A "diels-alderase" at last

Diels this Alder your thinking? Electrocyclization reactions are exceedingly rare in metabolic pathways. Why is this so? Recently a [4+2] cycloaddition catalyzed by a monofunctional "Diels-Alderase" has been characterized. How does a 500-fold rate enhancement shown by this enzyme stack up?     

希尔顿钢厂钢包自然开浇率和无附着率的提高

叙述了水口引流料，对自然开浇和无附着性能影响最大的因素是钢包的循环特性，钢包自然开浇和无附着性能的大幅度提高改进了希尔顿钢厂所有连铸的操作工艺，并提高了钢水质量。本文阐述了技术开发工作（即水口引流砂的选择、座砖形状和耐火材料的改进、砂的添加方法和其它工艺的改进）所取得的成果。     

Free convection mass transfer at horizontal electrodes

The free convection mass-transfer behaviour of upward-facing horizontal surfaces has been investigated experimentally using an electrochemical technique involving the measurement of limiting currents for the deposition of copper on copper electrodes from acidified cupric sulphate solutions. The data are well correlated by the equations

Sh = 0·64(Sc . Gr)^0·25 for 3 × 10⁴ < ScGr < 2·5 × 10⁷ and

Sh = 0·16(Sc . Gr)^0·33 for 2·5 × 10⁷ < ScGr < 10¹². Electrode diameters varied between 0·1 and 12 cm and cupric sulphate concentrations between 0·01 and 0·3 M.

The data compare favourably with previous experimental results for the corresponding heat-transfer case.     

Free convective mass transfer at up-pointing truncated cones

Free convective mass transfer at the individual surfaces and total surface of up-pointing truncated cones of different height and base diameter were experimentally studied using the limiting diffusion current technique. It was found that the mass transfer rate is highest for the upward-facing horizontal surface. The experimental mass transfer coefficient for the combination of downward-facing horizontal and conical surface is lower than that obtained from the equivalent summed separate surfaces due to the fact that the conical surface is exposed to solution which has already been depleted in cupric ions. The total mass transfer was successfully correlated using a single characteristic dimension (defined as a surface area divided by perimeter projected onto horizontal plane) which takes into account all the relevant dimensions of a truncated cone, i.e. the height and two diameters.     

Free Volume of Metaphosphate Glasses at the Transition Temperature

The free volume fraction (v_f) defined by Simha and Boyer was measured for metaphosphate and mixed metaphosphate glasses (MO · P₂O₅; M=Ca, Sr, Ba, Mg, Zn). The glasses were classified into groups (1) in which M =Ca, Sr, Ba (V_f≠0.1) and (2) in which M =Mg, Zn (v_f∼ 0.l). The V_f values of mixed glasses containing cations from both groups varied. Thus, these glasses are not in an iso-free-volume state at the glass transition temperature.     

Free radical polymerization of methyl methacrylate at high temperatures

Free-radical polymerization of methyl methacrylate in a tubular reactor has been conducted at above-Tg temperatures. A salient feature of these experiments is the very efficient control of reactor temperature by vapor-liquid equilibrium of the polymerizing mixture via monomer evaporation. The system pressure thus provides a powerful control variable, restricting the temperature in the entire reactor by changing the monomer evaporation rate. In the range of our experimental conditions, the temperature and pressure in the reactor follow the Antoine equation closely. High temperature runs also reduce the length requirement of the reactor. However, molecular weight averages of the products are not impressive, unless slow-burning initiators are used. Modeling of above-Tg reactions has been attempted at two-levels of sophistication. A plug-flow model gives predictions in good agreement with our experimental temperatures and conversion data. The predicted molecular weights are also consistent with the experimentally observed values. However, the more elaborate rheokinctic model suggests that the superficial agreement between model and experiment is due to initiator burn-out, which limits the final conversion to within 40 percent. The liquid layer next to the reactor wall can never be so viscous as to form a stagnant deposit, due to this conversion limitation. The velocity profiles are thus not very much distorted, and a plug-flow model is adequate. With a slow-burning initiator and a sufficiently long reactor, skewing of velocity profile and reactor channeling will eventually emerge. Hence, the rheokinetic model must be evoked to model the system under such conditions.     

