Biological Ligands for Metal Ions

More Complex Cofactors — MoCo, FeMoCo, P-clusters, H-clusters, and Cu2 Siderophores 

In Chapter 2, we explained the basic notions involved in the coordination chemistry of metal ions. We now consider potential ligands, which could be involved in binding metals in metalloproteins. We already defined ligand binding as the affinity of the metal ion for any atom, group, or molecule that is attached to the central metal ion. We can divide them into three categories  

As was pointed out in Chapter 2, biologically important metal ions and their ligands can be classified according to the hard-soft theory of acids and bases (Table 2.1). While there are exceptions, most metal ions bind to donor ligands as a function of preferences based on this concept, with hard acids (metal ions, like Na+, K+, Ca , Mg , and Fe ) binding preferentially to hard bases (ligands, like O), and soft acids (like Cu+) to soft bases (like S and N). 

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When the two partial sequences stemming from the central NH group are chemically and stereochemically identical, Ci symmetric products result. We consider such compoimds as candidates for biological smdies, as well as models of C2 chiral auxiliaries for asymmetric induction (ref. 23), or ligands for metal ions. Syntheses of C2 symmetric compounds requiring the use of chiral auxiliaries have been recently reported (ref. 24). 

Figure 1 Thiamine, vitamin B1, is the cofactor, as its pyrophosphate ester, for many important biologic reactions. These reactions involve the formation of an anion 2 that is stabilized by resonance with a carbene form 3. The related species 4 and derivatives have been developed as important ligands for metal ions in chemical synthesis.

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