Metal siderophore complex

Is the intact metal-siderophore complex always transported into the cell (as in the hydroxamate siderophores known as ferrichromes) or is ferric ion alone transferred to a cell-wall-bound transport system ... 

What are the detailed molecular geometries of various kinetically inert metal-siderophore complexes ... 

Answers to these questions may be obtained by studying the structure (by single crystal x-ray diffraction) and the transport of specific coordination isomers of substitution-inert metal-siderophore complexes. Related work on metal siderophores has been reviewed recently (3, 9). In this paper we summarize our results on studies of the synthesis and characterization of... 

As mentioned previously, siderophores must selectively bind iron tightly in order to solubilize the metal ion and prevent hydrolysis, as well as effectively compete with other chelators in the system. The following discussion will address in more detail the effect of siderophore structure on the thermodynamics of iron binding, as well as different methods for measuring and comparing iron-siderophore complex stability. The redox potentials of the ferri-siderophore complexes will also be addressed, as ferri-siderophore reduction may be important in the iron uptake process in biological systems. 

Another possible route for reduction of the iron center is photoreduction. This has been studied in a variety of marine siderophore systems, such as aquachelin, marinobactin, and aerobactin (2), where it was demonstrated that photolytic reduction was due to a ligand-to-metal charge transfer band of the Fe(III)-siderophore complex, eventually resulting in reduction ofiron(III) and cleavage of the siderophore (31,154,155). This suggests a possible role for iron reduction in iron release (71,155). 

Both A A and cis trans equilibria of siderophore complexes can exist in solution. The chirality of the ligand can impose a preferred metal-center chirality. In addition, the degree of this preference depends on the stereochemical rigidity of the ligand. In principle, the magnitude of the molar circular dichroism can be used as a measure for diastereoisomeric equilibria based on a comparison of the solid-state and solution ellipticity. Nevertheless, predictions of metal-center chiralities require theoretical calculations. For example, empirical-force-field calculations of iron(III) enterobactin show that the A orientation at the metal center is more stable than the A by 0.5 kcalmoH, which is consistent with the CD spectra. ... 

Some metals may need to be mobilized from the environment to make them bioavailable. Iron in particular must be rendered more soluble to be accessible for uptake. Microorganisms and some plants have evolved with secreted ligands known as siderophores (or phytosiderophores). These ligands bind Fe + with extraordinary affinity. For example, a complex of the siderophore enterobactin with ferric iron has a formal stability constant of 10 (19). Once siderophores compete with other environmental ligands for iron, the ferric iron-siderophore complex then binds to specific transport proteins at the microbial... 

As with hydroxamate siderophores, simple tris(catecholato) metallate(lll) complexes have served as models for enterobactin. Unlike hydroxamate, catecholate is a symmetric, bidentate ligand. Consequently, there are no geometrical isomers of simple tris(catecholato) metal complexes, and only A and A optical isomers are possible. However all siderophore catecholates are substituted asymmetrically on the catechol ring, such that geometric isomers may in principle exist. However, in the case of enterobactin molecular models show only the more symmetric cis chelate is possible, as the A or A form. 

As with the hydroxamate siderophores, our initial approach has been to study simple tris(catecholato)metallate(III) complexes as models for the tricatecholate siderophore enterobactin. Unlike hydroxamates, catecholate is a symmetric, bidentate ligand. 

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