Polymer Concentration-Controlled Substrate Specificity in Solvolysis of <i>p</i>-Nitrophenyl Alkanoates Catalyzed by 4-(Dialkylamino)pyridine-Functionalized Polymer in Aqueous Methanol Solution

Wang, Guang-Jia; Fife, Wilmer K.

doi:10.1021/ja970407c

Cited by 16 publications

(15 citation statements)

References 37 publications

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“…Without 1 , the solvolysis rate of 2 ( n = 2−18) is also very slow, and no substrate specificity is found in 1:1 (v/v) methanol−aqueous Tris buffer solution. As indicated in Figure , in the absence of 1 an increase of the alkanoate chain length in 2 causes minor decreases in the solvolysis rates, which is in accordance with previous observations in 1:1 (v/v) methanol−aqueous phosphate buffer solution . However, in the presence of 1 the solvolysis rates of 2 ( n = 2−18) are much faster and increase significantly with increasing the concentration of 1 , indicative of catalytic solvolysis reactions.…”

Section: Resultssupporting

confidence: 91%

“…4-(Dialkylamino)pyridine-functionalized polymers have been regarded as useful and simple model systems for obtaining a better understanding of the origins of enzymic efficiency and selectivity. − We have recently made an attempt to investigate such a model system to gain insight into the dominant control factors in solvolysis of p -nitrophenyl esters 2 ( n = 2−18) catalyzed by 4-(dialkylamino)pyridine-functionalized polymer 1 . − The mechanism of the reaction involves the attack by nucleophile 1 at the carbonyl group of substrates 2 and the formation of an N -acylpyridinium intermediate where the breakdown of the intermediate is the rate-determining step in the catalytic reactions . Strikingly, we have found ion-induced substrate specificity in the 1 -catalyzed solvolysis of 2 ( n = 2−18) in aqueous and methanol−water solutions. , Salting-in effects of the tris(hydroxymethyl)methylammonium ion in aqueous Tris buffer solution lead to the same substrate specificity for 2 ( n = 6) that is obtained with the enzyme, cholesterol esterase, for the same hydrolysis reaction …”

Section: Introductionmentioning

confidence: 99%

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Control of Substrate Specificity in Polymer-Catalyzed Solvolysis Reactions of p-Nitrophenyl Alkanoates by Changing the Buffer System

Wang

Fife

1999

Macromolecules

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The substrate specificity in solvolysis reactions of p-nitrophenyl alkanoates 2 (n = 2−18) catalyzed by 4-(dialkylamino)pyridine-functionalized polymer 1 was examined in buffered aqueous methanol solution at pH 8.0 and 30 °C. The chemical reactivity and substrate specificity in this catalytic system were found to be controlled by changing the buffer system. In 1:1 (v/v) methanol−aqueous phosphate buffer solution, macromolecule 1 exhibits substrate specificity for 2 (n = 14) below 1.0 × 10-5 unit mol L-1, and the substrate specificity changes from 2 (n = 14) to 2 (n = 12) as the concentration of 1 increases to 2.5 × 10-5 unit mol L-1 and changes again from 2 (n = 12) to 2 (n = 10) when the concentration of 1 increases further to 7.5 × 10-5 unit mol L-1. However, in 1:1 (v/v) methanol−aqueous Tris buffer solution, macromolecule 1 was found to demonstrate the same substrate specificity for 2 (n = 14) when the concentration of 1 is increased from 5.0 × 10-6 to 1.0 × 10-4 unit mol L-1. The control of substrate specificity by the change of the buffer system is believed to be unprecedented for catalysis of ester solvolysis.

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Section: Resultssupporting

confidence: 91%

Section: Introductionmentioning

confidence: 99%

Control of Substrate Specificity in Polymer-Catalyzed Solvolysis Reactions of p-Nitrophenyl Alkanoates by Changing the Buffer System

Wang

Fife

1999

Macromolecules

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“…The majority of antibody-catalyzed reactions have been aryl ester and amide hydrolyses. 1a,b Such reactions are not only of interest in the catalytic antibody field but have also found application in many other disciplines such as physical organic, 3 biological, 4 bioinorganic, 5 polymer 6 and supramolecular chemistry. 7 In particular, p-nitrophenyl esters have been used in nu- merous physical organic and biochemical studies due to the ease of detecting the p-nitrophenoxide anion produced in base-catalyzed hydrolysis.…”

Section: Introductionmentioning

confidence: 99%

Fidelity in Hapten Design: How Analogous Are Phosphonate Haptens to the Transition States for Alkaline Hydrolyses of Aryl Esters?

