Giant magnetoresistance of coiling polymers

We examine the magnetoresistance (MR) of conducting polymers with interest paid on the role of structural flexibility. Through Monte Carlo simulation and Green function method, we evaluate the electric transmission for a variety of polymer configurations. It is found that for a single polymer the transmission displays a complex oscillation and also a parity-dependent periodicity. For an ensemble of polymers the averaged transmission yields the nonlinear behavior of MR under varying magnetic fields. Interestingly, more flexible polymers are shown to achieve higher MR, depending on the population and the size of the loops.     

Solid-Phase Synthesis of a Monofunctional trans-a(2)Pt(II) Complex Tethered to a Single-Stranded Oligonucleotide

Cross-linking ability is possible with the oligonucleotide-tethered, monofunctional trans-Pt(II) complex shown. It was synthesized by a novel solid-phase approach comprising conjugation of immobilized tetrathymidylic acid with a trans-a(2)Pt(II) building unit, ammonolysis, and transformation of the resulting complex (R=1-N-cyclohexylmethylthyminate) into the chloro derivative (R=Cl). a=NH(2)CH(3), T=thymine.     

Curved Hammett plot in alkaline hydrolysis of O-aryl thionobenzoates: change in rate-determining step versus ground-state stabilization

Second-order rate constants have been measured for alkaline hydrolysis of O-aryl thionobenzoates (X-C(6)H(4)-CS-OC(6)H(4)-Y) in 80 mol % H(2)O-20 mol % DMSO at 25.0 +/- 0.1 degrees C. The Hammett plot for the reaction of O-4-nitrophenyl X-substituted thionobenzoates (X-C(6)H(4)-CS-OC(6)H(4)-NO(2), 1a-e) exhibits a downward curvature. However, a possible traditional explanation in terms of a change in the rate-determining step (RDS) has been considered but rejected. The proposed explanation involves stabilization of the ground-state (GS) through-resonance interaction between the electron-donating substituent X and the thionocarbonyl functionality on the basis of the linear Yukawa-Tsuno plot obtained for the same reaction. The Br?nsted-type plot for the reaction of O-aryl thionobenzoates (C(6)H(5)-CS-OC(6)H(4)-Y, 2a-i) is linear but exhibits many scattered points with a small beta(lg) (-0.35). The Hammett plot for the same reaction shows rather poor correlation with sigma(-) constants but much better correlation with sigma(o) constants. The alkaline hydrolysis of O-aryl thionobenzoates (1a-e and 2a-i) has been proposed to proceed through an addition intermediate in which bond formation is the RDS.     

Significant and differential acceleration of dephosphorylation of the insecticides, paraoxon and parathion, caused by alkali metal ethoxides

In the reaction of paraoxon with alkali metal ethoxides, ion-paired EtO-M+ species are more reactive than the dissociated EtO- with the reactivity order EtO-Li+ EtO-Na+ > EtO-K+ > EtO-, while in the reaction of parathion, the reactivity follows the order EtO-K+ > EtO- > EtO-Na+ > EtO-Li+.     

Optimizing Dilute-Acid Pretreatment of Rapeseed Straw for Extraction of Hemicellulose

Biological conversion of biomass into fuels and chemicals requires hydrolysis of the polysaccharide fraction into monomeric sugars prior to fermentation. Hydrolysis can be performed enzymatically or with mineral acids. In this study, dilute sulfuric acid was used as a catalyst for the pretreatment of rapeseed straw. The purpose of this study is to optimize the pretreatment process in a 15-mL bomb tube reactor and investigate the effects of the acid concentration, temperature, and reaction time. These parameters influence hemicellulose removal and production of sugars (xylose, glucose, and arabinose) in the hydrolyzate as well as the formation of by-products (furfural, 5-hydroxymethylfurfural, and acetic acid). Statistical analysis was based on a model composition corresponding to a 3³ orthogonal factorial design and employed the response surface methodology to optimize the pretreatment conditions, aiming to attain maximum xylan, mannan, and galactan (XMG) extraction from hemicellulose of rapeseed straw. The obtained optimum conditions were: H₂SO₄ concentration of 1.76% and temperature of 152.6 °C with a reaction time of 21 min. Under these optimal conditions, 85.5% of the total sugar was recovered after acid hydrolysis (78.9% XMG and 6.6% glucan). The hydrolyzate contained 1.60 g/L glucose, 0.61 g/L arabinose, 10.49 g/L xylose, mannose, and galactose, 0.39 g/L cellobiose, 0.94 g/L fructose, 0.02 g/L 1,6-anhydro-glucose, 1.17 g/L formic acid, 2.94 g/L acetic acid, 0.04 g/L levulinic acid, 0.04 g/L 5-hydroxymethylfurfural, and 0.98 g/L furfural.