Free convective mass transfer at vertical cylindrical electrodes of varying aspect ratio

A limiting current technique was used for the measurement of the free convection mass transfer rate at entire vertical cylinders with varying aspect ratio. The mass transfer rates at the single surfaces were in good agreement with the mass transfer data in the literature. Mass transfer rates at a combination of a downward-facing horizontal and a vertical surface was controlled by the downward-facing horizontal surface (for cylinder aspect ratios lower than 2). It was found that the entire surface measured mass transfer rate was lower than that predicted by summation of the mass transfer rate at the individual surfaces. An interference factor,f, was introduced for correlation of the total mass transfer data for the full range of cylinders together. The dependence of the interference factor on the aspect ratio of the cylinder (L/r) was established.Nomenclature A ₀ total surface area of cylinder - A _V vertical surface area of cylinder - A _H surface area of horizontal base of cylinder - c _b bulk concentration of copper ion - c _{CuSo 4} bulk concentration of copper sulphate - c _{H 2}SO₄ bulk concentration of sulphuric acid - d cylinder diameter - D diffusion coefficient of copper ion - f interference factor - F Faraday's constant - g gravitational acceleration - I _L limiting diffusion current - k mass transfer coefficient - L cylinder length - L _w characteristic dimension (Weberet al. [2]) - n charge number of copper ion - r cylinder radius - Ra Rayleigh number - Ra _L Rayleigh number based on cylinder length - Sh Sherwood number - Sh _L Sherwood number based on vertical cylinder length - Sh ₀ stagnant medium Sherwood number - T electrolyte temperature Greek symbols density difference between bulk solution and interface - density - dynamic viscosity     

Free convection heat transfer over a horizontal plate at low prandtl number

Boundary layer equations for free convection heat transfer along a semi-infinite horizontal plate are derived by giving more importance to the energy equation. The equations are obtained for low Prandtl number and two separate polynomials are used to approximate the temperature and velocity profiles in these regions. The rate of heat transfer is compared with the available analytical and numerical results based on conventional boundary layer equations.     

Gibbs Free Energies of Formations of the Magnesium and Calcium Germanates at High Temperatures

The relative chemical potentials and partial molar solution enthalpies of the quasi-binary systems MgO-GeO₂ and CaO-GeO₂, were determined by means of solid-state galvanic cells and high-temperature solution calorimetry, respectively. The standard free energies of formation of the corresponding germanates, MgGeO₃, CaGeO₃, CaGe₄O₉, are presented as a function of temperature.     

Free Cholesterol Accelerates Aβ Self-Assembly on Membranes at Physiological Concentration

The effects of membranes on the early-stage aggregation of amyloid β (Aβ) have come to light as potential mechanisms by which neurotoxic species are formed in Alzheimer’s disease. We have shown that direct Aβ-membrane interactions dramatically enhance the Aβ aggregation, allowing for oligomer assembly at physiologically low concentrations of the monomer. Membrane composition is also a crucial factor in this process. Our results showed that apart from phospholipids composition, cholesterol in membranes significantly enhances the aggregation kinetics. It has been reported that free cholesterol is present in plaques. Here we report that free cholesterol, along with its presence inside the membrane, further accelerate the aggregation process by producing aggregates more rapidly and of significantly larger sizes. These aggregates, which are formed on the lipid bilayer, are able to dissociate from the surface and accumulate in the bulk solution; the presence of free cholesterol accelerates this dissociation as well. All-atom molecular dynamics simulations show that cholesterol binds Aβ monomers and significantly changes the conformational sampling of Aβ monomer; more than doubling the fraction of low-energy conformations compared to those in the absence of cholesterol, which can contribute to the aggregation process. The results indicate that Aβ-lipid interaction is an important factor in the disease prone amyloid assembly process.     

弱酸性树脂吸附低浓度游离碱行为的研究

利用电导实验技术 ,跟踪观察弱酸性树脂吸附低浓度游离碱的行为 ,研究温度对吸附的影响 ,测定吸附活化能 ( Ea)和吸附剂 -吸附质相互作用能 ( U)。实验结果表明 ,温度升高 ,吸附剂 -吸附质相互作用能增加 ,表观吸附速率常数 ( k)增大 ,而且 U与 T存在良好的线性相关 ( r=0 .995 1 )     

室温条件下无溶剂研磨法合成取代芳基席夫碱类化合物

以芳香醛和芳胺为原料,采用室温无溶剂研磨法合成一系列标题化合物。无溶剂研磨法比传统的合成方法具有收率高、反应时间短、条件温和、操作简便、对环境友好等优点。所合成的化合物结构通过IR、1HNMR、13CNMR、MS和元素分析进行了鉴定和表征。