Kinetic Data Show (kinetic + data_show)

Distribution by Scientific Domains

Selected Abstracts

Effects of alcohols on micellization and on the reaction methyl 4-nitrobenzenesulfonate + Br, in cetyltrimethylammonium bromide aqueous micellar solutions

María Muñoz
The effects of n -hexanol, n -pentanol, and n -butanol on the critical micelle concentration (cmc), on the micellar ionization degree (,), and on the rate of the reaction methyl 4-nitrobenzenesulfonate + Br, have been investigated in cetyltrimethylammonium bromide (CTAB) aqueous solutions. An increase in the alcohol concentration present in the solution produces a decrease in the cmc and an increase in the micellar ionization degree. Kinetic data show that the observed rate constant decreases as alcohol concentration increases. This result was rationalized by considering variations in the equilibrium binding constant of the methyl 4-nitrobenzenesulfonate molecules to the micelles, variations in the interfacial bromide ion concentration, and variations in the characteristics of the water,alcohol bulk phase provoked by the presence of alcohols. When these operative factors are considered, kinetic data in this and other works show that the second-order rate constants in the micellar pseudophases of water,alcohol micellar solutions are quite similar to those estimated in the absence of alcohols. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 634,641, 2004 [source]

Expression and characterization of active site mutants of hevamine, a chitinase from the rubber tree Hevea brasiliensis

FEBS JOURNAL, Issue 3 2002
Evert Bokma
Hevamine is a chitinase from the rubber tree Hevea brasiliensis. Its active site contains Asp125, Glu127, and Tyr183, which interact with the ,1 sugar residue of the substrate. To investigate their role in catalysis, we have successfully expressed wild-type enzyme and mutants of these residues as inclusion bodies in Escherichia coli. After refolding and purification they were characterized by both structural and enzyme kinetic studies. Mutation of Tyr183 to phenylalanine produced an enzyme with a lower kcat and a slightly higher Km than the wild-type enzyme. Mutating Asp125 and Glu127 to alanine gave mutants with ,,2% residual activity. In contrast, the Asp125Asn mutant retained substantial activity, with an approximately twofold lower kcat and an approximately twofold higher Km than the wild-type enzyme. More interestingly, it showed activity to higher pH values than the other variants. The X-ray structure of the Asp125Ala/Glu127Ala double mutant soaked with chitotetraose shows that, compared with wild-type hevamine, the carbonyl oxygen atom of the N -acetyl group of the ,1 sugar residue has rotated away from the C1 atom of that residue. The combined structural and kinetic data show that Asp125,and Tyr183 contribute to catalysis by positioning the,carbonyl oxygen of the N -acetyl group near to the C1 atom. This allows the stabilization of a positively charged transient intermediate, in agreement with a previous proposal that the enzyme makes use of substrate-assisted catalysis. [source]

Nucleophilic heteroaromatic substitution: Kinetics of the reactions of nitropyridines with aliphatic amines in dipolar aprotic solvents

Chukwuemeka Isanbor
Rate data are reported for the reactions of 2-chloro-5-nitropyridine 2a, 2-chloro-3-nitropyridine 2b, and the corresponding 2-phenoxy derivatives 2c with n -butylamine, pyrrolidine and piperidine and 2d with n -butylamine and pyrrolidine in dimethyl sulfoxide (DMSO) as solvent. The same reactions in acetonitrile had been reported earlier (Crampton et al., Eur J Org Chem 2007, 1378,1383). Values in these solvents are compared with those of 2,4-dinitrochlorobenzene 3a, 2,6-dinitrochlorobenzene 3b, and the corresponding nitroactivated diphenyl ethers 3c and 3d. Reactions with n -butylamine in both solvents gave values of kobs, which increase linearly with amine concentration indicating that nucleophilic attack is rate limiting. The only exception is the reactions in acetonitrile with 2c where base catalysis was observed. Values of k1, the rate constant for the nucleophilic attack, decrease in the order pyrrolidine > piperidine > n -butylamine. In acetonitrile, kinetic data show that k/k ratios are more than unity while the inverse is the case in DMSO. With the phenoxy derivatives, substitution was the only process observed. Base catalysis detected in the reactions of the 1-phenoxy derivatives is attributed to rate-limiting deprotonation of the initially formed zwitterionic intermediate. Our results shed more light on fundamental aspects of activation, hydrogen bonding, and steric effects associated with an aza or a nitro group in the molecules investigated as it affects the nucleophilic aromatic substitution (SNAr) reaction pathways. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 125,135, 2008 [source]

Modulation of Spectrokinetic Properties of o -Quinonoid Reactive Intermediates by Electronic Factors: Time-Resolved Laser Flash and Steady-State Photolysis Investigations of Photochromic 6- and 7-Arylchromenes

Jarugu, Narasimha Moorthy Prof.
Abstract A variety of differently substituted 6- and 7-arylchromenes such as that depicted undergo photoinduced CO bond cleavage to yield colored o -quinonoid intermediates. A combined analysis of ,s,ms (laser flash) and real-time kinetic data show that the o -quinonoid intermediates decay faster when the C2-aryl and C6-/C7-aryl rings contain electron-donating and electron-accepting groups, respectively. Similarly, the decay occurs slowly for the reversed scenario, while intermediate decay rates are observed when both substituents are electron donating. A ready synthetic accessibility of a series of 6- and 7-arylchromenes via Pd0 -catalyzed Suzuki coupling protocol has permitted a comprehensive investigation of the thermal decay behavior of a broad set of photogenerated o -quinonoid reactive intermediates. It is shown that substantial mesomeric effect between the benzopyran nucleus and the aryl ring at C6 or C7 position of the former renders significant absorption beyond 350,nm such that they are readily photoactivated to yield colored o -quinonoid intermediates. The absorption spectra of the latter are found to be strongly influenced by the substituents on C2-, C6- and C7-aryl rings; indeed the colored absorptions can be conveniently tuned by appropriate choice of substituents. The thermal decay (bleaching phenomenon), which is important from the point of view of their application in ophthalmic lenses, was investigated in each case by ,s,ms as well as real-time absorption spectroscopy. By careful experimentation, we have extracted the decay rate constants for Z,E and E,E o -quinonoid isomers of all 6- and 7-arylchromenes in an attempt to establish a correlation between the electronic attributes with their thermokinetic behavior. From a combined analysis of ,s,ms (laser flash) and real-time kinetic data, it is shown that the colored o -quinonoid intermediates decay faster when the C2-aryl and C6-/C7-aryl rings contain electron-donating and electron-accepting groups, respectively. In the same vein, the decay was found to occur slowly for the reversed scenario, while intermediate decay rates are observed when both substituents are electron-donating. Thus, any substituent on the C2-aryl ring that contributes mesomerically to the development of charge on the quinonoid oxygen, and any substituent on the C6-/C7-aryl ring that exerts ,I effect appear to expedite thermal decay. Furthermore, evidence is obtained for the first time from ,s time-resolved laser-flash spectroscopy for the formation and characterization of the trans,cis (E,Z) o -quinonoid isomer, which has heretofore eluded spectral characterization in the photochromic phenomena of pyrans. [source]