Effects of Surfactants on Solvolysis of p-Nitrophenyl Alkanoates Catalyzed by 4-(Dialkylamino)pyridine-Functionalized Polymer in Aqueous Methanol Solution

Abstract

Catalysis of the solvolysis of p-nitrophenyl alkanoates 2 (n ) 2-18) by 4-(dialkylamino)pyridinefunctionalized polymer 1 in the presence of sodium n-dodecyl sulfate (SDS), n-dodecyltrimethylammonium bromide (DTAB), and heptaethylene glycol n-dodecyl ether (HEGDE) was investigated in 50:50 (v/v) methanol-water buffer (0.05 M H2PO4 -/HPO4 2-, pH 8.0) solution. Strikingly, a large rate enhancement is observed in the presence of anionic surfactant SDS for the solvolysis of 2 (n ) 10-18) as compared with the corresponding reaction catalyzed by 1 or SDS alone. SDS is believed to bind both substrate and catalyst within micellar aggregates and influence reactivity by a combination of catalyst-substrate proximity and micellar microenvironment. In the presence of HEGDE, no significant effects on the 1-catalyzed solvolysis of 2 (n < 8) were observed. At (2.5-7.5) × 10 -5 unit mol L -1 1, the solvolysis rate exhibits a modest decrease for 2 (n ) 10 and 12) and a modest increase for 2 (n ) 14, 16, and 18) with added nonionic surfactant HEGDE. The solvolysis rate for 1-catalyzed solvolysis of 2 (n ) 2-18) was found to be unchanged by cationic surfactant DTAB in aqueous methanol solution.

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

“…The macromolecule 1 is an amphiphilic polymer containing distinct hydrophobic and hydrophilic regions. The catalytic center of macromolecule 1 is the 4-(dialkylamino)pyridine group and the hydrophobic association of substrate to catalyst in the reaction medium is responsible for the rate enhancements observed in the 1 -catalyzed solvolysis reaction of 2 . − …”

“…Materials and Reagents. Synthesis of the poly(siloxane-bis(trimethylene)) supported 4-(dialkylamino)pyridine ( 1 ) has been described previously 6c…”

“…Depending on the copolymer composition and the surrounding medium, amphiphilic macromolecules can also form aggregates with multiple morphologies . Extensive studies have shown that the increase of hydrophobic effects of amphiphilic macromolecules, i.e., the increase in ratio of hydrophobic to hydrophilic components and the addition of salting-out agents, leads to changes of aggregate morphology from spheres to rods, and to vesicles in appropriate solvents. , 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. − Recently, we have reported ion-induced substrate specificity in solvolysis of p -nitrophenyl alkanoates 2 ( n = 2−18) catalyzed by polymer 1 containing the 4-(dialkylamino)pyridine functionality and a bis(trimethylene)disiloxane backbone (Scheme ) . The tris(hydroxymethyl)methylammonium ion as a salting-in ion induces the same substrate specificity for 2 ( n = 6) in aqueous Tris buffer solution that is obtained with cholesterol esterase for the same hydrolysis reaction .…”

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

“…The addition of salting-out agent NaCl leads to a change of substrate specificity from 2 ( n = 14) to 2 ( n = 12) in 1:1 (v/v) methanol−aqueous phosphate buffer solution , Moreover, we have reported an interesting example of electrostatic interactions that contribute to the catalytic activity and substrate specificity for the 1 -catalyzed solvolysis of 2 ( n = 2−18) in the presence of anionic surfactant sodium dodecyl sulfate (SDS), which provides significant insight into the mechanism of catalytic ester solvolysis. These results may be akin to the observations that many enzymes usually accelerate the hydrolytic reactions at hydrophobic binding sites by electrostatic stabilization of the transition state through interactions with nearby amino acid constituents. , To our knowledge, the control of substrate specificity by changing the buffer system has not yet been reported for catalysis of solvolysis reactions of p -nitrophenyl esters.…”

“…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 …”