Enterobactin Protonation and Iron Release: Structural Characterization of the Salicylate Coordination Shift in Ferric Enterobactin1

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Abstract
The siderophore enterobactin (Ent) is produced by many species of enteric bacteria to mediate iron uptake. This iron scavenger can be reincorporated by the bacteria as the ferric complex [FeIII(Ent)]3- and is subsequently hydrolyzed by an esterase to facilitate intracellular iron release. Recent literature reports on altered protein recognition and binding of modified enterobactin increase the significance of understanding the structural features and solution chemistry of ferric enterobactin. The structure of the neutral protonated ferric enterobactin complex [FeIII(H3Ent)]0 has been the source of some controversy and confusion in the literature. To demonstrate the proposed change of coordination from the tris-catecholate [FeIII(Ent)]3- to the tris-salicylate [FeIII(H3Ent)]0 upon protonation, the coordination chemistry of two new model compounds N,N‘,N‘‘-tris[2-(hydroxybenzoyl)carbonyl]cyclotriseryl trilactone (SERSAM) and N,N‘,N‘‘-tris[2-hydroxy,3-methoxy(benzoyl)carbonyl]cyclotriseryl trilactone (SER(3M)SAM) was examined in solution and solid state. Both SERSAM and SER(3M)SAM form tris-salicylate ferric complexes with spectroscopic and solution thermodynamic properties (with log β110 values of 39 and 38 respectively) similar to those of [FeIII(H3Ent)]0. The fits of EXAFS spectra of the model ferric complexes and the two forms of ferric enterobactin provided bond distances and disorder factors in the metal coordination sphere for both coordination modes. The protonated [FeIII(H3Ent)]0 complex (dFe-O = 1.98 Å, σ2stat(O) = 0.00351(10) Å2) exhibits a shorter average Fe−O bond length but a much higher static Debye−Waller factor for the first oxygen shell than the catecholate [FeIII(Ent)]3- complex (dFe-O = 2.00 Å, σ2stat(O) = 0.00067(14) Å2). 1H NMR spectroscopy was used to monitor the amide bond rotation between the catecholate and salicylate geometries using the gallic complexes of enterobactin: [GaIII(Ent)]3- and [GaIII(H3Ent)]0. The ferric salicylate complexes display quasi-reversible reduction potentials from −89 to −551 mV (relative to the normal hydrogen electrode NHE) which supports the feasibility of a low pH iron release mechanism facilitated by biological reductants.