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

34 J.P. Glusker
structures of magnesium-containing molecules in the CSD and PDB and by ab
initio molecular orbital calculations. The magnesium ion maintains water in its
inner coordination sphere in an orientation in which the Mg 2+ ◊◊◊OH angle is
120–130°. This then orients groups in the second coordination sphere that are
bound to these inner sphere water molecules. This means that magnesium ions
are involved in precise orientations of functional groups such as phosphate
groups in proteins. In a motif involving an additional water molecule (see
Fig. 26) [58], this orientational capability is further accentuated, as shown in
Fig. 27.
6.3
Zinc Binding
Zinc ions have approximately the same size and charge as magnesium ions, but
different chemical characteristics because zinc is a transition element and
magnesium is not. This difference is evident in its coordination behavior. Unlike
magnesium, zinc exhibits coordination numbers of 4, 5, and 6 with almost no
energy consequences for changes between these [70]. This is in line with the way
in which these metal ions are used in biological systems; magnesium ions generally
play a structural role, while zinc ions are more likely to be directly involved
a b
c
Fig. 26 a – c. Motifs found in magnesium-binding proteins: a the ring noted in Fig. 24; b an
aquated carboxylate motif; c stereoview of the motif found in cobalt formate [63]. These
motifs are also used by other metal ions that have similar sizes and ligand preferences