Effects of transition metal ions on the catalytic activity of carbonic anhydrase mimics

Many carbonic anhydrase (CA)-mimetic catalysts have active sites composed of a Zn ion coordinated with multiple nitrogens of azacyclic compounds. In this study, some divalent transition metal ions (Co²⁺, Cd²⁺, and Ni²⁺) were examined as potential substitutes for the Zn cofactor when preparing CA-mimetic catalysts by means of self-assembly of histidyl bolaamphiphiles. The self-assembly of histidyl bolaamphiphiles yielded a densely packed histidine imidazole sites offering metal complexation places similar to the triple-histidine imidazoles of natural CA active sites. In the catalytic hydrolysis of p-nitrophenyl acetate, the catalytic efficiency of the mimetic catalysts agreed with the order of catalytic activities of metal-substituted carbonic anhydrases: Zn > Co > Cd ≈ Ni; activation energy of the CA-mimetic catalysts and practical CO₂ hydration efficiency also had the same order. However, Co showed better catalytic activity than Zn when the concentrations of metal salts were higher than 0.1 mM because of weakened inhibition by the counter anion. This study elucidated the effect of transition metal ions on the catalytic efficiency of a CA-mimetic catalyst, with emphasis on the coordination of transition metal ions.