The degradation of cellulose to lmw samples with $\overline {DP} _{{\rm w}} $ varying from 15 to 130 is investigated. Cellulose samples prepared from the hydrolysis of regenerated cellulose fibers in dilute HCl possess $\overline {DP} _{{\rm w}} $ = 50. Applying homogenous degradation of microcrystalline cellulose in H3PO4 at RT for 3 weeks, samples with $\overline {DP} _{{\rm w}} $ = 35 and a PDI of 1.58 are obtained. Decreasing the hydrolysis temperature to 8 °C results in lmw cellulose with $\overline {DP} _{{\rm w}} $ > 70. Fractionation in DMA/LiCl provides samples with $\overline {DP} _{{\rm w}} $ = 12 to 130, together with a narrow molecular weight distribution. Detailed structural analysis by 2D NMR spectroscopy reveals that the prepared lmw celluloses are suitable as mimics for cellulose.
Sense codon recoding is the basis for genetic code expansion with more than two different noncanonical amino acids. It requires an unused (or rarely used) codon, and an orthogonal tRNA synthetase:tRNA pair with the complementary anticodon. The Mycoplasma capricolum genome contains just six CGG arginine codons, without a dedicated tRNAArg. We wanted to reassign this codon to pyrrolysine by providing M. capricolum with pyrrolysyl‐tRNA synthetase, a synthetic tRNA with a CCG anticodon (${{\rm tRNA}{{{\rm Pyl}\hfill \atop {\rm CCG}\hfill}}}$ ), and the genes for pyrrolysine biosynthesis. Here we show that ${{\rm tRNA}{{{\rm Pyl}\hfill \atop {\rm CCG}\hfill}}}$ is efficiently recognized by the endogenous arginyl‐tRNA synthetase, presumably at the anticodon. Mass spectrometry revealed that in the presence of ${{\rm tRNA}{{{\rm Pyl}\hfill \atop {\rm CCG}\hfill}}}$ , CGG codons are translated as arginine. This result is not unexpected as most tRNA synthetases use the anticodon as a recognition element. The data suggest that tRNA misidentification by endogenous aminoacyl‐tRNA synthetases needs to be overcome for sense codon recoding. 相似文献
The extraction of gold in membrane extractors was theoretically investigated. Extraction of gold in the form of $ {\rm Au(CN)}_2^- $ in a solution of n‐heptane and synergistic extractants of LIX79+TOPO was studied. The membrane extractor consists of three sections: the tube side, the membrane, and the shell side. Conservation equations were derived for $ {\rm Au(CN)}_2^- $ in the membrane module and were numerically solved based on finite element method. Simulations were conducted through solving the momentum and mass transfer equations simultaneously. It was indicated that as the feed flows within the tube side, it moves into the membrane due to the concentration difference, and then gets swept by the moving extractants within the shell side. The distribution of solute concentration in the membrane contactor was obtained. Simulation results showed increasing the feed flow rate reduces the extraction efficiency, while doing the same for the organic phase flow rate does not change the extraction efficiency. 相似文献
PET/PEN blends were prepared over the full composition range via a melt mixing process under various processing conditions. This resulted in transesterification reactions and formation of copolymer structures with various average sequence block lengths and degree of randomness (RD) determined by 1H NMR. It was seen that with an increase in time and temperature of mixing copolymer content (TEN%) and RD increased, whereas the , values were decreased. The differences in the extent of transreactions arising from different processing histories showed their systematic influence on rheological characteristics. Moreover due to progress of transreactions during the rheological measurements, convergence was seen in all the rheological characteristics at terminal zones in the high frequency regions. Similar convergence in the copolymer structural parameters was also obtained by NMR analysis. An increase in TEN% led to a systematic increase in viscosity of the blends. A decrease in the , values results in an increase in elasticity and relaxation time due to improvement of blend interface with increase in extent of copolymer formation.
