- Unexpected resistance to base-catalyzed hydrolysis of nitrogen pyramidal amides based on the 7-azabicyclic[2.2.1]heptane scaffold
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Non-planar amides are usually transitional structures, that are involved in amide bond rotation and inversion of the nitrogen atom, but some ground-minimum non-planar amides have been reported. Non-planar amides are generally sensitive to water or other n
- De Velasco, Diego Antonio Ocampo Gutiérrez,Su, Aoze,Zhai, Luhan,Kinoshita, Satowa,Otani, Yuko,Ohwada, Tomohiko
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- Structure-activity analysis of base and enzyme-catalyzed 4-hydroxybenzoyl coenzyme A hydrolysis
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In this study, the second-order rate constant k2 of base-catalyzed hydrolysis and the values of kcat, Km and kcat/Km of wild-type Pseudomonas sp. CBS3 4-hydroxybenzoyl coenzyme A (4-HBA-CoA) thioesterase-catalyzed hydrolysis of 4-HBA-CoA and its para-substituted analogs were measured. For the base-catalyzed hydrolysis, the plot of log k2 vs the σ value of the para-substituents was linear with a slope (ρ) of 1.5. In the case of the enzyme-catalyzed hydrolysis, the kcat/Km values measured for the para-substituted analogs defined substrate specificity. Asp32 was shown to play a key role in substrate recognition, and in particular, in the discrimination between the targeted substrate and other cellular benzoyl-CoA thioesters.
- Song, Feng,Zhuang, Zhihao,Dunaway-Mariano, Debra
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- Competitive endo- and exo-cyclic C-N fission in the hydrolysis of N-aroyl β-lactams
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The balance between endo- and exo-cyclic C-N fission in the hydrolysis of N-aroyl β-lactams shows that the difference in reactivity between strained β-lactams and their acyclic analogues is minimal. Attack of hydroxide ion occurs preferentially at the exocyclic acyl centre rather than that of the β-lactam during the hydrolysis of N-p-nitrobenzoyl β-lactam. In general, both endo- and exo-cyclic C-N bond fission occurs in the alkaline hydrolysis of N-aroyl β-lactams, the ratio of which varies with the aryl substituent. Hence, the Bronsted β-values differ for the two processes: -0.55 for the ring-opening reaction and -1.54 for the exocyclic C-N bond fission reaction. For the pH-independent and acid-catalysed hydrolysis of N-benzoyl β-lactam, less than 3% of products are derived from exocyclic C-N bond fission.
- Tsang, Wing Y.,Ahmed, Naveed,Hemming, Karl,Page, Michael I.
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p. 1432 - 1439
(2007/10/03)
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- Kinetics and mechanism of the pyridinolysis of S-4-nitrophenyl 4-substituted thiobenzoates in aqueous ethanol
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The pyridinolysis of S-4-nitrophenyl 4-X-substituted thiobenzoates (X = H, Cl, and NO2; 1, 2, and 3, respectively) is studied kinetically in 44 wt % ethanol-water, at 25.0 °C and an ionic strength of 0.2 M (KCl). The reactions are measured spectrophotometrically (420-425 nm) by following the appearance of 4-nitrobenzenethiolate anion. Pseudo-first-order rate coefficients (kobsd) are obtained throughout, under excess of amine over the substrate. Plots of kobsd vs [free amine] at constant pH are linear with the slope (kN) independent of pH. The Bronsted-type plot (log kN vs pKa of the conjugate acids of the pyridines) for the reactions of thiolbenzoate 1 is curved with a slope at high pK a, β1 = 0.20, and slope at low pKa, β2 = 0.94. The pKa value for the center of the Bronsted curvature is pKa0 = 9.7. The pyridinolysis of thiolbenzoates 2 and 3 show linear Bronsted-type plots of slopes 0.94 and 1.0, respectively. These results and other evidence indicate that these reactions occur with the formation of a zwitterionic tetrahedral intermediate (T±). For the pyridinolysis of thiolbenzoate 1, breakdown of T± to products (k2 step) is rate-limiting for weakly basic pyridines and T± formation (k1 step) is rate-determining for very basic pyridines. The k2 step is rate-limiting for the reactions of thiolbenzoates 2 and 3. The smallest pK a0 value for the reaction of 1 is due to a the weakest electron withdrawal of H (relative to Cl and NO2) in the acyl group, which results in the smallest k-1/k2 ratio. The pK a0 values for the title reactions are smaller than those for the reactions of secondary alicyclic amines with thiolbenzoates 1-3. This is attributed to a lower leaving ability from the T± of pyridines than isobasic alicyclic amines. The lower pKa0 value found for the pyridinolysis of 2,4-dinitrophenyl benzoate (pK a0 = 9.5), compared with that for the pyridinolysis of 1, is explained by the greater nucleofugality from T± of 2,4-dinitrophenoxide than 4-nitrobenzenethiolate, which renders the k -1/k2 ratio smaller for the reactions of the benzoate relative to thiolbenzoate 1. The title reactions are also compared with the aminolysis of similar thiolbenzoates in other solvents to assess the solvent effect.
