Siderophores anionic

Siderophore-ionophore supramolecular assembly formation via host-guest complexation of the pendant protonated amine arm of ferrioxamine B has been confirmed by X-ray crystallography (Fig. 28) (203). The stability and selectivity of this interaction as a function of ionophore structure, metal ion identity, and counter anion identity were determined by liquid-liquid extraction, isothermal calorimetry, and MS (204 -211). Second-sphere host-guest complexation constants fall in the range 103— 106M-1 in CHC13 and methanol depending on ionophore structure. 

Fe(III) displacement of Al(III), Ga(III), or In(III) from their respective complexes with these tripodal ligands, have been determined. The M(III)-by-Fe(III) displacement processes are controlled by the ease of dissociation of Al(III), Ga(III), or In(III) Fe(III) may in turn be displaced from these complexes by edta (removal from the two non-equivalent sites gives rise to an appropriate kinetic pattern) (343). Kinetics and mechanism of a catalytic chloride ion effect on the dissociation of model siderophore-hydroxamate iron(III) complexes chloride and, to lesser extents, bromide and nitrate, catalyze ligand dissociation through transient coordination of the added anion to the iron (344). A catechol derivative of desferrioxamine has been found to remove iron from transferrin about 100 times faster than desferrioxamine itself it forms a significantly more stable product with Fe3+ (345). 

W(VI) centers. At room temperature and under mild conditions, iron release from the complexes is observed upon reduction of the Fe(III) centers. This release is controlled by the ionic strength of the medium, the nature and concentration of the anions present in the supporting electrolyte, and by the pH of the solution. This behavior parallels those described for most siderophores that depend on the same parameters. 

Amidocryptands were until recently unknown in anion circles [39-41,54-56], However, Raymond and coworkers previously synthesized catechol-based amidocryptands as models for siderophores [57], The appeal of polyamide cryptands over polyamines is related to their solution properties, namely the former class of ligands is less susceptible... 

Iron transporters known as siderophores occur in various bacteria. They coordinate iron in a complex that involves three catechol residues. A natural host molecule called entero-bactin is shown along with a cryptand-like molecule (9) that is one of several that were devised to mimic this complexation behavior (reviewed in Roosen-berg, 2000 Raymond, 2003). The catechols deprotonate to the catecholate anions, which provide six oxygen donors for ferric ion The host thus completely envelops the cation permitting transport as the complex. 

Despite the considerable structural variation found in the siderophores, their common feature is to form six-coordinate octahedral complexes with ferric ion of great thermodynamic stability. The ligating groups usually contain the oxygen atoms of hydroxamate (a) or catecholate (b) anions. 

Iron is transported in forms in which it is tightly complexed to small chelators called siderophores (microorganisms) or to proteins called transferrins (animals) or to citrate or mugeneic acid (plants). The problem of how the iron is released in a controlled fashion is largely unresolved. The process of mineral formation, called biomineralization, is a subject of active investigation. Vanadium and molybdenum are transported as stable anions. Zinc and copper appear to be transported loosely associated with peptides or proteins (plants) and possibly mugeneic acid in plants. Much remains to be learned about the biological transport of nonferrous metal ions. 

Molecular Squares, Boxes, arid Cubes, p. 909 71-71 Stacking Theory and Scope, p. 1076 Preorganization and Complementarity, p. 1158 Pyrrole- and Polypyrrole-Based Anion Receptor-s. p. 1176 Rotaxanes arid Pseudorotaxanes, p. 1194 Self-Assernhling Catenanes, p. 1240 Siderophores, p. 1278 Spherands, p. 1344... 

Molecular Clefts and Tweezers, p. 887 Organometallic Anion Receptors, p. 1006 Phase-Transfer Catalysis in Environmentally Benign Reaction Media, p. 1042 Siderophores, p. 1278 Swfactants, Part 1 Fundamentals, p. 1458 Surfactants, Part 11 Applications, p. 1470 The Template Effect, p. 1493... 

The concept that a transition metal complex can interact in an orderly manner with neutral molecules or ions to give an outer sphere complex dates back over 100 years to Alfred Werner s original description of coordination chemistry (Figure 1). Werner fonnd the idea, we now call second-sphere coordination, essential to explain snch simple phenomena as (i) the dependence of optical rotation on the nature of the anion and solvent, (ii) the formation of adducts between amines and saturated complexes, and (iii) solvents of crystallization. Indeed, aspects of the second-sphere coordination are known to be important in such diverse areas as the biological activity of siderophores and the function of MRl contrast agents. ... 

Aerobic microorganisms also require iron, but cannot simply absorb it from their aqueous environment since Fe is precipitated as Fe(OH)3 (K p = 2.64 x 10 ). Evolution has provided these organisms with O-donor poly-dentate ligands called siderophores which scavenge for iron. Examples of siderophores are the anions derived... 

Siderophores produced by plants and soil microorganisms may play an important role in the complexation and weathering of Fe-bearing minerals. Several bacteria and fungi produce siderophores in order to enhance Fe solubility. Certain grasses also excrete phytosiderophores under Fe-deficient conditions the affinity of phytosiderophores for Fe (III) is less than that of microbial siderophores. All siderophores contain the anionic reactive group (R—CO—NO—), which enables these compounds to be extremely effective chelators of Fe. 

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