Structural and electronic properties of adsorbed nucleobases on Si-doped hexagonal boron nitride nanoflake: a computational study

Structural Chemistry - Effect of doping on surface reactivity and molecular adsorption mechanism is a key feature for many applications, including molecular sensing, molecular recognition, and...     

Double frictional receding contact problem for an orthotropic layer loaded by normal and tangential forces

With the increasing research in the field of contact mechanics, different types of contact models have been investigated by many researchers by employing various complex material models. To ascertain the orthotropy effect and modeling parameters on a receding contact model, the double frictional receding contact problem for an orthotropic bilayer loaded by a cylindrical punch is taken into account in this study. Assuming plane strain sliding conditions, the governing equations are found analytically using Fourier integral transformation technique. Then, the resulting singular integral equations are solved numerically using an iterative method. The weight function describing the asymptotic behavior of the stresses are investigated in detail and powers of the stress singularities are provided. To control the trustworthiness and correctness of the analytical formulation and to compare the resulting stress distributions and contact boundaries, a numerically efficient finite element method was employed using augmented Lagrange contact algorithm. The aim of this paper is to investigate the orthotropy effect, modeling parameters and coefficients of friction on the surface and interface stresses, surface and interface contact boundaries, powers of stress singularities, weight function and to provide highly parametric benchmark results for tribological community in designing wear resistant systems.

     

Solid-Phase Microextraction of Aliphatic Alcohols Based on Polyaniline Coated Fibers

The efficiency of polyaniline (PANI), coated gold wire was investigated for use as a fiber for solid-phase microextraction (SPME). Aniline monomers were electropolymerized on gold wires by applying a constant current to an acetate buffer containing NaClO₄ as supporting electrolyte for 30 min. These fibers were used for the extraction of some aliphatic alcohols from gaseous samples. The results obtained proved the ability of PANI fiber for sampling organic compounds from gaseous samples. From this work, optimum conditions for preparation and conditioning of fibers and for the extraction of analytes from gaseous samples were obtained. Under optimum conditions, one fiber was used for several equivalent analyses and the relative standard deviations (RSD) were <7% (n=6). However, fiber to fiber reproducibility was <9% (n=6). This fiber is firm and durable and is simply prepared. Calibration graphs were linear in the range: 0.1–10 g mL^–1 for aliphatic alcohols; the detection limit range was 15–75 ng mL^–1 (S/N=3) using a flame ionization detector.     

Reactivity of substituted charged phenyl radicals toward components of nucleic acids

Reactions of differently substituted phenyl radicals with components of nucleic acids have been investigated in the gas phase. A positively charged group located meta with respect to the radical site was employed to allow manipulation of the radicals in a Fourier-transform ion cyclotron resonance mass spectrometer. All of these electrophilic radicals react with sugars via exclusive hydrogen atom abstraction, with adenine and uracil almost exclusively via addition (likely at the C8 and C5 carbons, respectively), and with the nucleoside thymidine by hydrogen atom abstraction and addition at C5 in the base moiety (followed by elimination of (*)CH(3)). These findings parallel the reactivity of the phenyl radical with components of nucleic acids in solution, except that the selectivity for addition is different. Like HO(*), the electrophilic charged phenyl radicals appear to favor addition to the C5-end of the C5-C6 double bond of thymine and thymidine, whereas the phenyl radical preferentially adds to C6. The charged phenyl radicals do not predominantly add to thymine, as the neutral phenyl radical and HO(*), but mainly react by hydrogen atom abstraction from the methyl group (some addition to C5 in the base followed by loss of (*)CH(3) also occurs). Adenine appears to be the preferred target among the nucleobases, while uracil is the least favored. A systematic increase in the electrophilicity of the radicals by modification of the radicals' structures was found to facilitate all reactions, but the addition even more than hydrogen atom abstraction. Therefore, the least reactive radicals are most selective toward hydrogen atom abstraction, while the most reactive radicals also efficiently add to the base. Traditional enthalpy arguments do not rationalize the rate variations. Instead, the rates reflect the radicals' electron affinities used as a measure for their ability to polarize the transition state of each reaction.     

Control of electron demand in the cycloadditions of 2(H)-1,4-oxazin-2-ones

3-Methoxy-5-chloro-6-methyl-2(H)-1,4-oxazin-2-one 4, 3-methoxy-5-chloro-6-phenyl-2(H)-1,4-oxazin-2-one 5, 3-phenylsulfenyl-5-chloro-6-methyl-2(H)-1,4-oxazin-2-one 6, and 3-phenylsulfenyl-5-chloro-6-phenyl-2(H)-1,4-oxazin-2-one 7, are ambident dienes and undergo Diels-Alder cycloadditions with electron neutral, rich and deficient dienophiles.     

