Molecular studies of hydroxamate siderophore interactions with aqueous and solid phase iron

Abstract

This dissertation explores the functional group and dissolution properties of hydroxamate siderophores in aqueous solution. Microbially produced siderophores in soil and aqueous systems obtain (by complexation or iron oxide dissolution) ferric iron in limiting environments to sustain cellular growth in bacteria and fungi. For this reason, siderophores play a role in controlling the free ferric iron flux in the environment. Infrared (IR), ultra-violet/visible (UV/vis), and X-ray absorption (XA) spectroscopic techniques were used to probe the molecular structure of desferrioxamine B (desB (C₂₅H₄₈O ₈N₆); a hydroxamate siderophore) and acetohydroxamic acid (aHa (CH₃CONHOH); model monohydroxamic acid) as a function of pH in aqueous solutions. These experimental methods were complemented with calculated model compounds to correctly assign infrared and X-ray transitions. Infrared results from alla were difficult to apply to desB because of the large amount of coupling between the hydroxamate functional group and long chain methylene groups in the molecule. However, XA spectra of aHa greatly aided in the interpretation of desB. Upon deprotonation of the hydroxamate group (primarily an O-acid), the (C=O)NO core electronic charge is highly delocalized, with most of the negative charge drawn towards the two oxygen atoms. Calculated transitions in the pre-edge region of the XA spectra indicate that 2p-orbitals from all atoms in the hydroxamate core interact with the 3d-orbitals from ferric iron. The large amount of delocalization in the hydroxamate core is the likely cause for the strong binding constants of hydroxamate siderophores. Iron oxide dissolution studies with aHa and desB were conducted with inductively coupled plasma - atomic emission spectrometry (ICP-AES). The results indicate that siderophores preferentially dissolve disordered iron oxide phases rather than coarse-grained goethite, the most ubiquitous iron oxide phase in soil systems. This work is directly applicable to research that requires an advanced understanding of the functional group chemistry of siderophores in a wide pH range. This includes, but is not limited to, contaminant transport, actinide remediation at Superfund sites, and cellular dissociation mechanisms.