New phosphorylating reagents for deoxyribonucleosides and oligonucleotides

New phosphorylating reagents 1 and 2 were prepared in three steps from 4-hydroxybenzaldehyde. They showed good efficiency in the solid phase synthesis of 5′-phosphate monoester nucleosides. End-phosphate DNA sequence synthesis demonstrated the efficiency of the new reagents (1 and 2) according to the general procedure of automated DNA synthesis. The oxidation of P(III) to P(V) and the removal of benzyl protecting groups were achieved in a single step by treatment with a 0.02 M I₂/pyridine/H₂O solution. Due to this one-pot treatment, it is possible to use the phosphorylating reagents (1 and 2) for the synthesis of base-sensitive ODNs. The reagents 1 and 2 are unique among phosphorylating reagents.     

Epoxy lactones by photooxidative rearrangement of 6β-acetoxyvouacapane

The first study of photooxidation reaction of 6β-acetoxyvouacapane isolated from Caesalpinia platyloba is reported. The reaction yielded four new epoxy lactones, 6β-acetoxy-15,16α-epoxy-13-spirocassa-12,16-olide, 6β-acetoxy-15,16β-epoxy-13-spirocassa-12,16-olide, 6β-acetoxy-12,13β-epoxycassa-16,12-olide and 6β-acetoxy-12,13α-epoxycassa-16,12-olide. All the structures were supported by 1D and 2D NMR spectroscopy as well as mass spectrometry. The stereochemistry was established on the base of single crystal X-ray diffraction.     

Syntheses of N-aryl-protected glucosamines and their stereoselectivity in chemical glycosylations

N-Aryl-protecting groups were introduced in glucosamines to achieve β-selective glycosylation. Various N-aryl aminosugars were synthesized via Buchwald–Hartwig reaction. Glycosylation using glycosyl trichloroacetimidates of N-aryl aminosugars smoothly proceeded in the presence of trimethylsilyl trifluoromethanesulfonate. Use of a glycosyl donor comprising an electron-donating 2,4-dimethoxyphenyl (DMP) group led to the glycosylation proceeding with high β selectivity. This stereoselectivity seemed to be derived from the formation of an aziridine intermediate. The DMP-protecting group can be removed immediately by using ammonium hexanitratocerate (IV).     

X‐ray induced degradation of surface bound azido groups during XPS analysis

Mesoporous silica SBA‐15 was synthesized and silanized with azidopropyl triethoxysilane in order to design a clickable material. Fourier transform infrared analysis permitted to prove the attachment of the azidopropylene groups to SBA‐15 resulting in the reactive and functional material N₃‐SBA‐15. X‐ray photoelectron spectroscopy was used to determine the surface composition of SBA‐15. However, we unexpectedly found that the surface bound azido groups undergo X‐ray induced decomposition during the X‐ray photoelectron spectroscopy analysis resulting in the formation of nitrenes. These are very reactive groups able to intercalate C―C and C―H bonds of the propylene chains as judged from the N1s peak shape. Possible mechanisms of intercalation are suggested. C1s and N1s peaks were recorded at different exposure time. N/C, N⁺/N and N⁺/C undergo exponential decay. N⁺/N reaches the value of zero in less than 80 min of exposure to the X‐ray source. The N⁺/C decay plot was fitted with first‐order kinetics, and the decomposition kinetic constant (k_dec) was found to equal to 516.4 s^?1. This is a fast X‐ray induced degradation which must be considered with care when examining clickable materials with surface bound alkyl azido groups. Copyright © 2016 John Wiley & Sons, Ltd.     

Shape memory effect and recovery stress property of carbon nanotube/waterborne epoxy nanocomposites investigated via TMA

Shape memory polymers (SMPs) have received great attention and scientific interest in widespread technological development during last few decades. Besides the development of novel SMPs, various techniques have been practiced for characterization of shape memory effect (SME) of SMPs. In this study, the shape memory effect and recovery stress property of the carbon nanotube (CNT)/waterborne epoxy (WEP) nanocomposites below and above the glass transition temperature (T_g) of the nanocomposites and under isostrain and isostress were systematically investigated via thermal mechanical analysis (TMA), respectively. The experimental results showed that the nanocomposites exhibit excellent shape memory effect. The shape memory fixity and recovery ratios were approximately 100% even below glass transition temperature (T_g). A remarkable point is that the strain of the nanocomposites suddenly increased with the temperature decreasing in a certain period of the heating-cooling cycles under isostress condition and the strain increment increased with temperature in general. Especially at low temperature, the recovery stress was very sensitive to temperature under isostrain condition of ±0.25 °C temperature with differential of 25.5 °C developed pressure difference of 0.20 MPa. Moreover, TMA is a practical method for quantifying the SME and recovery stress properties of SMPs and their composites.     

