Binding and catalysis with a metal-induced ternary complex of an ethylenediamine-substituted cyclodextrin

Abstract

The binary complex (CDen₂Cu) from 6-(2-aminoethyl)amino-6-deoxy-β-cyclodextrin (CDen) and Cu²⁺ binds methyl orange (MO) about twice as efficiently as CDen alone, whereas brilliant yellow (BY), owing to the extended shape of this substrate, shows an approximately sevenfold increase in K. Deacylation of p-nitrophenyl benzoate (PNPB) shows, with an ethylenediamine model compound, the expected rate enhancement, which is retarded upon addition of sufficient Cu²⁺ to block the N lone pairs. In sharp contrast, deacylation of PNPB is accelerated by CDen₂Cu, whereas the ‘shorter’p-nitrophenyl acetate (PNPA), being less suited to formation of a ternary inclusion complex, again exhibits a rate decrease. Catalytic rate constants k_c and Michaelis–Menten constants K_D are obtained from Lineweaver–Burk or Eadie plots; equilibrium constants for MO and BY have been studied at different wavelengths. The problems associated with the evaluation of the multiple variables in such equilibria are illustrated.