Alkali metal cations can inhibit non-covalent catalysis

The study concerns the effect of inorganic salts on supramolecular catalysis. The model reaction is the acid hydrolysis of the ammonium phenyl acetate derivative promoted by cucurbit[7]uril macrocycle. When salt is absent, the macrocycle is insensitive to the ionic strength of the solution, and the reaction rate linearly depends on the concentration of hydronium ions (H3O+). After the addition of inorganic salts, in particular, Na+ and K+ ions, the catalytic effect of the macrocycle is suppressed. The kinetic and binding data collected by us evidence the formation of the ternary complexes between the cations, macrocycle, and substrate, which are less prone to H3O+ attack. This type of inhibition corresponds to a rare uncompetitive model in contrast to a more common competitive one that relies on the displacement of the substrate. This study shows that special care must be taken when studying catalysis in solutions that contain metal cations, such as regular water and inorganic buffers.