Environmentally benign green composites based on epoxy resin/bacterial cellulose reinforced glass fiber: Fabrication and mechanical characteristics

Bio-based bacterial cellulose (BC) epoxy composites were manufactured and their mechanical properties were examined. The BC was initially fabricated from Vietnamese nata de coco by means of alkaline pretreatment followed by solvent exchange. The obtained fibers were dispersed in epoxy resin (EP) by both mechanical stirring and ultrasonic techniques. The resulting blend was used as the matrix for glass-fiber (GF) composite fabrication using a prepreg method followed by multiple hot-press-curing steps. The morphology, mechanical characteristics and mode-I interlaminar fracture toughness of the fabricated composites were investigated. With a 0.3-wt% BC content, the mode-I interlaminar fracture toughness for both crack initiation and crack propagation were improved by 128.8% and 1110%, respectively. The fatigue life was dramatically extended by a factor of 12, relative to the unmodified composite. Scanning electron microscopy images revealed that the BC plays a vital role in increasing the interlaminar fracture toughness of a GF/EP composite via the mechanisms of crack reflection, debonding and fiber-bridging.     

Adsorbing behavior of polycarboxylate superplasticizer in the presence of the ester group in side chain

The effect of the ester group in side chain on adsorbing behavior and dispersion of polycarboxylate superplasticizer (PC) was studied by comparing the performance of two types of PC. The fluidity of cement paste was tested to discuss the dispersing ability and dispersing retention ability of PC. The total organic carbon analyzer was used to measure the adsorption amount, and the adsorption layer was obtained by x-ray photoelectron spectroscopy. Fourier-transform infrared spectroscopy, nuclear magnetic resonance, and pH value were used to verify the stability of the ester group, and the electrokinetic properties of cement particle were confirmed using the zeta potential measurement. The results show that the ester group in side chain reduces the initial dispersing ability while it increases the dispersing retention ability. The dispersing retention ability depends on the increase of adsorption amount and adsorption layer in 5–60 minutes, and the greater increase leads to the better dispersing retention ability. The ester group can be decomposed to release carboxyl group to enhance the adsorbing ability of PC under the condition of cement hydration, which is the main reason for the greater increase of adsorption amount and adsorption layer and the improvement in dispersing retention ability. It suggests that grafting the ester group is a good way to enhance the dispersing retention ability.     

Fiber-reinforced three-dimensional graphene aerogels for electrically conductive epoxy composites with enhanced mechanical properties

To enhance the mechanical properties of three-dimensional graphene aerogels with aramid fibers,graphene/organic fiber aerogels are prepared by chemical reduction of graphene oxide in the presence of organic fibers of poly(p-phenylene terephthalamide)(PPTA) and followed by freeze-drying. Thermal annealing of the composite aerogels at 1300 ° C is adopted not only to restore the conductivity of the reduced graphene oxide component but also to convert the insulating PPTA organic fibers to conductive carbon fibers by the carbonization. The resultant graphene/carbon fiber aerogels(GCFAs) exhibit high electrical conductivities and enhanced compressive properties, which are highly efficient in improving both mechanical and electrical performances of epoxy composites. Compared to those of neat epoxy, the compressive modulus, compressive strength and energy absorption of the electrically conductive GCFA/epoxy composite are significantly increased by 60%, 59% and 131%, respectively.     

Unusual electro-optical behaviour of the nematic polyacrylate

ABSTRACT

High sensitivity of liquid crystals to the electric field makes them highly demanded and widely used in different applications. Despite the large number of the electro-optical research on the low-molar-mass liquid crystals electro-optics of the liquid crystalline (LC) polymers is much less studied. Herein, the comparative electro-optical behaviour of two nematic comb-shaped polyacrylates with phenylbenzoate mesogenic side groups was studied in detail. These two polyacrylates have completely the same structure of polymer backbone and spacer length but different in the direction of the ester group in the phenylbenzoate fragments. It was found that this difference predetermines their completely opposite electro-optical properties.

The influence of the electric field of different strength and frequency on the orientation of the mesogenic groups of these polymers is studied. It is shown that application of the electric field at temperatures above the glass transition temperature (~25°C) induces reorientation of the mesogenic groups along or perpendicular to the electric field direction depending in its turn on the ester group direction. For one of the polyacrylates an unusual textural transition is found; during cooling of the polymer sample under applied field at definite temperature a sharp change in the mesogen’s orientation from homeotropic to planar one is found. This electro-optical phenomenon is observed for the first time and probably associated with sharp change in sign of anisotropy of dielectric permittivity from positive (at high temperatures) to negative one (at lower temperatures). Kinetics of the electro-optical switching at different temperatures, influence of the molar masses of the polymer and frequency of the applied AC field on electro-optical behaviour of the polymers are studied. The possibility of the fixation of the electroinduced homeotropic alignment of the mesogenic groups by photopolymerisation of the diacrylate dissolved in the polymer is demonstrated.     

Synthesis and properties of allyloxy-based tolane liquid crystals with high negative dielectric anisotropy

A series of tolane liquid crystals (LCs) containing 2,3-difluorophenyl and allyloxy terminal groups have been synthesised via multi-step reactions. Their thermotropic mesophases, birefringences and dielectric anisotropy properties are discussed by comparison with the non-fluorinated analog. The results show that the allyloxy-based tolane LC 3TOV reveals a high value of the birefringence (0.29), negative dielectric anisotropy (?4.44) and a broad nematic mesophase with a low melting point and high clearing point. The effects of the terminal alkyl chains, the lateral fluoro substituents, the allyloxy terminal group and alkyne bridge on the mesomorphic and physical properties were also discussed. Meanwhile, Density Functional Theory (DFT) calculations of molecular conformation and polarisability were used to correlate the experimental findings.