Calcium Ion Coordination: A Comparison with That of Beryllium, Magnesium, and Zinc

The coordination geometry of divalent calcium ions has been
investigated by analyses of the crystal structures
of small molecules containing this cation that are found in the
Cambridge Structural Database, protein crystal structures
in the Protein Databank, and by ab initio molecular orbital
calculations on hydrated structures of the form
Ca[H2O]n2+·mH2O,
in which there are n water molecules in the first
coordination shell and m water molecules in the
second coordination shell (hydrogen bonded to water molecules in the
first shell). Calcium ions in crystal structures
generally bind to oxygen atoms in ligands (rather than any other
element), and their preferred coordination numbers
range from 6 to 8. In protein crystal structures the tendency of
calcium to bind water molecules is less than for
magnesium (1.5 versus 2.2 water molecules on the average per metal ion
site, respectively). The ratio of bidentate
to monodentate binding of calcium ions to carboxylate groups is similar
for small molecules and protein structures
in that no bidentate binding occurs if the coordination number of
Ca2+ is 6, but its occurrence rises to near 20%
for
coordination numbers 7 and 8. Complexes of the form
Ca[H2O]52+·H2O
and
Ca[H2O]42+·2H2O
were found (by ab
initio molecular orbital calculations in vacuo) to be
significantly higher in energy than
Ca[H2O]62+ (by 8.2 and
15.0
kcal/mol, respectively). For Ca2+ surrounded by
seven or eight water molecules, the differences in energy
between
Ca[H2O]62+·H2O
and Ca[H2O]72+ and among
Ca[H2O]62+·2H2O,
Ca[H2O]72+·H2O,
and Ca[H2O]82+ are
extremely
small when diffuse functions are included in the basis set. Thus,
the net energy penalty for changing the number of
water molecules in the first coordination shell between 6 and 8 is
small. Molecular orbital calculations also indicate
that the effect of a calcium ion on the H−O−H angle to bound water
is less (at normal coordination numbers) than
that of magnesium, zinc, or beryllium.