Kinetics of Gallium Removal from Transferrin and Thermodynamics of Gallium-Binding by Sulfonated Tricatechol Ligands

Ambiguity of NRPS Structure Predictions: Four Bidentate Chelating Groups in the Siderophore Pacifibactin

Identified through a bioinformatics approach, a nonribosomal peptide synthetase gene cluster in Alcanivorax pacificus encodes the biosynthesis of the new siderophore pacifibactin. The structure of pacifibactin differs markedly from the bioinformatic prediction and contains four bidentate metal chelation sites, atypical for siderophores. Genome mining and structural characterization of pacifibactin is reported herein, as well as characterization of pacifibactin variants accessible due to a lack of adenylation domain fidelity during biosynthesis. A spectrophotometric titration of pacifibactin with Fe(III) and ¹³C NMR spectroscopy of the Ga(III)-pacifibactin complex establish 1:1 metal:pacifibactin coordination and reveal which of the bidentate binding groups are coordinated to the metal. The photoreaction of Fe(III)-pacifibactin, resulting from Fe(III) coordination of the β-hydroxyaspartic acid ligands, is reported.

Iron(III) Coordination Chemistry of Alterobactin A: A Siderophore from the Marine Bacterium Alteromonas luteoviolacea

Alterobactin A is a siderophore produced by the oceanic bacterium Alteromonas luteoviolacea. The thermodynamic stability constant of the ferric alterobactin A (Alt-A) complex was estimated from electrochemical measurements on the basis of a previously reported linear relationship between the reduction potentials and the pH-independent stability constants for known iron(III) complexes. The reduction potential of the ferric alterobactin A complex determined by square wave voltammetry is -0.972 V vs SCE and reversible, corresponding to a thermodynamic stability constant of 10(51+/-2). Potentiometric titration of Fe(III)-Alt-A shows the release of six protons on complexation of Fe(III) to Alt-A. The 1H NMR resonances of the Ga(III)-Alt-A complex show that the C-4, C-5, and C-6 catecholate protons and the C(alpha) and C(beta) protons of both beta-hydroxyaspartate moieties are shifted downfield relative to the free ligand, which along with the potentiometric titration data is consistent with a complex in which Fe(III) is coordinated by both catecholate oxygen atoms and both oxygen atoms of each beta-hydroxyaspartate. The UV-vis spectrum of Fe(III)-Alt-A is invariant over the pH range 4-9, indicating the coordination does not change over a wide pH range. In addition, in the absence of a coordinated metal ion, the serine ester of Alt-A hydrolyzes forming Alt-B.

Silybin, a new iron-chelating agent

Silybin, a natural occurring flavolignan isolated from the fruits of Silibum marianum, has been reported to exert antioxidant and free radical scavenging abilities. It was suggested to act also as an iron chelator. The complexation and protonation equilibria of the ferric complex of this compound have been studied by potentiometric, spectrophotometric and electrochemical techniques. The formation of the complex silybin-Ga(III) in anhydrous DMSO-d6 has been studied by 1H NMR spectroscopy. Mass spectrometry and infrared spectroscopy on silybin-Fe(III) complex confirm all data obtained by 1H NMR spectroscopy. The experimental results show that silybin binds Fe(III) even at acidic pH. Different ternary complexes were observed at increasing methoxide ion concentration and their stability constants have been calculated. The results show the possible role of silybin in relation to the chelation therapy of chronic iron overload, as occurs in the treatment of Cooley's anemia.