Chromium rhodotorulic acid

Chromium(III) complexes of a number of polyhydroxamic acids, microbial iron sequestering and transport agents (siderochromes) have been reported.797,798 The kinetic inertness of the chromium(III) complexes allows the facile separation of isomers for the model complex tris(iV-methyl-( - )-methoxyacetylhydroxamato)chromium(III), D-cis, L-cis and the l/d-trans isomers have been separated.798 The chromium complexes of desferrioxamine B (191) have been investigated the possible isomers are illustrated below (192-196). The cis isomer was isolated in relatively pure form.799 Thiohydroxamate800 and dihydroxamate (rhodotorulic acid) complexes have also been studied.801... 

The transport or release of iron has been much discussed. One view is that the iron is transferred from the siderophore complex at the outer membrane to another membrane-bound protein, for example in the uptake of iron by rhodotorulic acid in Rhodotorula. The other view is that the intact Fein-siderophore complex is taken up into the cell. This is supported by studies with inert chromium(III) complexes in E. coli and also by labelling studies. 

Raymond and co-workers have synthesized and separated optical and geometrical isomers of simple tris hydroxamate chroniium(III) and tris phenolate chromium(III) or rhodium(III) complexes and assigned the absolute configurations of these isomers based on criteria such as chromatographic behavior due to differences in dipole moment, theoretical symmetry considerations, and X-ray crystallographic data 174). The absolute configuration of isomers of chromium(III) complexes of ferrichrome, ferrichrysin 175), ferrioxamine B and Dx 176), rhodotorulic acid 177), enterobactin 178),... 

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). 

---