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

Abstract

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.