首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 15 毫秒
1.
The hydrolysis kinetics ofp-nitrophenyl acetate by basic polypeptides with a variety of amino acid residues were analyzed. The results suggest that the tertiary structure of microenvironment brought about by random coil assembly is more important as a binding site than the number of basic residues (catalytic sites).  相似文献   

2.
Coupled nonenzymatic cleavage/ligation of oligoribonucleotides catalyzed by magnesium ions afforded longer RNA molecules with a new sequence. The efficiency of formation of ligation products reaches 6%. The possible role of this reaction in the evolution of the RNA world is discussed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2413–2419, December, 2007.  相似文献   

3.
Rate constants have been obtained for the hydrolysis of the trifluoroethyl, phenyl, and p-nitrophenyl esters of 2-aminobenzoic acid at 50 degrees C in H(2)O. The pseudo-first-order rate constants, k(obsd), are pH independent from pH 8 to pH 4 (the pK(a) of the amine group conjugate acid). The 2-aminobenzoate esters hydrolyze with similar rate constants in the pH-independent reactions, and these water reactions are approximately 2-fold slower in D(2)O than in H(2)O. The most likely mechanism involves intramolecular general base catalysis by the neighboring amine group. The rate enhancements in the pH-independent reaction in comparison with the pH-independent hydrolysis of the corresponding para substituted esters or the benzoate esters are 50-100-fold. In comparison with the hydroxide ion catalyzed reaction, the enhancement in k(obsd) at pH 4 with the phenyl ester is 10(5)-fold. Intramolecular general base catalyzed reactions are assessed in respect to their relative advantages and disadvantages in enzyme catalysis. A general base catalyzed reaction can be more rapid at low pH than a nucleophilic reaction that has a marked dependence on pH and the leaving group.  相似文献   

4.
The last few years have seen a considerable increase in our understanding of catalysis by naturally occurring RNA molecules called ribozymes. The biological functions of RNA molecules depend upon their adoption of appropriate three-dimensional structures. The structure of RNA has a very important electrostatic component, which results from the presence of charged phosphodiester bonds. Metal ions are usually required to stabilize the folded structures and/or catalysis. Some ribozymes utilize metal ions as catalysts, whereas others use the ions to maintain appropriate three-dimensional structures. In the latter case, the correct folding of the RNA structures can perturb the pKa values of the nucleotide(s) within a catalytic pocket such that they act as general acid/bases catalysts.  相似文献   

5.
两种双核配合物催化PNPP水解反应的机理研究   总被引:1,自引:0,他引:1  
Cu-Cu(Ⅱ)和Cu-Zn(Ⅱ)草酰胺桥联双核配合物被合成和表征.研究了该同核和异核配合物催化PNPP水解的动力学和机理,建立了一种双核配合物催化PNPP水解的动力学数学模型.结果表明:在缓冲溶液中随着溶液pH的增大,草酰胺桥联双核配合物催化PNPP水解速率提高;配合物中的两个金属离子在催化PNPP水解过程中具有协同效应;这两种草酰胺桥联双核配合物在催化PNPP水解中表现出较好的催化活性.  相似文献   

6.
Kinetics of the basic hydrolysis of 1,2-glyceryl dinitrate (1,2-DNG) and 1,3-glyceryl dinitrate (1,3-DNG) esters were investigated in CO2-free aqueous calcium hydroxide solutions. The hydrolysis reactions were carried out in a temperature controlled reactor vessel with provision for continuous N2 sparging of the reaction mixture. Both glyceryl dinitrate esters hydrolyzed via second-order reaction at 25°C. 1,2-DNG in basic solutions isomerized to 1,3-DNG which subsequently hydrolyzed to yield products. The main hydrolysis product of 1,3-DNG was identified as glycidyl nitrate. Other products formed during the basic hydrolysis of DNGs were nitrites and nitrates.  相似文献   

