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.
Quantitative Interpretation of Multifrequency Multimode EPR Spectra of Metal Containing Proteins, Enzymes, and Biomimetic Complexes
