198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

Directional Aspects of Intermolecular Interactions 31
Fig. 23 a – c. The a-hydroxycarboxylate group [66]: a ionized group; b the effect of binding a
metal ion; c the effect of replacing the hydroxyl group by fluorine
5.4
Surroundings of -Hydroxycarboxylate Groups
Chelating groups that bind metals include hydroxamates, some of which are
powerful enough to extract iron from stainless steel, and the a-hydroxycarboxylate
groups, HO-CR 2-COO – (R=any group), found in many important biochemical
compounds such as citrates and malates [65]. X-ray crystallographic
studies indicate that, if the cation is of a suitable size, the entire chelating group
is approximately planar, even though the O◊◊◊O distance is shorter than the sum
of the van der Waals radii for oxygen atoms. A study of one form of potassium
citrate revealed, surprisingly, that the potassium ion did not chelate the
a-hydroxycarboxylate, but that the hydroxyl hydrogen atom formed an internal
hydrogen bond. This was verified by a neutron diffraction study [66]. When the
central hydroxyl group of citric acid is replaced by fluorine, the a-hydroxycarboxylate
group is still a good chelating group, but no hydrogen atom is available
to form an internal hydrogen bond. Therefore, in dipotassium 3-fluorodeoxycitrate,
a potassium cation is chelated by the a-fluorocarboxylate group. This is
shown in Fig. 23.
6
Metal Ion-Based Directional Interactions
The role of hydrogen bonding as a means of binding molecules together in a
directional manner has been described. The effect of replacing the hydrogen
atom by a metal ion, which is much larger, will now be explored. As will be
shown, a metal ion can bring molecules together in a manner analogous to that
of a hydrogen bond. A specific example is provided by the magnesium-binding
site of the protein integrin [67], diagrammed in Fig. 24. The role of metal ions in