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

32 J.P. Glusker
Fig. 24. The magnesium-binding site in integrin [67]. Note that, two carboxylate groups, two
hydroxy groups and one water molecule bind in such a manner that two ring motifs (each
involving eight atoms) are formed. This coordination of magnesium ions serves to bind two
portions of the molecule together
protein folding is similar; while the interaction in this case may be termed
“intramolecular,” the components on the protein may be removed in sequence
from each other. Positively charged metal ions act as electrophiles, seeking electron
density on other atoms or ions. Therefore, in order to achieve local electrical
neutrality, metal cations attract around them those atoms that have some negative
charge.The interatomic distances between cations and anions will depend on
the strength of this charge-charge interaction. Thus the shorter a metal ion-oxygen
distance, the higher the negative charge on the oxygen. Assuming that the
cations and liganding anion atoms are hard spheres, the shortest distance
between them can be presumed to be the sum of their ionic radii. Ionic or atomic
radii are measured as half of the closest distance that two identical nonbonded
atoms or ions approach each other in crystal structures. Additional ionic radii
can be deduced from minimum distances between different ions.
The number of ions or atoms that pack around a metal ion in a crystal is referred
to as its coordination number of the metal ion. Its value is related to the ratio
of the radius of the cation to that of the anion that packs around it [26]. Small,
highly charged cations, such as Be 2+ and Al 3+ have low coordination numbers,
although they are usually higher in crystals than in gases or liquids. Larger
cations have higher coordination numbers, 6 being the most common value