STEREOCHEMICAL CHARACTERIZATION OF RHIZOFERRIN AND IDENTIFICATION OF ITS DEHYDRATION PRODUCTS

The polycarboxylate siderophore, rhizoferrin, and its dehydration products were separated by preparative HPLC and characterized by C-13-NMR, H-1-NMR, UV, circular dichroism (CD) and IR spectroscopy, and also by capillary electrophoresis. Assignment of all carbon atoms and protons by NMR spectra confirmed the structure of rhizoferrin and gave evidence for the presence of the cyclized dehydration byproducts, imidorhizoferrin and bis-imidorhizoferrin. The imido forms were also characterized by their mobility during capillary electrophoresis. UV spectra revealed a 1:1 iron:ligand ratio above pH 3. Based on the absorption maximum of the metal ligand charge transfer band at 335 nm a molar extinction coefficient of 2300 M-1 cm-1 was calculated for ferric rhizoferrin. CD measurements revealed that the quarternary carbon atoms of the two citric acid residues possess an R,R configuration and that the iron complex of rhizoferrin adopts a LAMBDA configuration around the metal center.