Aqueous Solution Speciation of Fe(III) Complexes with Dihydroxamate Siderophores Alcaligin and Rhodotorulic Acid and Synthetic Analogues Using Electrospray Ionization Mass Spectrometry

Abstract

Aqueous solutions of Fe3+ complexes of cyclic (alcaligin) and linear (rhodotorulic acid) dihydroxamate siderophores and synthetic linear eight-carbon-chain and two-carbon-chain dihydroxamic acids ([CH3N(OH)C=O)]2(CH2)n; H2Ln; n = 2 and 8) were investigated by electrospray ionization mass spectrometry (ESI-MS). Information was obtained relevant to the structure and the speciation of various Fe(III)-dihydroxamate complexes present in aqueous solution by (1) comparing different ionization techniques (ESI and FAB), (2) altering the experimental parameters (Fe3+/ligand ratio, pH, cone voltage), (3) using high-stability hexacoordinated Fe(III) siderophore complex mixtures (ferrioxamine B/ferrioxamine E) as a calibrant to quantify intrinsically neutral (H+ clustered or protonated) and intrinsically charged complexes, and (4) using mixed-metal complexes containing Fe3+, Ga3+, and Al3+. These results illustrate that for all dihydroxamic acid ligands investigated multiple tris- and bis-chelated mono- and di-Fe(III) species are present in relative concentrations that depend on the pH and Fe/L ratio.

Magnetic susceptibility of Mn(III) complexes of hydroxamate siderophores

“…respectively [25]. The 2:2 complex formed at pH 11 is reported to be an anti-ferromagnetically coupled bis-µ-oxo dimer, Fe 2 (alcaligin) 2 O 2 , [25] and not detected at lower pH by ESI-MS [33].…”

Collision-activated dissociation, infrared multiphoton dissociation, and electron capture dissociation of the Bacillus anthracis siderophore petrobactin and its metal ion complexes

“…Electrospray ionization mass spectrometry (ESI MS) and tandem mass spectrometry (MS/MS) have proved to be powerful techniques for detecting and identifying siderophores [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. For example, Gledhill demonstrated the utility of these techniques for characterizing hydroxamate-type siderophores, including ferrioxamine, ferrichrome, and iron(III) rhodotoluate [9].…”

Accurate Mass MS/MS/MS Analysis of Siderophores Ferrioxamine B and E1 by Collision-Induced Dissociation Electrospray Mass Spectrometry

“…It is interesting to note that while most fragments result from cleavage at two positions, with loss of a monohydroxyl unit being typical, one fragment, m/z = 314.0560, retains a hydroxamate -OH as a (bis) Fe (III)-dihydroxamate species (SI Figure 3). This fragment has been previously observed, predominantly at low pH (<2) and at nominal mass accuracy [14,15]. Fragments such as these can be expected to differ within suites of siderophores or novel compound structures.…”

“…This was curious given the noncyclic structure of the MS 2 fragments of Fer E1, suggesting that MS 3 data can be correlated back to the parent geometry for the cyclized trihydroxamates. It is postulated that the cyclic, Fer E1, retains more oxime interactions because of the proximity of the Fe (III) to these units in the cyclic geometry [15]. Our data suggest that the result of these iron-oxygen interactions is less oxime breakage (aa, bb) and a variable fragmentation compared with the more flexible, linear Fer B.…”