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.