- Castro, Enrique A.,Vivanco, Melissa,Aguayo, Raul,Santos, Jose G.
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p. 5399 - 5404
(2007/10/03)
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- Preliminary characterization of four 2-chlorobenzoate-degrading anaerobic bacterial consortia
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Dechlorination was the initial step of 2CB biodegradation in four 2-chlorobenzoate-degrading methanogenic consortia. Selected characteristics of ortho reductive dehalogenation were examined in consortia developed from the highest actively dechlorinating dilutions of the original 2CB consortia, designated consortia M34-9, P20-9, P21-9 and M50-7. In addition to 2-chlorobenzoate, all four dilution consortia dehalogenated 4 of 32 additional halogenated aromatic substrates tested, including 2-bromobenzoate; 2,6-dichlorobenzoate; 2,4-dichlorobenzoate; and 2-chloro-5-hydroxybenzoate. Dehalogenation occurred exclusively at the ortho position. Both ortho chlorines were removed from 2,6-dichlorobenzoate. Benzoate was detected from 2-bromobenzoate and 2,6-dichlorobenzoate. 4-Chlorobenzoate and 3-hydroxybenzoate were formed from 2,4-dichlorobenzoate and 2-chloro-5-hydroxybenzoate, respectively. Only benzoate was further degraded. Slightly altering the structure of the parent "benzoate molecule" resulted in observing reductive biotransformations other than dehalogenation. 2-Chlorobenzaldehyde was reduced to 2-chlorobenzyl alcohol by all four consortia. 2-chloroanisole was O-demethoxylated by three of the four consortia forming 2-chlorophenol. GC-MS analysis indicated reduction of the double bond in the propenoic side chain of 2-chlorocinnamate forming 2-chlorohydrocinnamate. None of the reduction products was dechlorinated. The following were not dehalogenated: 3- and 4-bromobenzoate; 3- and 4-chlorobenzoate; 2-, 3-, and 4-fluorobenzoate; 2-, 3-, and 4-iodobenzoate; 2-, 3-, and 4-chlorophenol; 2-chloroaniline; 2-chloro-5-methylbenzoate; 2,3-dichlorobenzoate; 2,5-dichlorobenzoate; 2,4,5-trichlorophenoxyacetic acid; and 2,4-dichlorophenoxyacetic acid. Consortia M34-9, P20-9, P21-9, and M50-7 dechlorinated 2-chlorobenzoate at -9 followed by those of M50-7 with rates declining above 2 and 3 mm 2CB, respectively. The major physiological types of microorganisms in consortia M34-9, P20-9, P21-9, and M50-7 were sulfate-reducing and hydrogen-utilizing anaerobes.
- Genthner, Barbara R. Sharak
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- Nucleophilic displacement at the benzoyl centre: A study of the change in geometry at the carbonyl carbon atom
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The second-order rate constants for the reaction between hydroxide ion and phenoxide ion with 4-nitrophenyl esters of substituted benzoic acids in 10% acetonitrile-water (v/v) solution obey Hammett σ correlations. The values of the Hammett ρ of 1.67 (κArO) and 2.14 (κOH) are consistent with a large change in hybridization at the central carbon by comparison with the ρ value for a standard reaction where a full sp2 to sp3 change occurs. The transition state for the concerted reaction thus has a substantially tetrahedral geometry. The observation of the anti-Hammond effect whereby the ρ value for the hydroxide ion exceeds that of the less reactive phenoxide ion is consistent with a concerted, ANDN, mechanism for these reactions. A stepwise mechanism, AN + DN, is unlikely to yield a measurable break in the Hammett correlation for a change in the benzoyl substituent if the partitioning of the putative tetrahedral intermediate involves forward and reverse reactions with Hammett correlations possessing similar ρ values.