7.
Summary The tetrabutylammonium bromide catalyzed esterification of salts of carboxylic acids1 a–e with 2-propenyl halides2 a,b is described.
Phasentransferkatalysierte Synthese von 2-Propenylestern von Carbonsäuren (Kurze Mitt.)
Zusammenfassung Es wird die tetrabutylammoniumkatalysierte Veresterung von Carbonsäuresalzen1 a–e mit 2-propenylhalogeniden2 a,b beschrieben
  相似文献   

8.
The comparative kinetic investigation of the hydrolysis of p-nitrophenyl picolinate (PNPP) and p-nitrophenyl acetate (PNPA) catalyzed by the tetracoordinate macrocyclic Schiff base complex of zinc(II) (1) at 30 °C is reported. The results indicate that the (1) catalyzed hydrolyses of PNPP and PNPA are acid-base catalytic processes and that the active species is the metal bound hydroxide ion, namely, ZnL—OH. (1) promoted hydrolysis of PNPP proceeds much faster than that of PNPA. At pH 7.51, the apparent second-order rate constants k c for hydrolysis of PNPP and PNPA are 0.254 and 7.28 × 10–3 mol–1 dm3 s–1, respectively. The difference in hydrolytic rates may be attributed to the difference of hydrolytic mechanisms by which the PNPP and PNPA operate. The reasons are discussed in detail.  相似文献   

9.
We have continued our kinetics investigation of the iodosobenzoate (IBA) catalysis of the hydrolysis of p-nitrophenyl diphenyl phosphate (PNDP), in microemulsion media composed of hexadecane in water stabilized by cetyltrimethylammonium bromide and 1-butanol over a range of water mass fractions. We have examined two iodosobenzoic acid derivatives (5-nitro-2-iodosobenzoic acid and 5-octyloxy-2-iodosobenzoic acid) as catalysts. In addition, we have determined by31P FT-NMR techniques that the major product of the hydrolysis of PNDP, both in IBA catalyzed and in uncatalyzed media, is diphenylphosphate.  相似文献   

10.
The mechanism of the nucleotidyl transfer reaction catalyzed by yeast RNA polymerase II has been investigated using molecular mechanics and quantum mechanics methods.Molecular dynamics(MD) simulations were carried out using the TIP3 water model and generalized solvent boundary potential(GSBP) by CHARMM based on the X-ray crystal structure.Two models of the ternary elongation complex were constructed based on CHARMM MD calculations.All the species including reactants,transition states,intermediates,and products were optimized using the DFT-PBE method coupled with the basis set DZVP and the auxiliary basis set GEN-A2.Three pathways were explored using the DFT method.The most favorable reaction pathway involves indirect proton migration from the RNA primer 3’-OH to the oxygen atom of-phosphate via a solvent water molecule,proton rotation from the oxygen atom of-phosphate to the-phosphate side,the RNA primer 3’-O nucleophilic attack on the-phosphorus atom,and P-O bond breakage.The corresponding reaction potential profile was obtained.The rate limiting step,with a barrier height of 21.5 kcal/mol,is the RNA primer 3’-O nucleophilic attack,rather than the commonly considered proton transfer process.A high-resolution crystal structure including crystallographic water molecules is required for further studies.  相似文献   

11.
12.
A series of new alkyl mono‐ and bimetallic aluminum complexes supported by novel amidinate ligands has been prepared in very high yields. These complexes were fully characterized by spectroscopic methods. Alkyl aluminum complexes 1 – 6 were investigated as catalysts for the ring‐opening polymerization and copolymerization of ε‐caprolactone and L‐lactide. Under the optimal reaction conditions, complex 5 acts as an efficient single‐component initiator for the ring‐opening polymerization and copolymerization of cyclic esters to yield biodegradable polyester materials with narrow polydispersities. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2397–2407  相似文献   

13.
Phosphodiester cleavage of BNPP can be accelerated by various degradative receptors including enzymes. In the hydrolytic cleavage of BNPP, a bell-shaped receptor concentration-rate profile composed of a positive-catalysis phase at low receptor concentrations and a negative-catalysis phase at excess receptor concentration was observed. The negative catalysis can be explained by nonproductive binding, leading to an increase of the activation energy barrier and a decrease of the reaction rate.  相似文献   

