Electrostatic terms cooperativity

This review article summarizes our more recent investigations of the esterolytic action of water soluble imidazole containing polymers. In our previous work we have explained increased reactivity of polymeric reactants in terms of electrostatic and cooperative effects. Recently, we have directed our efforts in emphasizing the apolar interaction as it has reflected dramatic rate enhancements and proven a predominant factor in determining the maximum catalytic efficiency. Polymeric catalysts which are included in this review are poly[l-alkyl-4(5)-vinyl-imidazoles] and their copolymers, hydro-phobic terpolymers of 4(5)-vinylimidazole, copoly[vinylamine/4(5)-vinylimidazole], and poly(ethylenimine-g L-histidine). All of these catalysts share the common property of water solubility and the capability of attaining high catalytic efficiencies which is attributed to apolar interactions. 

The interaction of Ca2+ with pectins is discussed. The role of carboxylic acid salt formation and the degree of polymerization are first considered in terms of electrostatic and/or cooperative specific interactions. Then the effect of the degree of esterification and that of the pattern of carboxylic group distribution are discussed pectin esterase forms blocks which behave as fully hydrolyzed polymers and favor aggregation. Finally, the role of the calcium addition on the degree of aggregation was established. All the data show the important role of molecular structure of the pectins on calcium interactions. 

In the area of catalysis, the esterolysis reactions of imidazole-containing polymers have been investigated in detail as possible models for histidine-containing hydrolytic enzymes such as a-chymotrypsin (77MI11104). Accelerations are observed in the rate of hydrolysis of esters such as 4-nitrophenyl acetate catalyzed by poly(4(5)-vinylimidazole) when compared with that found in the presence of imidazole itself. These results have been explained in terms of a cooperative or bifunctional interaction between neighboring imidazole functions (Scheme 19), although hydrophobic and electrostatic interactions may also contribute to the rate enhancements. Recently these interpretations, particularly that depicted in Scheme 19, have been seriously questioned (see Section 1.11.4.2.2). 

In the PEC system of interest, complexation effects are considered to be characterized by several interactions cooperative, concerted, complementary and those due to microdomains [42]. Individual contributions are represented by a free energy thermodynamic function. For a PEC, the predominant term is the electrostatic interaction. Other terms include hydrogen bonding, hydrophobic interactions and van der Waals forces. Because individual components are difficult to evaluate [43] and their ratio is impossible to control independently, a superposition of different interactions is suggested. This approach is used in the present work. 

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