Siderophore metallate complexes

Visca P, Colotti G, Serino L, Verzili D, Orsi N, Chiancone E (1992) Metal Regulation of Siderophore Synthesis in Pseudomonas aeruginosa and Functional Effects of Siderophore-Metal Complexes. Appl Environ Microbiol 58 2886... 

Several excellent review articles on the biological 18 22> medical10 13-23-24> and agricultural aspects8,9,25) have been published. However, no comprehensive treatment of siderophore metal complex chemistry has appeared. That is the intention of this paper. 

Anderegg G, Raber M (1990) Metal Complex Formation of a New Siderophore Desferrithi-ocin and of Three Related Ligands. J Chem Soc Chem Commun 1194... 

Although their role in nature is still not clear, there are extracellular microbial products, such as biosurfactants, bioemulsifiers and siderophores, that complex or chelate metals quite efficiently. Examples of these compounds are shown in Figure 10.1. 

Figure 16-1 Structures of several siderophores and of their metal complexes. (A) Enterobactin of E. coli and other enteric bacteria 12 (B) parabactin (R = H) from Paracoccus denitrijkans and agrobactin (R = OH) from Agrobacterium tumefaciens 20 (C) ferrichrome 9 (D) pyochelin from Pseudomonas aeruginosa.21...

Chromic Ferrichrome Complexes. The spectra for the model chromic hydroxamate complexes are reproduced in Figure 6. Since the visible and CD spectra of the isomers are wholly dominated by the metal complex chromophore, these data can be used to characterize and to identify coordination isomers of complexes formed by the siderophores. The preparation and characterization of the chromic complexes of des-ferriferrichrome and desferriferrichrysin have been reported (3). Although an examination of molecular models for both complexes shows two coordination isomers are possible (A-cis and A-cis), both chromic complexes consist exclusively of the A-cis isomer. These results agree with x-ray crystallographic investigations which have shown that both ferri-chrysin and ferrichrome A crystallize as only the A-cis isomer (14, 15). Both chromic complexes have identical CD spectra which are the same as the A-cis Cr(men)3 spectrum (Figure 6). 

Figure 16-1 Structures of several siderophores and of their metal complexes. (A) Enterobactin of E. coli and other enteric bacteria ...

Ferrioxamines are the only known siderophores with no chiral center, so ferrioxa-mines are the only siderophores with no intrinsic optical activity of the metal complex. Ferrioxamine E, a cyclic ferrioxamine, crystallizes as a racemic mixture of A-cis and A-cis isomers as determined by X-ray crystallography 187). From an examination of molecular models of ferrioxamine B five enantiomeric pairs of non-redundant isomers are distinguishable. By ion exchange chromatography the more polar cis... 

Rhodotorulic acid (RA), a dihydroxamate siderophore, forms dimeric complexes with iron, aluminium and chromium of the stoichiometry M2(RA)3 at neutral pH 36 188). The coordination chemistry of this siderophore is probably the most complicated of the siderophores. The combination of cis-trans, A and A configurations of two iron miters, connected by three RA molecules, makes 42 non-redundant isomers theoretically possible each can be simulated by molecular models. Recently three different isomers or mixtures of isomers of Cr2RA3 were separated by reversed phase HPLC-chromatography177). The visible spectrum of the most abundant fraction corresponds to the cis isomer the two other fractions are very similar to the visible spectrum of the trans Cr(men)3 isomer. The CD spectra, in comparison with the Cr(men)3 model complex, show two different optical isomers, assigned as A -trans and A -trans. The A isomer preparation seems also to contain a certain amount of the A configuration. This is the first time that two different, kinetically stable optical isomers have been isolated from the metal complexes of a siderophore 177). 

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. 

Helical structures in nature also are formed through complexation with metal cations Some microbes, including Escherichia coli, produce enterobactin a siderophor, which complexes Fe(III) with a very high complex constant of 10 . ... 

Certain secondary metabolites act as metal transport agents. One group is composed of the siderophores (also known as sideramines) which function in uptake, transport, and solubilization of iron. Siderophores are complex molecules which solubilize ferric ion which has a solubility of only 10 18 mo 1/1 at pH 7.4... 

Transition-metal complexes of a-hydroxy acids can be photolabile. The oil-degrading marine bacterium Marinobacter hydrocarbonoclasticus produces a siderophore, which appears to exploit photodecarboxylation to facilitate iron release. Petrobactin (Figure 4) forms a stable ferric complex through iron chelation by two catecholate moieties and a citryl group. Decarboxylation of the citryl moiety via photolysis of ferric petrobactin yields a less stable ferric complex than the... 

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