Hydrolytic metalloenzyme models. Micellar effects on the activation of the hydroxyl groups of N-alkyl-2-(hydroxymethyl)-imidazole ligands by Cu2+ in the transacylation of p-nitrophenyl picolinate.

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The activation of the hydroxyl groups of two N-methyl/dodecyl-2-(hydroxymethyl)imidazole ligands by complexation with Cu2+ was investigated kinetically by observing the rates of the release of p-nitrophenol in the transacylation of p-nitrophenyl picolinate (PNPP). The kinetics were carried out in aqueous buffers of pH ranging from 4.5 to 8.0 at 25 °C in the absence of surfactant micelles for a hydrophilic N-methyl ligand, and in the presence of micelles for a lipophilic N-dodecyl ligand. The kinetic analyses indicated that the 1:1 and 2:1 complexes of the ligand and Cu2+ are active nucleophiles with the N-methyl and N-dodecyl ligand, respectively. The pKa's of the hydroxyl groups of these complexes were determined to be 7.00 in water and 6.41 in aqueous CTABr micelles, respectively, by analyses of the pH-rate profiles. The rate constants, which show the nucleophilic reactivities of ionized hydroxyl anions of complexes toward PNPP, were determined to be kN=1.11×104 and 1.25×106 mol−1 dm3 s−1 with N-methyl and N-dodecyl ligands, respectively, indicating that the latter micellar complex is 100-times more active than the former non-micellar counterpart.
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