Density functional theory study of the oxidative dehydrogenation of propane on the (001) surface of V2O5

The density functional theory using a plane‐waves basis set and pseudopotential has been used to study the reaction pathways for ODH of propane on the V₂O₅(001) surface. The calculations indicated that propane adsoprtion step was initiated by the insertion of vanadyl oxygen O (1) into methylene C? H bond forming an iso‐propanol structure. This step is the rate‐determining step with an activation energy of 23.3 kcal/mol. The subsequent step involved the abstraction of the second hydrogen by O (1) site leading the formation of propene. This process had an activation energy of 22.5 kcal/mol. The elimination of surface bound water molecule at the O (1) was a barrierless process. The energy required for this process was compensated from O2 dissociative adsorption. Finally, the electronic density of state has been applied to prove the reality of the calculated results. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Enzymatic transformation of platycosides and one‐step separation of platycodin D by high‐speed countercurrent chromatography

Platycosides, the saponins found in the roots of Platycodon grandiflorum (Platycodi Radix), are typically composed of oleanane triterpenes with two side chains. In platycosides, platycodin D, a glucose unit at C‐3, is a major component, which has several pharmacological activities. Because of the high demand for this compound, we attempted to enzymatically convert platycodin D₃ and platycoside E, having two and three glucose units at C‐3, respectively, into platycodin D. In this study, we tested the ability of several glycosidases to transform platycosides, or more specifically, the ability to transform platycoside E and platycodin D₃ into platycodin D. To obtain pure platycodin D on a preparative scale, high‐speed countercurrent chromatography with a solvent system of ethyl acetate/n‐butanol/water (1.2:1:2, v/v/v) was used for the separation of the enzymatically transformed product. Approximately 39.4 mg of platycodin D (99.8% purity) was obtained from 200 mg of the product in a one‐step separation. The results strongly support the advantage of enzymatic transformation of the platycosides for the efficient enrichment of platycodin D in the complicated extract of the medicinal plant.     

Rapid method for the determination of capsaicin and dihydrocapsaicin in Gochujang using ultra-high-performance liquid chromatography

A sensitive and specific heating block method coupled with ultra-HPLC (u-HPLC) was developed for the analysis of capsaicin in Gochujang and validated by comparing with a conventional HPLC (AOAC Method 995.03). The method validation parameters yielded good results, including linearity, precision, accuracy, and recovery. The u-HPLC separation was performed on a reversed C18 column (50 x 2 mm id, particle size 2 microm), followed by fluorescence detection (excitation 280 nm, emission 325 nm). Methanol was used as the extracting solvent, and the amount of sample taken was approximately 0.2 g; the optimum amount of extraction solvent and extraction time were 15 mL and 1 h, respectively. The recovery of capsaicin in Gochujang was more than 93%, and the LOD and LOQ of the u-HPLC analysis were 0.05 and 0.16 microg/g for capsaicin and 0.05 and 0.16 microg/g for dihydrocapsaicin. The calibration graphs for capsaicin and dihydrocapsaicin were linear from 0.2 to 10.0 microg/mL for u-HPLC. The interday and intraday precisions (RSD values) were < 6.27%.     

An electrochemically enhanced solid-phase microextraction approach based on a multi-walled carbon nanotubes/Nafion composite coating

In this paper, we proposed an approach using a multi-walled carbon nanotubes (MWCNTs)/Nafion composite coating as a working electrode for the electrochemically enhanced solid-phase microextraction (EE-SPME) of charged compounds. Suitable negative and positive potentials were applied to enhance the extraction of cationic (protonated amines) and anionic compounds (deprotonated carboxylic acids) in aqueous solutions, respectively. Compared to the direct SPME mode (DI-SPME) (without applying potential), the EE-SPME presented more effective and selective extraction of charged analytes primarily via electrophoresis and complementary charge interaction. The experimental parameters relating to extraction efficiency of the EE-SPME such as applied potentials, extraction time, ionic strength, sample pH were studied and optimized. The linear dynamic range of developed EE-SPME-GC for the selected amines spanned three orders of magnitude (0.005–1 μg mL⁻¹) with R² larger than 0.9933, and the limits of detection were in the range of 0.048–0.070 ng mL⁻¹. All of these characteristics demonstrate that the proposed MWCNTs/Nafion EE-SPME is an efficient, flexible and versatile sampling and extraction tool which is ideally suited for use with chromatographic methods.     

Separation of post-translational modifications in monoclonal antibodies by exploiting subtle conformational changes under mildly acidic conditions

Chromatographic separation plays a key role in the identification, quantification, and characterization of protein variants. Here we describe separation of species containing two post-translational modifications (glycosylation and methionine oxidation) in the Fc fragment of a monoclonal antibody. The method is based on cation-exchange chromatography under mildly acidic conditions that destabilize mainly the CH2 domain. Our data suggest that the separation is not mediated by the chemical modification itself, but rather by subtle structural changes induced by the chemical modification in the domain-decoupled conformation that monoclonal antibodies adopt around pH 4. Compared to other procedures already described in the literature, this method demonstrates an improved separation and allows purification of species in the native fold for additional functional characterization. This approach of separation under conditions where the protein assumes an alternative conformation could find a more general utility for the separation of chemical modifications in proteins.     

Learning a novel phonological contrast depends on interactions between individual differences and training paradigm design

Studies evaluating phonological contrast learning typically investigate either the predictiveness of specific pretraining aptitude measures or the efficacy of different instructional paradigms. However, little research considers how these factors interact--whether different students learn better from different types of instruction--and what the psychological basis for any interaction might be. The present study demonstrates that successfully learning a foreign-language phonological contrast for pitch depends on an interaction between individual differences in perceptual abilities and the design of the training paradigm. Training from stimuli with high acoustic-phonetic variability is generally thought to improve learning; however, we found high-variability training enhanced learning only for individuals with strong perceptual abilities. Learners with weaker perceptual abilities were actually impaired by high-variability training relative to a low-variability condition. A second experiment assessing variations on the high-variability training design determined that the property of this learning environment most detrimental to perceptually weak learners is the amount of trial-by-trial variability. Learners' perceptual limitations can thus override the benefits of high-variability training where trial-by-trial variability in other irrelevant acoustic-phonetic features obfuscates access to the target feature. These results demonstrate the importance of considering individual differences in pretraining aptitudes when evaluating the efficacy of any speech training paradigm.     

Equivalent contributing depth investigated by a lateral wave with axial transmission in viscoelastic cortical bone

Cortical bone is a viscoelastic heterogeneous medium which may be assessed with axial transmission. This work aims at evaluating the average depth investigated by the lateral wave for radial variations of material properties in relatively thick cortical bone. The equivalent contributing depth (ECD) is derived from the finite element simulation results for spatial variations of a viscoelastic coefficient (η(11)) and of porosity. A value of ECD equal to around 1.6 mm is obtained for a spatial variation of η(11). The method fails to predict accurate values of the ECD for a spatial variation of porosity, because all parameters vary simultaneously.     

越南火龙果冷藏现状分析

随着全球化经济的快速发展,食品冷藏技术日益重要.文中综述了越南火龙果的现有冷藏技术及其存在的问题,并提出解决方案.