Isomeric siderophores

From Pseudomonas mendocina five siderophores were isolated by chromatography. They are reported to have identical molecular masses of 1,152 Da (the also reported 3a) value of 929 Da is an error L. E. Hersman, private communication) and an identical amino acid composition, which has not been revealed 141a). Color reactions show the presence of a hydroxamate, but not of a catecholate grouping. A gene analysis suggests a partial sequence acyl-Asp-Dab-Ser-formylOHOm-Ser-formylOHOm where asparagine could be OHAsp and the C-terminal ornithine cOHOm 9b). In which way the five isomeric siderophores with identical molecular masses differ from each other is not clear. 

Itou Y, Okada S, Murakami M (2001) Two Stmctural Isomeric Siderophores from the Freshwater Cyanobacterium Anabena cylindrica (NIES-19). Tetrahedron 57 9093... 

Isomeric siderophores from the freshwater cyanobacterium Anabaena cylindrica. Itou et al reported the structural characterization of a pair of iron chelating siderophores isolated from the freshwater cyanobacterium Anabaena cylindrica. Both direct and long-range HSQC and HMBC data were used in the characterization of the structures. 

The stmcture of pyochelin (for a detailed bibliography, see (57)), a secondary siderophore of Pseudomonas aeruginosa and of Burkholderia cepacia was established (75) as 2-(2-o-hydroxyphenyl-2-thiazolin -yl)-3-methylthiazolidine-4-carboxylic acid. It consists of a mixture of two easily interconvertible stereoisomers (pyochelin I and II) differing in the configuration of C-2". They can be separated by chromatography, but in methanolic solution (not in DMSO) the equilibrium (ca. 3 1) is restored quickly. For a discussion of the mechanism of isomerization, see (57, 577). 

Micacocidin (Fig. 22, 68) from Pseudomonas sp. complexes Fe " and other metal ions 189,190). Whether it acts as a siderophore has not been investigated. A stereospecific synthesis was elaborated 161,161a), but the same isomerization problems at C-4 and C-2" were encountered as had been observed with pyochelin (see Note 14 in 161)). 

We conclude that plural mechanisms exist for siderophore iron utilization in the enteric bacteria. The iron may be rapidly removed with ( . coli) or without (S. typhimurium) effective transport of the ligand. The rate of this process is such that labilization of the iron by reduction appears most likely. The intact ferric chelates also pass the cell envelope, but by a generally slower mechanism. An estimate of the number of atoms of iron acquired per bacterium indicates true uptake rather than adsorption to the cell surface has taken place. In both organisms the A-cis chromic coordination isomer of ferrichrome is active, indicating that dissociation and/or isomerization is not obligatory for its transport. 

The isomers of Cr(men)3 isomerize with half-lives (several hours) similar to the Cr(benz)3 complex. The rate of isomerization of the tris-(hydroxamate) complexes is therefore not particularly sensitive to the substituent of the hydroxamate nitrogen atom, since the men ligand contains an alkylated nitrogen atom, and the benz ligand contains an unsubstituted nitrogen atom. In the absence of an induced strain, the corresponding siderophore complexes must isomerize much more slowly because of the steric constraints of the ligand. 

The chromic-substituted siderophore complexes can be prepared and, in contrast to the naturally occurring ferric complexes, are kinetic-ally inert to isomerization or ligand substitution. 

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