Tantillo

Houk

1999

J. Org. Chem.

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Is antibody catalysis of hydrolysis reactions limited by imperfect mimicry of transition states by haptenic analogs? This question was explored by comparison of calculated transition states for the hydrolyses of aryl esters and the haptens intended to mimic them. The addition-elimination reaction of phenyl acetate and hydroxide ion in the gas phase was examined using ab initio calculations at the RHF/6-31+G(d)//RHF/6-31+G(d) and MP2/6-31+G(d)//RHF/6-31+G(d) levels of theory. Although transition states for both addition and elimination were located, the barrier to elimination disappeared when zero point energy or electron correlation was included. The reaction is therefore concerted, and the transition state for addition is the only relevant transition state in the gas phase. When aqueous solvation is considered through PCM and SCI-PCM calculations, the barrier to addition is shown to increase dramatically, and a stepwise addition-elimination pathway is predicted. A single transition state for the analogous reaction with p-nitrophenyl acetate was also located, and this reaction is predicted to be concerted in both the gas phase and in solution. The calculated geometries, CHELPG atomic charges, and electrostatic potential surfaces of stationary points involved in hydrolysis were compared with those of model phosphonate and phosphinate haptens. The haptens are better mimics of the two transition states and the tetrahedral intermediate than of the reactants or products-exactly the situation desired for catalyst generation-yet they are more analogous to the tetrahedral intermediate and the transition state for elimination than to the rate-determining addition transition state. In addition, the hydrogen-bonding pattern and asymmetry of the stationary points are not faithfully reproduced by the haptens. These considerations suggest that catalysis may be improved by hapten redesign or antibody mutagenesis.

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“…First, temperature dependence of enzymatic catalysis is gradual, whereas gel catalysts can perform an infinitely sharp, discontinuous transition. Although there are quite a few studies successfully mimicking various aspects of enzymes by using synthetic polymers, only gradual change in catalytic activity in response to changing environment has been reported (19)(20)(21)(22). Second, the environmental condition for enzymatic activity is extremely limited and narrow.…”

Section: Discussionmentioning

confidence: 99%

Gel catalysts that switch on and off

Wang

Kuroda

Enoki

et al. 2000

Proc. Natl. Acad. Sci. U.S.A.

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We report development of a polymer gel with a catalytic activity that can be switched on and off when the solvent composition is changed. The gel consists of two species of monomers. The major component, N-isopropylacrylamide, makes the gel swell and shrink in response to a change in composition of ethanol͞water mixtures. The minor component, vinylimidazole, which is capable of catalysis, is copolymerized into the gel network. The reaction rate for catalytic hydrolysis of p-nitrophenyl caprylate was small when the gel was swollen. In contrast, when the gel was shrunken, the reaction rate increased 5 times. The activity changes discontinuously as a function of solvent composition, thus the catalysis can be switched on and off by an infinitesimal change in solvent composition. The kinetics of catalysis by the gel in the shrunken state is well described by the Michaelis-Menten formula, indicating that the absorption of the substrate by the hydrophobic environment created by the N-isopropylacrylamide polymer in the shrunken gel is responsible for enhancement of catalytic activity. In the swollen state, the rate vs. active site concentration is linear, indicating that the substrate absorption is not a primary factor determining the kinetics. Catalytic activity of the gel is studied for substrates with various alkyl chain lengths; of those studied the switching effect is most pronounced for p-nitrophenyl caprylate.N atural enzymes catalyze chemical reactions, and equally importantly, regulate them by reversibly and repeatedly switching on and off the catalytic activities. To mimic this special feature of enzymes, we designed a polymer gel consisting of two species of monomers, each having a specific role. The major component allows the gel to reversibly swell and shrink in response to changes in environmental parameters such as temperature and solvent (1-9). On swelling and shrinking, the local density of the hydrophobic moiety changes and the affinity to substrate molecules is altered accordingly. The minor component capable of catalysis of substrate decomposition is copolymerized with the responsive monomers into the gel network. The catalytic activity of the gel is expected to be switched on and off as the substrate is reversibly bound and released during the cycle of gel swelling and shrinking. Materials and MethodsTo demonstrate the feasibility of the idea, we chose hydrolysis of p-nitrophenyl esters with different alkyl chain lengths as the chemical reaction to examine (Fig. 1a). This reaction is known to be catalyzed by 4(5)-vinylimidazole (10). We, therefore, used it as the minor catalytic monomer component of the gel (Fig. 1b). As the major component, we chose the widely used hydrophobic monomer N-isopropylacrylamide (NIPA). The substrate and gel monomer components were selected because (i) the substrate and one of the hydrolysis products, p-nitrophenol, were easily detected by using UV spectroscopy; (ii) the NIPA gel underwent a discontinuous swelling͞shrinking transition in response to changes in environmen...

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Polymer Concentration-Controlled Substrate Specificity in Solvolysis of p-Nitrophenyl Alkanoates Catalyzed by 4-(Dialkylamino)pyridine-Functionalized Polymer in Aqueous Methanol Solution

Cited by 16 publications

References 37 publications

Control of Substrate Specificity in Polymer-Catalyzed Solvolysis Reactions of p-Nitrophenyl Alkanoates by Changing the Buffer System

Control of Substrate Specificity in Polymer-Catalyzed Solvolysis Reactions of p-Nitrophenyl Alkanoates by Changing the Buffer System

Fidelity in Hapten Design: How Analogous Are Phosphonate Haptens to the Transition States for Alkaline Hydrolyses of Aryl Esters?

Gel catalysts that switch on and off

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