Hard and Soft X-ray Absorption Spectroscopic Investigation of Aqueous Fe(III)−Hydroxamate Siderophore Complexes

Abstract

Microorganisms release organic macromolecules, such as siderophores, to obtain Fe(III) from natural systems. While the relative stabilities of Fe(III)-siderophore complexes are well-studied, the structural environments of Fe(III) and ligands in the complex are not well-understood. Using the X-ray absorption spectroscopy (XAS) at the Fe- and N-K absorption edges, we characterized the nature of Fe(III) interactions with a hydroxamate siderophore, desferrioxamine B (desB), and its small structural analogue, acetohydroxamic acid (aHa), as a function of pH (1.4-11.4). These experimental studies are complemented with DFT calculations. The Fe-XAS studies suggest that Fe(aHa)3 is the dominant species in aqueous solutions in the pH range of 2.8-10.1, consistent with thermochemical information. However, the N-XAS and resonance Raman studies show that the chemical state of the ligand in the Fe(aHa)3 complex changes significantly with pH, and these variations are correlated with further deprotonation of the Fe(aHa)3 complex. The N-XAS studies also indicate that the overlap of Fe 3d orbitals with the molecular orbitals of the hydroxamate group is significant. The Fe- and N-XAS studies of Fe(III)-desB complexes indicated that Fe(desB)+ is the dominant species between pH values of 1.4 and 11.4, consistent with predicted stability constants. This information is useful in understanding the role of iron in bacterial transport, siderosis treatment, and actinide sequestration at contaminated sites. This is the first N-XAS study of aqueous metal ligand complexes, which demonstrates the applications of soft-XAS in studying the electronic structure of metal complexes of organic macromolecules in aqueous solutions.

Photoreduction of Terrigenous Fe‐Humic Substances Leads to Bioavailable Iron in Oceans

“…The normalized spectra of HS were fitted (Supporting Information, Table S3) to model compounds 4 (Fe III O 6 ), 5 (Fe III O 6 ), and 13 (Fe II O 6 ). The use of model compounds very similar to 4 and 5 were shown to give the best fitting results in an earlier study of HS 23. The fitting results of initial NR (48 % 4 , 52 % 5 ) and CB0 13 10 s (42 % 4 , 58 % 5 ) supports the previous assignment of Fe III O 6 coordination.…”

“…At roughly 2.95 and 3.92 Å contributions from Fe−C single scattering and Fe−O/N−C multiple scattering were observed, respectively. These Fe−C distances are present in complexes of Fe III and small organic ligands,23, 26 and have been shown to exist in iron‐containing organic soils 12c. Several types of functional groups containing oxygen atoms can be attributed to the Fe III coordination environments, such as carboxyl, hydroxy and catecholic groups.…”

X-ray spectroscopic and scattering methods applied to the characterisation of cobalt-based Fischer–Tropsch synthesis catalysts

“…Although significantly less penetrating than hard X-rays, soft Xray energies in the range of 200 -4000 eV allow for the examination of the K-edges of 'ligand elements' such as C, O, S and Cl. Which are able to act as 'reporter' elements for metal sites determining the electronic state of the ligand and its metal complex [86], or in the case of FTS may themselves be present in adsorbed species on the catalyst (C or O K-edges fall in this region). Additionally, soft X-rays allow for the probing of Co L-edges, which are typically more sensitive to Co oxidation and coordination state.…”

Iron oxyhydroxide mineralization on microbial extracellular polysaccharides

“…The details of the spectral calculations can be found in Edwards and Myneni (2005). Briefly, the optimized structures were input into StoBe (Hermann et al, 2002) to compute a NEXAFS spectrum at the carbon 1s edge for each calculated structure.…”