New Methods for the Synthesis of Glycosides and Oligosaccharides—Are There Alternatives to the Koenigs-Knorr Method? [New Synthetic Methods (56)]

Glycoproteins, glycolipids, and glycophospholipids (glycoconjugates) are components of membranes. The oligosaccharide residue is responsible for intercellular recognition and interaction; it acts as a receptor for proteins, hormones, and viruses and governs immune reactions. These significant activities have stimulated interest in oligosaccharides and glycoconjugates. With their help it should be possible to clarify the molecular basis of these phenomena and to derive new principles of physiological activity. Major advances in the synthesis of oligosaccharides have been made by the use of the Koenigs-Knorr method, in which glycosyl halides in the presence of heavy-metal salts are employed to transfer the glycosyl group to nucleophiles. The disadvantages of this procedure have led to an intensive search for new methods. Such methods will be discussed in this article. Emphasis is placed on glycoside and saccharide formation by 1-O-alkylation, on the trichloroacetimidate method, and on activation through the formation of glycosylsulfonium salts and glycosyl fluorides.     

A dual-electrode approach for highly selective detection of glucose based on diffusion layer theory: experiments and simulation

A dual-electrode configuration for the highly selective detection of glucose in the diffusion layer of the substrate electrode is presented. In this approach, a glassy carbon electrode (GCE, substrate) modified with a conductive layer of glucose oxidase/Nafion/graphite (GNG) was used to create an interference-free region in its diffusion layer by electrochemical depletion of interfering electroactive species. A Pt microelectrode (tip, 5 microm in radius) was located in the diffusion layer of the GNG-modified GCE (GNG-G) with the help of scanning electrochemical microscopy. Consequently, the tip of the electrode could sense glucose selectively by detecting the amount of hydrogen peroxide (H2O2) formed from the oxidization of glucose on the glucose oxidase layer. The influences of parameters, including tip-substrate distance, substrate potential, and electrolyzing time, on the interference-removing efficiency of this dual-electrode approach have been investigated systematically. When the electrolyzing time was 30 s, the tip-substrate distance was 1.8 a (9.0 microm) (where a is the radius of the tip electrode), the potentials of the tip and substrate electrodes were 0.7 V and 0.4 V, respectively, and a mixture of ascorbic acid (0.3 mM), uric acid (0.3 mM), and 4-acetaminophen (0.3 mM) had no influence on the glucose detection. In addition, the current-time responses of the tip electrode at different tip-substrate distances in a solution containing interfering species were numerically simulated. The results from the simulation are in good agreement with the experimental data. This research provides a concept of detection in the diffusion layer of a substrate electrode, as an interference-free region, for developing novel microelectrochemical devices.     

Theoretical Study of Rotational Isomerism in Ethyl Pseudohalides

The structure and conformational stability of ethyl pseudohalides CH₃CH₂ — XCN (X = O, S, Se) were investigated using ab initiocalculations at the MP2 level of theory with a triple- basis set augmented with polarization and diffusion functions. Full optimization was performed on the minimum energy structures as well as on the transition state forms. The relative stabilities of rotational conformers were calculated at the MP4 level using MP2 optimized reference geometries. The nature of all considered stationary points was verified by calculation of the harmonic vibrational frequencies. The calculated bond lengths, bond angles, dipole moments, and rotational constants of optimized global minima structures agree very well with the corresponding experimental data obtained from microwave spectroscopic studies. Also, available experimental frequencies are in good accord with the theoretical values. For ethyl cyanate CH₃CH₂ — OCN, the antiperiplanar (trans) form is predicted to be more stable than the synclinal (gauche) form, and the synperiplanar (cis) form corresponds to the transition state. For both ethyl thiocyanate CH₃CH₂ — SCN and ethyl selenocyanate CH₃CH₂ — SeCN, the gaucheform is the global minimum while the trans-conformer is a local minimum and the cis-form is a transition state.     

The Studies of the Heterogeneous Properties of Solid Surfaces by Means of the Derivatograph Q-1500 D

A simple method has been adopted to obtain an important information on the heterogeneous properties of materials studied, adsorbed liquid films and liquid-solid interactions. The method utilizes Q-TG mass loss and the first derivative Q-DTG mass loss curves with respect to temperature and time obtained during programmed liquid thermodesorption in quasi-isothermal conditions. The values of the adsorption capacity, total porosity of material, the value of active centers, desorption energy distribution and mesopore-size distribution functions obtained by this method are in good agreement with those estimated on the basis of independent methods. The theoretical and experimental results provided novel and unique data on the heterogeneity of solid surfaces, properties of liquid adsorbed films and thermal stability of the liquid/solid interfaces. This paper presents the more important results obtained so far in the studies the liquid/solid systems by means of the Paulik's classical and quasi-isothermal techniques. This revised version was published online in July 2006 with corrections to the Cover Date.     

线性Sobolev方程的半离散H1-Galerkin混合有限元分析

给出了线性Sobolev方程初边值问题的半离散H^1-Galerkin混合有限元格式，通过误差分析，得到了未知函数的L^2模和梯度函数的散度空间模的最优阶误差估计。     

非线性VOLTERRA积分微分方程PETROV-GALERKIN有限元方法

讨论了非线性Volterra积分微分方程初值问题的PGFE方法，并给出了PGFE解的存在性和唯一性。     

K6H2［CoW11TiO40］纳米脂质体复合物的制备及性质研究

制备了包封多酸化合物K6H2[CoW11TiO40] (缩写为CoW11Ti)的纳米脂质体复合物,对其物理化学性质、包封率和含药量、粒度粒径、体外释药量及其体外抗肿瘤活性进行了研究.实验表明,多酸纳米脂质体复合物包封率为45%,含药量为23%,粒径分布在62.2～76.7nm范围内,平均粒径为69.4nm.在4℃可以稳定存在3个月.多酸化合物被包封于纳米脂质体后,活性明显高于原多酸化合物.     

关于完善会计专业判断的思考

专业判断是会计固有的职责,具有不可替创向应是培养会计人员的判断能力,使其熟练掌握会计专业判断的原则和方法,并通过构筑和完善专业判断的支撑环境,实现专业判断的合理化,加快国际化进程.     

不同电场频率对羊柴种子发芽率和幼苗根长的影响

用不同电场强度和不同频率的匀强电场处理羊柴种子5 min,分别测定电场对其发芽率和根系的影响.实验结果表明:羊柴种子受电场作用后能提高种子发芽率.     

生物入侵的危害、原因及防治措施

从生物入侵对经济的影响,对生态系统及人类健康等方面的危害入手,对生物入侵的原因及入侵机理进行了分析,并对其防治措施提出一些建议.