The stress relaxation behaviour of liquid crystal-forming ethyl celllulose (EC) solutions in m-cresol was determined by means of a cone-plate type viscometer at 30°C. The effect of molecular weight (MW) on the behaviour was also determined. The relaxation behaviour could be fitted with the following equation: where σi and σf are steady-state shear stresses at shear rate $\dot \gamma _{\rm i}$ and $\dot \gamma _{\rm f}$, σ(t) is time- dependent stress, A1 and A2 are constants, τ1 and τ2 are relaxation times, t is time, and tc is a characteristic time. When log σ* was plotted against time, one straight line was obtained for isotropic solutions, whereas anisotropic solutions yielded two straight lines. This suggests that the liquid crystalline solutions have two separate relaxation processes: Process 1 has a relatively short relaxation time, and process 2 has a long one. The parameters τ1, τ2, and A2 were greatly dependent on polymer concentration, combination of $\dot \gamma _{\rm i}$ and $\dot \gamma _{\rm f}$, and MW, whereas A1 was independent thereof and was close to unity. The process 1 was supposed to be valid for individual molecules, and process 2 for liquid crystalline domains or randomly aggregated or entangled molecules. 相似文献
A technique based on monitoring oxygen consumption was applied to study 11 natural and model polyphenols (PP, QH2) as well as four typical monophenolics as a chain‐breaking antioxidant during the controlled chain oxidation of methyl linoleate (ML) in bulk at 37°C. The antioxidant activities of QH2 were characterized by two parameters: the rate constant k1 for reaction of QH2 with the peroxy radical ${\rm LO}_{2}^{{\bf .}} $ : (i) QH2 + ${\rm LO}_{2}^{{\bf .}} $ → ${\rm QH}^{{\bf .}} $ + LOOH and the stoichiometric factor of inhibition, f, which shows how many kinetic chains may be terminated by one molecule of QH2. The rate constant k1 were reduced significantly by factor of 4 –28 as compared to these determined during the oxidation of styrene in bulk; the effect was typically more pronounced for catechol derivatives than for pyrogallol derivatives. At the same time, f for QH2 was found to be close to two independent of the number of active OH groups, similar to that determined earlier during the inhibited oxidation of styrene. The formation of H bond between OH group of QH2 and carboxyl group of ML is suggested as a reason for reducing effect of ML on k1. Practical applications : This work reports rate constants for the reaction of lipid peroxyl radical with phenolics and stoichiometric coefficient of inhibition, which characterize the antioxidant activity (AOA) of 15 natural and model PP, QH2 during the controlled peroxidation of ML. The reactivity of PP, QH2 during the oxidation of ML is routinely lower than the reactivity during the oxidation of non‐polar model hydrocarbons. This information may be useful to estimate the AOA of natural PP, QH2 in real systems of practical significance including plant oils, fats, food‐stuffs, biological objects, and similar. 相似文献
A detailed rheological study of cellulose nitrate in ethylacetate had been carried out in the dilute concentration (c) regime, covering a degree of polymerization (DP) range between 300 < DPη < 7000 and shear rates ($ \dot \gamma $) between 100 s?1 < $ \dot \gamma $ < 2000 s?1. The results show a strong dependence of the transition Newtonian to non-Newtonian behavior on the three variables $ \dot \gamma $, DP, and c, similar to that found recently on solutions of synthetic polymers. Emphasis has been put on the critical concentrations corresponding to the standard shear rate 1000 s?1 to correspond to the standard conditions ($ \dot \gamma $ ? 1000 s?1; 0.3 < [η] · c < 0.6; DS = 2.90 ± 0.02) proposed for the determination of the intrinsic viscosity [η] of cellulose nitrates. It is shown that solutions with concentrations adjusted according to the above given conditions still exhibit Newtonian behavior, up to the highest range of DP. It follows, therefore, that applying the standard conditions, an extrapolation to $ \dot \gamma $ = 0 as has been proposed often for the intrinsic viscosity determination of cellulose nitrate is not advisable and results in considerable error. Considering the relationship between [η] and DP, the present results indicate that the decrease of the exponent ( a ) from a = 1.0 to a = 0.76, taking place above a DP ? 1000, is not a consequence of the applied shear rate but rather of the molecular properties of the solutes themselves. 相似文献
Co‐solvents can minimize two of the major problems associated with the use of ionic liquids (ILs) as solvents for homogeneous derivatization of cellulose: high viscosity and limited miscibility with non‐polar reagents or reaction products. Thus, the effects of 18 solvents and 3 binary solvent mixtures on cellulose solutions in three ILs were systematically studied with respect to the solution phase behavior. The applicable limits of these mixtures were evaluated and general guidelines for the use of co‐solvents in cellulose chemistry could be advanced: Appropriate co‐solvents should have $E_{{\rm T}}^{{\rm N}} $ values (normalized empirical polarity) > 0.3, very low “acidity” (α < 0.5), and relatively high “basicity” (β ≥ 0.4). Moreover, novel promising co‐solvents and binary co‐solvent mixtures were identified.
