Polyethylenimines with Nucleophile Adducts

With a polyethylenimine containing 10% of its residues alkylated with dodecyl groups and 15 % alkylated with methyleneimidazole substituents, esterolysis is truly catalytic [16]. Table 3.2 compares the catalytic effectiveness of this polymer biocatalyst (synzyme) with that reported for other substances that accentuate nitrophenyl ester hydrolysis [17, 18]. Clearly, this polymer is nearly 300 times as effective as free imidazole, but it does not match chymotrypsin, even with the activated unnatural nitrophenyl ester substrate, let alone peptide substrates. 

Nucleophiles other than imidazole have also been coupled to polyethylenimines and other polymers. These include hydroxamate [19-23], mercaptan [24, 25], aldoxime [26], and triazine [27] functionalities. In general, these moieties are active in transferring the acyl group of nitrophenyl esters to the nucleophile, but deacylation of the acylnu-cleophile is slow. 

Conversely, dimethylaminopyridines attached to polymers are remarkably effective catalysts of the solvolysis of esters, with true turnover [28-30], 

Pyridine itself attached to polyethylenimines (through a reduced Schiff base linkage) shows no catalysis, evidently because in the polymer environment both the pKj and nucleophilicity of the ring nitrogen are reduced substantially. However, 4-N,N-dialky- 

The pKj of the aminopyridines attached to the polymer (Table 3.3) are invariably lower than those of the corresponding isolated small molecules. The cationic character of polyethylenimine produces an electrostatic field that weakens the basicity of the nucleophile by 2-3 pK units. A lowering of an additional unit ocurrs when a long apolar group is attached to the aminopyridine moiety. Evidently, in this molecule the apolar environment favors further the uncharged, nonprotonated form of the pyridine. 

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The influence of the polymer on the intrinsic effectiveness of the nonprotonated pyridine moiety adduct can be assessed by comparison of the normalized rates k2(norm.), i.e., those expressed in terms of the concentration of >N- species in the solution. The intrinsic activity of the most effective nucleophiles (Table 3.3) is enhanced by a factor of 35, when it is attached to polyethylenimine. This increase is in addition to that due to the ability of the polymer at any pH to increase the fraction of aminopyridine in the uncharged >N- form. For a specific nucleophile, for which polymer adducts have been prepared with different aminopyridine contents, the catalytic effect, per pyridine, increases with decreasing residue concentration of the nucleophile (Table 3.3). 

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