Kinetic study of catalytic CO 2 hydration by metal-substituted biomimetic carbonic anhydrase model complexes

The rapid rise of the CO 2 level in the atmosphere has spurred the development of CO 2 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 (Zn 2+ , Cu 2+ and Ni 2+ ) 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(OH 2 )] 2+ < [(TPA)Ni(OH 2 )] 2+ < [(TPA)Zn(OH 2 )] 2+ . The effect of pK a on the CO 2 hydration rate was investigated by stopped-flow spectrophotometry. Because the water ligand in [(TPA)Zn(OH 2 )] 2+ had the highest pK a , it would be more difficult to deprotonate it than those coordinated to Cu 2+ and Ni 2+ . It was, therefore, expected that the complex would have the slowest rate for the reaction of the deprotonated water with CO 2 to form bicarbonate. However, it was confirmed that [(TPA)Zn(OH 2 )] 2+ had the fastest CO 2 hydration rate because the substitution of bicarbonate with water (bicarbonate release) occurred easily.