Transient-state kinetic approach to mechanisms of enzymatic catalysis

Transient-state kinetics by its inherent nature can potentially provide more directly observed detailed resolution of discrete events in the mechanistic time courses of enzyme-catalyzed reactions than its more widely used steady-state counterpart. The use of the transient-state approach, however, has been severely limited by the lack of any theoretically sound and applicable basis of interpreting the virtual cornucopia of time and signal-dependent phenomena that it provides. This Account describes the basic kinetic behavior of the transient state, critically examines some currently used analytic methods, discusses the application of a new and more soundly based "resolved component transient-state time-course method" to the L-glutamate-dehydrogenase reaction, and establishes new approaches for the analysis of both single- and multiple-step substituted transient-state kinetic isotope effects.     

酶法合成脂肪酸甲酯的研究进展

简要归纳了脂肪酸甲酯（FAME）的主要制备方法,其中生物酶法是最具工业化生产前景的方法。对酶法合成FAME的研究进展进行了详细叙述,并对其前景进行了展望。     

酶催化区域选择性合成蔗糖酯的研究进展

详细综述了非水相酶催化合成蔗糖酯的研究进展。主要介绍了酶的种类、反应介质、底物性质、外加辅助手段等因素对蔗糖酯产率以及对酶区域选择性的影响。酶催化区域选择性合成蔗糖酯的难点在于蔗糖具有多个可酰化羟基及其与酰基供体的不相溶性,提出通过介质工程、底物修饰改性、外加物理场如微波和超声波辅助等改善底物相溶性,通过筛选酶、蔗糖修饰等可以得到高区域选择性的蔗糖酯。最后指出酶催化合成蔗糖酯存在的问题、发展前景等。     

Enhancement of enzymatic catalysis of Bacillus amyloliquefaciens α-amylase by nonionic surfactant micelles

Some enzymes are considerably more stable when formulated with nonionic surfactants than when formulated with anionic surfactants. The effect of a nonionic surfatant, polyoxyethylene mono-N-dodecyl ether (Brij 35; number of units of ethylene oxide moieties, 23), on the kinetic behavior of hydrolysis of amylopectin with Bacillus amyloliquefaciens α-amylase (BAA) was studied at a temperature of 25°C and a pH of 7.0. The hydrolytic rate was accelerated by the addition of the nonionic surfactant above its critical micelle concentration. Lineweaver-Burk plots for the enzymatic hydrolysis in the absence and presence of the nonionic surfactant at 0.5 to 2.5% (wt/vol) had linear relationships, and the kinetic parameters, K _m and k _cat were obtained. The value of k _cat was increased with an increased concentration of Brij 35, whereas the K _m value was approximately constant. Therefore, the increase in k _cat contributed to the acceleration of the apparent hydrolytic rate. The interaction of amylopectin with the surfactant was examined by a surface tension measurement, and the result confirmed the corresponding binding between the substrate and the surfactant. A fluorescence analysis due to tryptophan in BAA suggested that BAA bound to the nonionic micelles. The increase in k _cat suggested that hydrolytic catalysis at the micellar pseudophase was more efficient than that at the aqueous pseudophase. The enhancement of the catalytic rate contributed to the effective removal of food stains containing starch when BAA was added with Brij 35 in a laundry detergent washing test.     

Synthesis by enzymatic catalysis. III. Copolymerisation of amino acids and dipeptides using papain

After studying the oligomerisation reaction of L-tyrosine, we investigated polymerisation of a mixture of methionine and tyrosine, which also led to the formation of a polypeptide. Various substrate concentrations and substrate/enzyme ratios were investigated. A hexapeptide was isolated containing three tyrosine and three methionine residues. Extension of the method to enzymatic polymerisation of dipeptide esters was also examined. After optimisation of the reaction, we obtained a peptide made up of four monomers using methyl L-leucyl-L-tyrosinate as substrate. The results are discussed in the light of a possible reaction mechanism.     

A nanoscopic view at the spherulitic morphology of isotactic polypropylene by atomic force microscopy

Summary The spherulitic morphology of isotactic polypropylene (i-PP) was investigated by a combination of optical and atomic force microscopes. Thin films of i-PP were prepared in situ by using a hot stage. The crystallization conditions were carefully selected to produce different types of spherulites including -I, -II, mixed, and -III types. Atomic force microscopy (AFM) scans unveiled morphological details such as mother/daughter lamellae, cross-hatching, and thickness and orientation of the lamellae. In a few cases, macromolecules were visualized. However, due to poor resolution, only the expected chain direction within the lamellae could be determined. The advantages and disadvantages of using AFM to study the surface of spherulites is discussed.     

Sieve trays: A view on the state of the art

Sieve tray performance is reviewed with reference to flow regimes and recently released commercial scale experimental data. Correlations are presented for regime transitions, pressure drop, axial mixing, entrainment, capacity, weeping and tray efficiency. The flow regimes, i.e. spray regime and emulsion flow regime, appear to have a dominant effect on nearly all of these performance variables.     

NPE 2012展示的最新挤出与混配技术

在NPE2012展览会上,整个挤出加工领域的展出主题集中在能耗效率、灵活性和生产效率上。吹膜高塔已经比较少见,但是不缺乏旨在帮助加工企业提高生产效率的新技术。     

The instantaneous image effect: A novel contribution to the mechanism of heterogeneous catalysis

A novel contribution to the mechanism of heterogeneous catalysis is discussed for a model H₂-on-nickel (100) system. The effect, which corresponds in a first approximation to the dispersion energy between adsorbate molecule and metal, is calculated through (1) admixture of an ionic component in the wave function of the adsorbate; (2) interaction between this ionic component and the image dipole which it creates in the polarizable metal. A single VB model, using an Anderson-type evaluation of the delocalization energy between adsorbate and metal, shows the image contribution to be 10 to 15% of the total adsorbate metal interaction energy.Presented in part at the 3rd International Symposium on the Mechanisum of Heterogeneous Catalysis Berkeley, Cal., 12–14 November, 1990.     

Biostatistics in the new millennium: a consulting statistician's perspective

This article attempts to predict the future for biostatistics and biostatisticians in the twenty-first century. Life will certainly be more complex and there will be growth in population, large corporations, and globalization generally. However, there will continue to be problems relating to biology, medicine, health and the environment, so biostatisticians can surely play an important role, if they are willing to adapt to changing circumstances. Personal views are expressed concerning those areas of most recent development that seem likely to be continued in the coming years: applications (clinical trials, epidemiology, vital statistics), philosophies, models, advances in computing, and the profession of biostatistics.