A Macrocyclic Enzyme Model System. Catalytic Properties of 10-Amino-[20]paracyclophane in the Deacylation of p-Nitrophenyl Carboxylates

Abstract

Abstract The catalytic efficiency of 10-amino[20]paracyclophane in the deacylation of p-nitrophenyl carboxylates was investigated in 10.9 or 20.8% (v/v) aqueous ethanol at μ=0.10 (KCl). The present catalyst exhibited marked catalytic effects not only in the free amine but in the ammonium form. The observed saturation-type kinetics is consistent with a reaction mechanism which involves pre-equilibrium complexation between the aminoparacyclophane and the substrate at a 1 : 1 molar ratio, followed by pseudo-intramolecular catalysis effected by the amino group of the macrocycle. Studies on the inhibition effect by 1-dodecanol and the modification of the catalyst by 2,4-dinitrofluorobenzene confirmed the effective binding ability of the present paracyclophane toward hydrophobic substrates. The free amine form of the catalyst acted as an effective nucleophile to give the acylated aminoparacyclophane as confirmed by the product analysis. On the other hand, the protonated amine form also enhanced the ester degradation, retaining a turnover behavior. On the basis of the kinetic solvent isotope effect and the exceedingly minor kinetic effect of [10-oxo[20]paracyclophan-22(23)-ylmethyl]trimethylammonium chloride in the ester degradation in the neutral pH region, a plausible reaction mechanism has been discussed.