Collision-Activated Dissociation, Infrared Multiphoton Dissociation, and Electron Capture Dissociation of the Bacillus anthracis Siderophore Petrobactin and Its Metal Ion Complexes

Siderophores are high-affinity iron-chelating ligands produced by microorganisms to scavenge vital Fe3+ from the environment. Thus, siderophores constitute potential therapeutic targets and their structural determination is important for exploiting their therapeutic value. Here, the virulence-associated siderophore petrobactin from Bacillus anthracis was characterized with electron capture dissociation (ECD). Fragmentation of doubly protonated petrobactin was investigated and compared to sustained off-resonance irradiation collision-activated dissociation (SORI CAD) and infrared multiphoton dissociation (IRMPD) of both the singly and doubly protonated species. These experiments demonstrate that ECD provides additional information (complementary bond cleavages) on the structure of petrobactin compared to both SORI CAD and IRMPD. Furthermore, complexes of petrobactin with divalent (Ca2+, Fe2+, and Co2+) and trivalent (Fe3+ and Ga3+) metal cations were also subjected to SORI CAD and ECD. Again, most structural information was obtained from the ECD spectra. However, significant differences were found in both SORI CAD and ECD of metal complexes, dependent on the nature of the metal ion. Intriguingly, unique behavior, consistent with a recently proposed solution-phase structure, was observed for the highly preferred Fe3+-petrobactin complex.