HYDRATION OF ZINC IONS - A COMPARISON WITH MAGNESIUM AND BERYLLIUM IONS

The coordination geometry of divalent zinc cations has been investigated by analyses of the crystal structures of small molecules containing this cation that are found in the Cambridge Structural Database and by ab initio molecular orbital calculations on hydrated structures of the form Zn[H2O](2+)(n). mH(2)O, in which there are n water molecules in the first coordination shell and m water molecules in the second coordination shell. Zinc ions in crystal structures are more commonly found to bind nitrogen and sulfur atoms, in addition to oxygen, while magnesium ions have a tendency to bind oxygen atoms. While most magnesium ion complexes have a metal ion coordination number for six, zinc ion complexes show coordination numbers that are generally four, five, and six. The higher of these coordination numbers for zinc (six) is primarily found when oxygen (or to a lesser extent, nitrogen) is bound, and the lowest when sulfur is bound. Ab initio molecular orbital studies of aquated zinc ions show that the total molecular energies of the three gas-phase complexes Zn[H2O]6(2+), Zn[H2O](2+)(5). H2O, and Zn[H2O](2+)(4). 2H(2)O and Mg[H2O](2+)(5). H2O, respectively, while Be[H2O](2+)(4). 2H(2)O is 22 kcal/mol lower in energy than Be[H2O](2+)(6), and no stable form with five water molecules in the first coordination sphere of a beryllium ion could be found. Thus the energy penalty for changing the local environment (coordination number) of divalent zinc ions surrounded by water is significantly less than that for the corresponding magnesium and berryllium ions. This is in line with the modes of utilization of these cations in enzyme systems, where magnesium ions play a more structural role than do zinc ions which, when bound to oxygen or nitrogen, tend to be involved in catalytic processes, possibly involving coordination number change. The effects of Be2+, Mg2+, and Zn2+ ions on water molecules bound in the first coordination sphere have been assessed by use of values of the H-O-H angles from the ab initio molecular orbital studies. It is found that this angle is increased from 105.5 degrees in an isolated water molecule to average values of 106.7 degrees for magnesium, 107.1 degrees for zinc, and 108.8 degrees for beryllium complexes. These values are even larger when other water molecules in the second hydration sphere that are hydrogen bonded to water molecules in the first hydration sphere are taken into account in the calculations, but the overall trend remains the same. This order of the effect of these cations presumbly expresses the extent of polarization of water molecules by each metal cation.