Inhibition of the kinetics of chelation of nickel (II) with malonate ion by β-cyclodextrin

Relaxation times, determined by the pressure-jump technique, for the kinetics of chelation of nickel (II) by malonate ion are significantly prolonged by the addition of β-cyclodextrin to aqueous solutions.The effect is due to inclusion of the malonate into the β-cyclodextrin cavity. In fact addition of glucose in a 7:1 ratio to malonate ion does not affect the kinetics of complexation with Ni²⁺. The effect is also specific to the size of the ring since α-cyclodextrin and γ-cyclodextrin do not show any effect on the above relaxation times.The kinetic data are interpreted to indicate a competitive inhibition process. β-cyclodextrin is bound to the anion and prevents its chelation reaction with Ni²⁺. The mechanism treats the data quantitatively with an inclusion formation constant between malonate and β-cyclodextrin of the order of 10²M^?1.