Kinetic study of catalytic CO₂ hydration by metal-substituted biomimetic carbonic anhydrase model complexes

The rapid rise of the CO₂ level in the atmosphere has spurred the development of CO₂ capture methods such as the use of biomimetic complexes that mimic carbonic anhydrase. In this study, model complexes with tris(2-pyridylmethyl)amine (TPA) were synthesized using various transition metals (Zn²⁺, Cu²⁺ and Ni²⁺) to control the intrinsic proton-donating ability. The pK_a of the water coordinated to the metal, which indicates its proton-donating ability, was determined by potentiometric pH titration and found to increase in the order [(TPA)Cu(OH₂)]²⁺ < [(TPA)Ni(OH₂)]²⁺ < [(TPA)Zn(OH₂)]²⁺. The effect of pK_a on the CO₂ hydration rate was investigated by stopped-flow spectrophotometry. Because the water ligand in [(TPA)Zn(OH₂)]²⁺ had the highest pK_a, it would be more difficult to deprotonate it than those coordinated to Cu²⁺ and Ni²⁺. It was, therefore, expected that the complex would have the slowest rate for the reaction of the deprotonated water with CO₂ to form bicarbonate. However, it was confirmed that [(TPA)Zn(OH₂)]²⁺ had the fastest CO₂ hydration rate because the substitution of bicarbonate with water (bicarbonate release) occurred easily.