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Bio-inorganic siderphores

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1. Siderophores and related synthetic analogues can be used in the treatment of patients, such as those with thalassemia, suffering from iron overload. The structure of one such potential ligand (1) is shown below:

Please see the attached document for the diagram.

(a) Identify the chelating group present in this molecule.
(b) This molecule forms a stable complex with Fe(III), with a formation constant of 10^43. Account for the high stability of this complex compared with that of the complex formed using the related ligand 2 which has an overall formation constant of 10^40.

Please see attached document for the diagram.

(c) If ethyl groups are added to the central benzene ring, as shown below, the stability constant of the complex increases to 10^47. Explain why this is the case.

Please see attached document for the diagram.

(d) The structure of desferrioxamine B (ligand 4) is shown below and this compound is also used to treat patients suffering from iron overload. Based on what you know about sideophores, identify the atoms in the ligand which are most likely to coordinate with Fe(III). What denticity does the ligand exhibit? Hence, determine the likely geometry around the Fe atom in the complex.

Please see attached document for the diagram.

Please note that you are NOT expected to include a diagram in your answer.

The formation constant for ligand 4 with Fe(III) is 10^30.6, which is higher than transferrin (10^20). Why is this important for the treatment of iron overload?

(e) Complexes of ligand 4 with Co(III) and Mn(III) have been studied using EXAFS spectroscopy.

The spectrum of the Co(III) complex showed 6 O atoms at a distance of 190 pm from the cobalt ion. However, the complex with Mn(III) produced an EXAFS spectrum showing 4 O at 195 pm and 2 O at 220 pm.

Account for the different geometries around the metal ion in the two complexes. You can assume that both complexes are weak-field.

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Solution Summary

Some problems centered on siderophores are solved.

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1a. All OH groups will chelate as they have lone pairs available for donation.

b. Both ligands can chelate through the OH groups. But because the 1st ligand has more chelating OH groups than the 2nd one, one would expect a more stable complex to form with ligand 1. Second, ligand 1 has an added aromatic ring in the center that can donate electron density into the outer rings which will then increase the chelation of the outer rings to the metal. This is not possible in ligand 2. Thirdly, because ligand 1 is macrocyclic, the metal is stabilized more due to the electron rich environment of ...

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