- Colthurst, Matthew J.,Williams, Andrew
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p. 1493 - 1497
(2007/10/03)
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- Dissociation Constants of Weak Organic Acids in Protic Solvents Obtained from Their First Hyperpolarizabilities in Solution
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The first hyperpolarizabilities (β) of some weak aromatic organic acids have been measured in protic solvents by the hyper-Rayleigh scattering (HRS) technique at low concentrations.The measured hyperpolarizability (βm) varies between the two extreme limits: the hyperpolarizability of the acid form (βHA) at the lower side and that of the basic form (βA-) at the higher side.The degree of dissociation (α) of the acid in a solvent is related to the measured hyperpolarizability, βm, by the following relationship: βm2 = (1 - α)βHA2 + αβA-2.The calculated β's including solvent effects in terms of an Onsager field do not reproduce the experimentally measured hyperpolarizabilities.Other solvent-induced effects like hydrogen bonding and van der Waals interactions seem to influence the first hyperpolarizability and, thus, indirectly the extent of dissociation of these weak acids in these protic solvents.
- Ray, Paresh Chandra,Das, Puspendu Kumar
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p. 17891 - 17895
(2007/10/03)
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- Hammett Equation and Micellar Effects upon Deacylation
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Substituent effects upon second order rate constants of reaction of OH- with phenyl p-substituted benzoates at surface of micelles of cetyltrialkylammonium bromide (alkyl = Me, Et, Pr and Bu) and tetradecylquinuclidinium bromide fit the Hammett equation.Values of ρ increase with increasing bulk of the surfactant head group and are considerably larger than in water, corresponding to decreases in micellar surface polarity.The effects of substitution of the propyl group into the phenyl and benzoyl groups show that substrate orientation at micellar surface plays no significant kinetic role.
- Bartoletti, Antonella,Bartolini, Simona,Germani, Raimondo,Savelli, Gianfranco,Bunton, Clifford A.
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p. 723 - 728
(2007/10/02)
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- Kinetics and mechanism of the basic hydrolysis of indomethacin and related compounds: A reevaluation
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The kinetics of the hydrolysis of indomethacin and related compounds were studied in an alkaline medium at 25°. The pseudo-first-order rate constants were evaluated from log absorbance versus time plots in the ultraviolet. These compounds showed a second-order rate constant at low concentrations of hydroxide ion and a first-order rate constant at higher concentrations of hydroxide ion.
- Cipiciani,Ebert,Linda,Rubessa,Savelli
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p. 1075 - 1076
(2007/10/02)
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- Oxygenation by Superoxide Ion of CCl4, FCCl3, HCCl3, p,p'-DDT, and Related Trichloromethyl Substrates (RCCl3) in Aprotic Solvents
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In dimethylformamide (DMF) superoxide ion (O2-) oxygenates compounds with the trichloromethyl group: CCl4, HCCl3, and FCCl3 yield bicarbonate ion; PhCCl3 yields a mixture of PhC(O)OO- and PhC(O)O-; CF3CCl3 and HOCH2CCl3 give their carboxylate anions, RC(O)O-; and p,p'-DDT yields its dechlorinated product, DDE, which in turn reacts with O2- to give (p-ClPh)2C=O.Alkyl trichloromethyl compounds are unreactive within a 10-min reaction time at millimolar concentrations.The relative rates of reaction have been measured by the rotated ring-disc voltammetric method.On the basis of the relationship between the relative reaction rates and the electrophilic character of the substrates, as measured by the peak reduction potentials (Ep), the initial step is believed to be an electron transfer from the nucleophile to the electrophilic trichloromethyl group (a nucleophilic attack on chlorine with a concerted reductive displacement of Cl- and formation of RCCl2OO*).
- Roberts, Julian L.,Calderwood, Thomas S.,Sawyer, Donald T.
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p. 7691 - 7696
(2007/10/02)
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