Hydrophobic interactions in catalysis by imidazole-containing polymers

Abstract

The catalytic activity of imidazole-containing polymers in the hydrolyses of substrates with poor leaving groups was examined. Hydrolyses of p -methoxyphenyl esters (S n ) catalyzed by poly[4(5)-vinylimidazole] (pvIm) in relation to imidazole (Im) indicates that both cooperative and hydrophobic interactions are operative. Hydrolyses of 3-methoxy-4-acyloxybenzoic acid substrates (S n - ) catalyzed by pvIm and a water-soluble copolymer, copoly[1-methyl-4-vinylimidazole/4(5)-vinylimidazole], exhibit many characteristics of enzyme-catalyzed reactions like saturation kinetics, bellshaped pH-rate profiles, and nonproductive binding. The importance of general-base, cooperative interactions for substrates with poor leaving groups and hydrophobic interactions in the formation of a stable catalyst-substrate complex in the case of long-chain esters is demonstrated. The interesting similarity between the Michaelis constants K m for the two polymer catalysts, pvIm and the copolymer, suggests that the common underlying principle involved in binding is hydrophobic interactions and the presence of N[bond]CH 3 group in the copolymer does not increase the hydrophobicity significantly. The pH-rate profiles for the hydrolyses of S n - by pvIm show that the optimum pH is around neutrality, which indicated that the presence of neutral Im units to serve as nucleophiles and protonated Im units to serve as electrostatic binding sites is essential for maximum catalytic efficiency. The rate enhancement as a function of acyl chain length at different pH leads to the conclusion that a cooperative effect between electrostatic and hydrophobic interactions would reduce the nonspecificity of hydrophobic interactions and result in their better realization. An approximate calculation shows that the binding of S 12 - , in relation to S 7 - , with the copolymer in aqueous solution corresponds to the additional interactions of two methylene units. Our recent studies 1–10 in the field of catalysis by polymeric imidazoles are oriented mainly toward the recognition of the significance of hydrophobic interactions. The role of hydrophobic interactions in enzymatic catalysis 11–13 and synthetic macromolecular catalysts 1–10,14–29 has been understood only recently. Hydrophobic interactions describe the tendency of nonpolar groups to associate themselves in aqueous solution 30,31 . Because the catalytic mechanism generally involves the prior complexation of the catalyst with the substrate and the catalyst-substrate complex is apolar, the influence of hydrophobic interactions in catalysis is conceivable. The favorable free energy of formation of hydrophobic interaction leads to better binding and better catalysis. In the present study we examined the hydrolytic reactions of the following substrates with poor leaving groups, catalyzed by imidazole-containing vinyl polymers, poly[4(5)-vinylimidazole] (pvIm), and a water-soluble copolymer, copoly[1-methyl-4-vinylimidazole/4(5)-vinylimidazole], ∼(1:1) M , Esters of varying acyl chainlength are chosen to determine the influence of hydrophobic interactions.