14.
A palladium-catalyzed intramolecular iodoetherification of alkenes is reported. The reaction is efficient and highly diastereoselective for disubstituted alkenes. The tether length between the alcohol and alkene can be varied to produce tetrahydrofuran and tetrahydropyran rings. Diphosphine palladium(II) salts are highly active catalysts enabling future studies on the development of an enantioselective process.  相似文献   

15.
Remarkable dependencies of the rate constants and enantioselectivities on the substituents and protecting groups were demonstrated by the catalyzed hydrolysis of p-nitrophenyl esters of various N-protected L-amino acids by L or D-LauHis in the presence of mixed micelles with CTABr.  相似文献   

16.
Cyclodextrins and their derivatives act as inverse phase-transfer catalysts in the hydrolysis of carboxylic acid esters. The reaction is affected by temperature, choice of cyclodextrin and also by the stirring rate. Lipophilic esters are better hydrolyzed under inverse phase-transfer catalysis conditions than under classic phase-transfer catalysis (PTC) conditions. Some typical features of the reaction are emphasized and compared with classical PTC reactions.  相似文献   

17.
The mechanism of hydrolysis of n-nitrophenyl acetate (NPA), butyrate (NPB), caprylate (NPC), and o-methoxycinnamate (NPOMC) catalysed by benzyl-containing polyethyleneimines of linear and branched structures was investigated in aqueous media. The reaction seems to proceed via a general basic mechanism of catalysis and does not involve acylation of the catalyst. Benzyldiethylamine is an analogue of the active centres in polymers with pKa = 8 · 35 ± 0 · 1, localized in the polymer globules at sites of higher hydrophobity.The reaction has a three-step mechanism involving binding of the substrate to an active centre (to give Michaelis sorption complex), substrate conversion and desorption of products. For each step, rate constants were determined. The effect of polymer (K2/Km)/KII increases from NPA to NPC; in the latter case, it is of order 105.  相似文献   

18.
2-Boron substituted 1,3-dienes have been prepared and used in preliminary experiments to demonstrate that it is possible to effect rhodium catalyzed Diels-Alder/hydrolysis tandem reactions.  相似文献   

19.
Insoluble microporous polystyrene-bound benzyltriethyl ammonium chloride has been used as a catalyst in the alkylation of phenylacetone with 1-bromobutane, and the kinetics of this reaction was investigated under phase-transfer catalytic conditions. The observed reaction rates depend on many experimental parameters, viz., stirring speed, substrate amount, basicity of aqueous NaOH, amount of 1-bromobutane, temperature, order of addition of the reactants and particle size, percent active site, and percent crosslinking of the polymer. The rates are nearly 12 times higher at lower concentrations of base than at higher concentrations and do not vary appreciably with a variation in stirring speed from 200 to 700 rpm. The rate of alkylation increases with a decrease in the particle size of the catalyst and crosslinking of the polymer. Based on the results obtained, a suitable mechanism in which a combination of intraparticle diffusion and intrinsic reactivity limit the reaction rates has been proposed. © 1993 John Wiley & Sons, Inc.  相似文献   

20.
The syntheses of three ligands are reported: N,N,N′,N′-tetra(2-hydroxyethyl)-1,3-propylene-diamine (1), N,N,N′,N′-tetra(2-hydroxyethyl)-1,10-decadiamine (2), N,N,N′,N′-tetra(2-hydroxyethyl)-1,4-xylyldiamine (3). The catalytic hydrolysis of p-nitrophenyl picolinate (PNPP) by the bivalent metal ion Cu(II) complexes of these ligands was studied kinetically in a buffered CTAB or Brij35 micellar solutions at 25 °C and different pH values. The results indicate that 1:2 and 2:1 complexes of these ligands and metal ion are the active species for the catalytic hydrolysis of PNPP in CATB and Brij35 micellar solutions. The ternary complex kinetic model for metallomicellar catalysis was employed to obtain the relative kinetic and thermodynamic parameters. The effects of the structure of the ligands and the microenvironment of reaction on the hydrolytic reaction of PNPP have been discussed in detail.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号