Theoretical Study of the Water Exchange Reaction on Divalent Zinc Ion using Density Functional Theory

Recent ab initio studies reported in the literature have challenged the mechanistic assignments made on the basis of volume of activation data 1,2]. In addition to that ab initio molecular orbital calculations on hydrated zinc(II)-ions were used to elucidate the general role of this ion in metalloproteins 3]. Due to our interest in both inorganic reaction mechanisms and enzymatic catalysis we started a systematic investigation of solvent exchange processes on divalent zinc-ion using density functional calculations. Our investigations cover aqua complexes of the general form Zn(H₂O)_n]²⁺·mH₂0 with n=3-6 and m=0-2, where n and m represent the number of water molecules in the coordination and solvation sphere, respectively.The complexes Zn(H₂O)₅]²⁺·2H₂O and Zn(H₂O)₄]²⁺·2H₂O turnend out to be the most stable zinc complexes with seven and six water molecules, respectively. This implies that a heptacoordinated zinc(II) complex, where all water molecules are located in the co-ordination sphere, should be energetically highly unfavorable and that Zn(H₂O)₆]²⁺ can quite readily push two coordinated water molecules into the solvation sphere. For the pentaqua complex Zn(H₂O)₅]²⁺ only one water molecule is easily lost to the solvation sphere, which makes the Zn(H2O)₄]²⁺·H₂O complex the most favorable in order to consider the limiting dissociative and associative water exchange process of hexacoordinated zinc(II). The dehydration and hydration energies using the most stable zinc(II) complexes Zn(H₂O)₄]²⁺·2H₂O, Zn(H₂O)₅]²⁺·2H₂O and Zn(H₂O)₄]²⁺·H₂O were calculated to be 24.1 and -21.0 kcal/mol, respectively.