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91.
Teréz Balogh 《Tetrahedron》2004,60(3):679-682
Our studies are presented to replace alcohols as solvents in reverse hydrolytic reactions catalyzed by immobilized β-glucosidase to synthesize O-substituted β-d-glucopyranosides in preparative-scale. We found that 1,2-diacetoxyethane is a suitable solvent and O-alkyl or aryl β-d-glucosides were synthesized in moderate yields (after isolation 12-19%). In these reactions proportion of glucose and glucosyl acceptor hydroxy compounds was 1:20. We suggest that 1,2-diacetoxyethane can be useful not only for alcohols but for other glucosyl donor compounds unsuitable for the role of solvent (e.g., phenols) in the synthesis of O-β-d-glucosides by reverse hydrolysis. 相似文献
92.
考察了阳离子、非离子和阴离子表面活性剂存在下水杨酸-2’-乙基己基酯(EHS)的吸收光谱和激发态分子内质子转移(ESIPT)荧光光谱. 结果表明, EHS可增溶在胶束中, 2’-乙基己基碳链朝向胶束内核, 而水杨酸基朝向胶束-水界面; 胶束环境有利于EHS分子对紫外光的吸收和分子内氢键的形成, 从而使ESIPT 荧光显著增强, 激发态分子以发射可见光和非辐射去活化方式衰减; 并根据EHS和表面活性剂分子的结构和大小, 解释了EHS分子在胶束中的结合位点, 荧光猝灭和酯水解的光谱测量进一步为此结合位点提供了佐证. 相似文献
93.
M. R. Castellar M. A. Taipa J. M. S. Cabral 《Applied biochemistry and biotechnology》1997,61(3):299-314
A kinetic study ofChromobacterium viscosum lipase was undertaken, and compared withPseudomonas glumae lipase. Optimum operation conditions were pH 9.0 and 50°C for both enzymes. A substrate specificity study was also developed. Both enzymes showed higher activity on triglycerides with a long chain of fatty acid; the specific activity was always higher for C.viscosum lipase. Stability of both enzymes in aqueous medium at 60°C and pH 9.0 was evaluated. C.viscosum lipase was three times more stable than P.glumae lipase, with at 1/2 value of 0.75 h. In addition, the activity of C.viscosum lipase with substrate concentration was studied with a triolein emulsion. A dependence of the intrinsic characteristics of the emulsion was observed. Therefore, stability ofC. viscosum lipase B with reaction products was assayed in a micellar system. Acid products reduced the specific activity of the enzyme. Glycerol and high buffer concentration were stabilizers of enzyme deactivation. Finally, substrate specificity ofC. viscosum lipase B in a micellar system was developed with tributyrin, tricaprylin, and triolein. Only tributyrin showed an apparent Michaelis-Menten kinetic with Vmax app = 958 U/mg and Kma app = 75.5 mM. Tricaprylin and triolein showed diffusion limitations at low substrate concentration and substrate inhibition at high substrate concentration. Diffusion parameters were calculated for both these substrates. Mass transfer coefficients (k1) were 0.314 Å/min and 1.53 Å/min for tricaprylin and triolein, respectively. Effectiveness factors (η) were 0.536 and 0.768 for tricaprylin and triolein, respectively. 相似文献
94.
In order to obtain a catalyst support with a high surface area, ZrO2 and ZrO2-Y2O3 were prepared by the hydrolytic decomposition of the corresponding isopropoxide dissolved in benzene. The hydrolysis was carried out at 80°C using an excess amount of distilled water in flowing dry nitrogen. The precipitates thus obtained were dried at 100°C followed by calcination at 500°C in air or nitrogen for 1 h. The specific surface areas for both of the ZrO2 and ZrO2-Y2O3 increased with increasing amount of water added for hydrolysis, and the surface areas for ZrO2-Y2O3 increased with increasing yttrium content. A ZrO2 having a surface area of 130 m2/g was produced, and a stabilized tetragonal ZrO2 with 15 mol% Y3+ having a surface area of 200 m2/g was produced. Furthermore, despite the difference in the ZrO2 and ZrO2-Y2O3 crystal structures, the lattice-strain of ZrO2 has been unequivocally related to the surface area. 相似文献
95.
