Coordination Chemistry of Microbial Iron Transport Compounds. 22. pH-Dependent Moessbauer Spectroscopy of Ferric Enterobactin and Synthetic Analogues

Article abstract of DOI:10.1021/ja00352a017

Iron complexes of the siderophore enterobactin (ent) and the synthetic analogue N,N',N''-tris(2,3-dihydroxybenzoyl)-1,3,5-tris(aminomethyl)benzene (MECAM) have been shown through Moessbauer spectroscopy to remain as Fe(III) complexes from pH 2 to 10 in aqueous solution.At high pH and low temperature these two complexes and N,N',N''-tris(2,3-dihydroxybenzyl)-1,3,5-tricarbamoylbenzene (TRIMCAM) exhibit both Moessbauer and EPR spectra indicative of high-spin ferric iron in a low-symmetry enviroment: a broad 4.3 type EPR signal is observed and the 4.2 K Moessbauer spectra are magnetic six-line patterns.At low pH, the magnetic Moessbauer spectrum collapses to a broad quarupole pair (ΔE_Q = 0.8 mm/s, δ = 0.5 mm/s), indicating that the spin relaxation rate is fast compared with the nuclear precession frequency.At low pH (>=2) titration data show that ^3- and its close analogue ^3- undergo stepwiise protonation in aqueous solution.THe ultimate products of this protonation are the insoluble compounds ⁰ and ⁰, in which protonation of one catechol oxygen results in a shift to a salicylate mode of coordination involving the ortho carbonyl of the 2,3-dihydroxybenzoyl rings.Chemical analysis of the supernatant solutions from these solids show no iron is present.From pH 10 to 2, in solution and the solid state, the complexes remain intact and the iron atom remains as high-spin Fe(III) in a six-coordinate, rhombic enviroment (g = 4.3).For ^3-, a structural isomer of the MECAM complex which dies not have a carbonyl group attached to the catechol ring and hence is not capable of the salicylate coordination mode, lowering the pH results in stepwise dissociation of the complex, in contrast to the corresponding enterobactin or MECAM complexes.While ^3- in neutral aqueous solution cannot readily release its iron by reduction, the potential drops rapidly with pH and reductive release of iron under acid conditions is feasible.In the absence of a specific reductase enzyme, either the internal reduction of ferric ion by enterobactin ligand (to give Fe²⁺ and semiquinone) in nonaqueous solution or the reducton in an acidic, aqueous enviroment of the Fe(III)trs(salicylate) species Fe(H3ent), which forms below pH 4, provides an alternative to the esterase mechanism of iron acquisition by microbes which use enterobactin in iron uptake.In contrast to the aqueous complex, Fe(ent) in methanol at low pH exhibits a quasi-reversible redox chemistry; at a pH meter reading of 1, approximately 45percent of the iron is in the ferrous form (ΔE_Q = 3.44 mm/s, δ = 1.38 mm/s).