycol)-poly(vinyl acetate) graft copolymers

A large variety of hydrophilic copolymers is applied in different fields of chemical industry including bio, pharma and pharmaceutical applications. For example, poly(ethylene glycol)-poly(vinyl alcohol) graft copolymers that are used as tablet coatings are responsible for the controlled release of the active compounds. These copolymers are produced by grafting of vinyl acetate onto polyethylene glycol (PEG) and subsequent hydrolysis of the poly(ethylene glycol)-poly(vinyl acetate) graft copolymers. The poly(ethylene glycol)-poly(vinyl acetate) copolymers are distributed with regard to molar mass and chemical composition. In addition, they frequently contain the homopolymers polyethylene glycol and polyvinyl acetate. The comprehensive analysis of such complex systems requires hyphenated analytical techniques, including two-dimensional liquid chromatography and combined LC and nuclear magnetic resonance spectroscopy. The development and application of these techniques are discussed in the present paper.     

Poly(ethylene oxide)-poly(styrene oxide)-poly(ethylene oxide) copolymers: Micellization, drug solubilization, and gelling features

Two new poly(ethylene oxide)-poly(styrene oxide) triblock copolymers (PEO-PSO-PEO) with optimized block lengths selected on the basis of previous studies were synthesized with the aim of achieving a maximal solubilization ability and a suitable sustained release, while keeping very low material expense and excellent aqueous copolymer solubility. The self-assembling and gelling properties of these copolymers were characterized by means of light scattering, fluorescence spectroscopy, transmission electron microscopy, and rheometry. Both copolymers formed spherical micelles (12-14 nm) at very low concentrations. At larger concentration (>25 wt%), copolymer solutions showed a rich phase behavior, with the appearance of two types of rheologically active (more viscous) fluids and of physical gels depending on solution temperature and concentration. The copolymer behaved notably different despite their relatively similar block lengths. The ability of the polymeric micellar solutions to solubilize the antifungal drug griseofulvin was evaluated and compared to that reported for other structurally-related block copolymers. Drug solubilization values up to 55 mg g⁻¹ were achieved, which are greater than those obtained by previously analyzed poly(ethylene oxide)-poly(styrene oxide), poly(ethylene oxide)-poly(butylene oxide), and poly(ethylene oxide)-poly(propylene oxide) block copolymers. The results indicate that the selected SO/EO ratio and copolymer block lengths were optimal for simultaneously achieving low critical micelle concentrations (cmc) values and large drug encapsulation ability. The amount of drug released from the polymeric micelles was larger at pH 7.4 than at acidic conditions, although still sustained over 1 day.     

Self-aggregation and phase behavior of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers in aqueous solution

The phase behavior and aggregation properties of block copolymers of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (Pluronics, poloxamers) in aqueous solution have recently attracted much attention. Both experimental and theoretical studies are reviewed, not comprehensively, but with the focus on studies, partly cooperative, partly independent, performed by groups in Uppsala (light scattering and fluorescence), Roskilde (rheology and calorimetry), Risø (SANS), Graz (x-ray and speed of sound), and Lund (theoretical model calculations).The phase behavior of these copolymers is similar in many respects to that of conventional nonionic surfactants, with the appearance of hexagonal, cubic, and lamellar liquid crystalline phases at high concentrations. In the isotropic solution phase the critical concentration for micelle formation is strongly temperature dependent, and at a given concentration the monomer to micelle transition occurs gradually over a broad temperature range, partly due to the broad size polydispersity of both the PO- and EO-blocks. For some Pluronic copolymers a transition from globular to long rod-like micelles occurs above a transition temperature, resulting in a strong and sudden increase of viscosity and viscoelasticity of the solution.Size and aggregation numbers have been determined for the globular micelles in some cases, and also the rod-like micelles have been characterized. NMR and fluorescence measurements have provided further information on the properties of the micellar core and mantle. In combination, results from different measurements on the same Pluronics material indicate that the aggregation number of the micelles increases with the temperature, whereas the hydrodynmic radius varies much less. The PEO-mantle of the micelles seems to contract with increasing temperature. The core appears to contain appreciable amounts of PEO in addition to PPO (and also some water). The segregation between core and mantle is not as distinct as in normal micelles, a conclusion which is in line with the predictions from the model calculations.     

Degradation behaviors of star-shaped poly(ethylene glycol)-poly(?-caprolactone) nanoparticles in aqueous solution

Star-shaped poly(ethylene glycol) (PEG)-poly(?-caprolactone) (?-PCL) block copolymers were synthesized via a ring-opening polymerization. Nanoparticles prepared by the precipitation/solvent evaporation technique exhibit a core-shell structure. The hydrolytic degradation of 3-arm PEG-PCL copolymeric nanoparticles was studied by dynamic light scattering (DLS), nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC). It was found that copolymers with shorter PCL block length degraded faster. The sizes of nanoparticles fluctuated during the initial degradation period, and then increased slightly before finally dropping off. The degradation mainly occurred at CL-CL linkages firstly then at the EO-CL linkages. The CL/EO molar ratios and the molecular weights of copolymers decreased as degradation time and a zero-order degradation behavior was observed.