Trace metal complexation by the triscatecholate siderophore protochelin: structure and stability

Abstract

Although siderophores are generally viewed as biological iron uptake agents, recent evidence has shown that they may play significant roles in the biogeochemical cycling and biological uptake of other metals. One such siderophore that is produced by A. vinelandii is the triscatecholate protochelin. In this study, we probe the solution chemistry of protochelin and its complexes with environmentally relevant trace metals to better understand its effect on metal uptake and cycling. Protochelin exhibits low solubility below pH 7.5 and degrades gradually in solution. Electrochemical measurements of protochelin and metal-protochelin complexes reveal a ligand half-wave potential of 200 mV. The Fe(III)Proto(3-) complex exhibits a salicylate shift in coordination mode at circumneutral to acidic pH. Coordination of Mn(II) by protochelin above pH 8.0 promotes gradual air oxidation of the metal center to Mn(III), which accelerates at higher pH values. The Mn(III)Proto(3-) complex was found to have a stability constant of log β(110) = 41.6. Structural parameters derived from spectroscopic measurements and quantum mechanical calculations provide insights into the stability of the Fe(III)Proto(3-), Fe(III)H(3)Proto, and Mn(III)Proto(3-) complexes. Complexation of Co(II) by protochelin results in redox cycling of Co, accompanied by accelerated degradation of the ligand at all solution pH values. These results are discussed in terms of the role of catecholate siderophores in environmental trace metal cycling and intracellular metal release.

Metallophores and Trace Metal Biogeochemistry

“…It is unknown how a possible salicylate-type coordination shift may affect the binding of Mo and V, compared to a catecholate-type coordination. The later has been found for Fe(III) and Mn(III) protochelin complexes at pH \ 7 (Harrington et al 2012a), as well as for other catecholamide siderophores (Abergel et al 2006a).…”

“…However, there are a number of specific examples that require that we distinguish between the two. In the class of catecholate metallophores, protochelin is a triscatecholate ligand that exhibits a log b = 44.6 for Fe(III), actually slightly lower than the log b 3 of catecholate (log b 3 = 44.9) and slightly higher than aminochelin (log b 3 = 41.3) (Harrington et al 2012a;Khodr et al 2002;Martell et al 2005). Enterobactin (log b = 49) has an additional increase in complex stability due to the preorganization of the donor groups, suggesting that the two effects should be distinguished from each other (Loomis and Raymond 1991).…”

The Siderophore Metabolome of Azotobacter vinelandii

“…5). A possible formation mechanism in aerobic A. vinelandii cultures involves oxidation of catechols to form cross-linked aromatic compounds, perhaps catalyzed by the presence of Fe (56,57).…”

“…A recent development in the discovery of siderophores is the use of high-resolution liquid-chromatography electrospray ionization mass spectrometry (HR-LC-MS) methods that exploit the characteristic 54 Fe- 56 Fe isotope pattern associated with organic Fe chelates (18)(19)(20). Data mining techniques are available for filtering the relevant Fe isotope patterns associated with Fe complexes even at low abundances and in highly complex matrixes, as well as for detecting the corresponding apo siderophores (18).…”

“…1. The HR-LC-MS data set was filtered for the characteristic 54 Fe- 56 Fe isotope pattern that is associated with Fe complexes using the software ChelomEx (18) (III), ⌬m/z ϭ Ϫ53.91928 Ϯ (0.0015 ϩ 2 ppm); for Fe(II), ⌬m/z ϭ Ϫ52.91145 Ϯ (0.0015 ϩ 2 ppm)]. To collect MS/MS spectra of possible siderophores, a parent ion list was generated using an extended list of possible siderophores by including also species that may not have shown an identified Fe isotope pattern because the 54 Fe intensity was below the detection limit but still showed coelution with species corresponding to the possible free ligand, whereby the intensity of the free ligand had to exceed that of the Fe complex more than 5-fold.…”