Three new diterpenes and the biological activity of different extracts of Jasonia montana

Three new diterpenes, namely jasonin-a (1), jasonin-b (2), and jasonin-c (3) were isolated from the aerial parts of Jasonia montana (Asteraceae). Their structures were elucidated on the basis of spectral data as [(1E)-2-((2S)-1,2,5-trimethylbicyclo[3.2.l]octan-8-yl)vinyl] benzene-3-carboxylic acid (1), [3-((2S, 5S)-1,2, 5-trimethylcycloheptanyl)propyl]benzene-3-carboxylic acid (2), and [(1E)-3-((7R)-1,7-dimethy-4-methylenecycloheptanyl)prop-1-enyl] benzene-3-carboxylic acid (3). In addition, the previously reported 5,7,3'-trihydroxy-3,6,4'-trimethoxy flavone designated as centaureidin (4), was also isolated and characterized from this source. The different extracts of the plant were also screened for hypoglycemic, antidiabetic, and antimicrobial activities, wherein the petroleum ether and ethanolic extracts exhibited hypoglycemic and antidiabetic activity, and the petroleum ether and chloroform extracts showed antimicrobial activity.     

Modified Polypyrrole with Tetrasulfonated Nickel Phthalocyanine as a Fiber for Solid-Phase Microextraction. Application to the Extraction of BTEX Compounds from Water Samples

Gold wire was coated with polypyrrole (PPY) by electropolymerization and used as a solid-phase microextraction (SPME) fiber. The adsorptive property of the coating was modified by doping with tetrasulfonated nickel phthalocyanine (NiPcTS). The efficiency and reliability of this fiber was investigated for the extraction of BTEX compounds from the headspace of water samples. Monitoring of extraction efficiency was performed by capillary GC-FID. Effects of several factors such as electropolymerization time, salt addition, exposure time and stirring speed on extraction efficiency were studied. The calibration graphs were linear in the range of 0.06 to 50 ng mL^?1 and the detection limits for BTEX compounds were 20–50 pg mL^?1. Comparing the results obtained using these fibers with results reported in the literature with polydimethylsiloxane (PDMS) fibers shows that under optimum conditions, the detection limits are comparable.     

Extraction of gold,palladium and silver ions using organically modified silica-coated magnetic nanoparticles and silica gel as a sorbent

Microchimica Acta - Two new adsorbents were synthesized by covalently grafting 2,3-Bis(2-formylphenoxy)-propan-1-ol (BH) onto the surface of silica-coated magnetic nanoparticles and silica gel by a...     

Micellar Nanocarriers Assembled from Doxorubicin‐Conjugated Amphiphilic Macromolecules (DOX–AM)

Amphiphilic macromolecules (AMs) have unique branched hydrophobic domains attached to linear PEG chains. AMs self‐assemble in aqueous solution to form micelles that are hydrolytically stable in physiological conditions (37 °C, pH 7.4) over 4 weeks. Evidence of AM biodegradability was demonstrated by complete AM degradation after 6 d in the presence of lipase. Doxorubicin (DOX) was chemically conjugated to AMs via a hydrazone linker to form DOX–AM conjugates that self‐assembled into micelles in aqueous solution. The conjugates were compared with DOX‐loaded AM micelles (i.e., physically loaded DOX) on DOX content, micellar sizes and in vitro cytotoxicity. Physically encapsulated DOX loading was higher (12 wt.‐%) than chemically bound DOX (6 wt.‐%), and micellar sizes of DOX‐loaded AMs (≈16 nm) were smaller than DOX–AMs (≈30 nm). In vitro DOX release from DOX–AM conjugates was faster at pH 5.0 (100%) compared to pH 7.4 (78%) after 48 h, 37 °C. Compared to free DOX and physically encapsulated DOX, chemically bound DOX had significantly higher cytotoxicity at 10^?7 M DOX dose against human hepatocellular carcinoma cells after 72 h. Overall, DOX–AM micelles showed promising characteristics as stable, biodegradable DOX nanocarriers.

     

Dynamical response of a prestrained system comprising a substrate and bond and covering layers to a moving load

The dynamical response of a system consisting of a prestressed substrate and covering and bond layers to a moving load is investigated within the scope of a piecewise-homogeneous body model by using a three-dimensional linearized theory of wave propagation in initially stressed bodies. It is assumed that the materials of the constituents are isotropic and homogeneous, and the subsonic speed of the moving load acting on the covering layer is constant. The investigations are carried out for a plane strain state under complete and in complete contact conditions. For various values of problem parameters, numerical results for the critical speed are presented and discussed.