Combinatorial development of new catalysts with phosphodiesterase and esterase activities requires specific fluorescent probes for rapid visual detection of hydrolytic activity. Such fluorescent probes have been synthesized with special attention to solubility in water and stability towards spontaneous hydrolysis at a given pH. The probes reported here include compound 5 based on a fluorescein fluorophore, compound 12 for FRET-detection of phosphodiester hydrolysis and compound 25 based on a quinolinium fluorophore. 相似文献
96.
It is commonly observed that the rate of enzymatic hydrolysis of solid cellulose substrates declines markedly with time. In
this work the mechanism behind the rate reduction was investigated using two dominant cellulases of Trichoderma reesei: exoglucanase Cel7A (formerly known as CBHI) and endoglucanase Cel7B (formerly EGI). Hydrolysis of steam-pretreated spruce
(SPS) was performed with Cel7A and Cel7B alone, and in reconstituted mixtures. Throughout the 48-h hydrolysis, soluble products,
hydrolysis rates, and enzyme adsorption to the substrate were measured. The hydrolysis rate for both enzymes decreases rapidly
with hydrolysis time. Both enzymes adsorbed rapidly to the substrate during hydrolysis. Cel7A and Cel7B cooperate synergistically,
and synergism was approximately constant during the SPS hydrolysis. Thermal instability of the enzymes and product inhibition
was not the main cause of reduced hydrolysis rates. Adding fresh substrate to substrate previously hydrolyzed for 24 h with
Cel7A slightly increased the hydrolysis of SPS; however, the rate increased even more by adding fresh Cel7A. This suggests
that enzymes become inactivated while adsorbed to the substrate and that unproductive binding is the main cause of hydrolysis
rate reduction. The strongest increase in hydrolysis rate was achieved by adding Cel7B. An improved model is proposed that
extends the standard endo-exo synergy model and explains the rapid decrease in hydrolysis rate. It appears that the processive
action of Cel7A becomes hindered by obstacles in the lignocellulose substrate. Obstacles created by disordered cellulose chains
can be removed by the endo activity of Cel7B, which explains some of the observed synergism between Cel7A and Cel7B. The improved
model is supported by adsorption studies during hydrolysis. 相似文献
97.
Kallol K. Ghosh Pokhraj Sharma Samiksha Taramkar Santosh Kumar Sar 《Reaction Kinetics and Catalysis Letters》2004,81(1):161-168
The acid catalyzed hydrolysis of two N-substituted hydroxamic acids (C6H5CON(OH)R, R = C6H5 (PBHA), R = C6H5CH2(BBHA)) in mixed systems containing -cyclodextrin (—CD) and a surfactant (sodium dodecyl sulfate, SDS) has been studied. The reactions are inhibited by —CD. The inhibition is attributed to the formation of inclusion complex and competition between the micellization and complexation processes.This revised version was published online in December 2005 with corrections to the Cover Date. 相似文献
98.
Rafael Fogel Rafaela Rodrigues Garcia Rebeca da Silva Oliveira Denise Neves Menchero Palacio Luciana da Silva Madeira Nei Pereira Jr. 《Applied biochemistry and biotechnology》2005,122(1-3):741-752
The dilute-acid hydrolysis of sugarcane bagasse was optimized using a statistical experimental design resulting in hydrolysates
containing 57.25 g/L of xylose, which were fermented with a high inoculum concentration (10 g/L of the yeast Candida guilliermondii IM/UFRJ 50088). The addition of urea reduced the time of conversion (t
C) to 75 h (without nitrogen source addition t
C>127 h), and, consequently, improving the rates of xylitol bioproduction. Fermentator experiments, using the optimized conditions,
resulted in enhanced conversion rates, reducing t
C to 30 h. The stability of the yeast in the hydrolysate was also verified in a 480-h cultivation. 相似文献
99.
核磁共振法研究CH2=CHSi(OET)3的水化反应 总被引:1,自引:0,他引:1
高强水泥的生产近年来巳成为许多国家发展水泥生产的一个主要方向。在水泥中加入有机单体可以改善混凝土的质量,如加重量比为3%的有机硅烷偶联剂乙烯基三乙氧基硅烷CH_2=CHSi(OEt)_3(简称VTES)于水泥中,即可在较大程度上增加水泥石的强度。 相似文献
100.