The siderophores are naturally occurring small molecule chelates secreted by many microorganisms. One important example of a siderophore is desferrioxamine (DFO, desferral) (Figure 10.4C). A large amount of (hard) oxygen donor atoms in an octahedral geometry render it highly specific toward iron(III), with a stability constant of not less than 1031 M-1. The chelate is used in the treatment of acute iron poisoning and in certain cases of anemia where iron is accumulated in liver and heart. Binding constants for the corresponding Fe(II) complexes are much smaller due to larger ionic radius and smaller charge, and release of iron can be induced simply by reduction of Fe(III). Since the coordination chemistry of Al3+ is similar to that of Fe3+, DFO can also be used in cases of aluminum poisoning. Incidentally, chelate formation also lies behind the body's strategy of producing fever in cases of infections. The higher temperature reduces the bacteria's ability to synthesize particular iron-chelating ligands.

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