engleski

Iron(III) Uptake by Chrysobactin, a Siderophore from the Phytopathogenic Bacterium Erwinia chrysanthemi

The stability of iron(III) complexes with the bidentate, catechol-type siderophore chrysobactin ( -N-(2, 3-dihydroxybenzoyl)-D-lysyl-L-serine) from Erwinia chrysanthemi was investigated using potentiometric and spectrophotometric titrations. Protonation constants of a free ligand and stability constants of its ferric complexes with different stoichiometries (1:1 to 1:3 metal:ligand ratio) and protonation degrees were determined at 25 °C and 0.10 M ionic strength. The stoichiometry of the complex species was confirmed by means of electrospray mass spectrometry. The kinetic studies of ferric chrysobactin 1:1 complex formation in acidic medium were carried out at 25 °C and ionic strength of 0.1 M using a stopped-flow spectrophotometer. Both protonation forms of the ligand with respect to the terminal carboxylic acid group of serine were found to be involved in the reaction mechanism together with two iron(III) hydroxo species, viz. FeOH(2+) and Fe2(OH)2(4+). The corresponding rate constants were determined. Comparison of these values indicated expected higher reactivities of both iron(III) species with the less positive (neutral) form of chrysobactin. The iron(III) hydroxy dimer was shown to be more reactive than FeOH(2+) regardless of the charge of the reacting ligand species, although that was less pronounced in the case of the completely protonated, positively charged chrysobactin. The enhanced reactivity is proposed to be due to the appearance of a stabilised precursor dimeric complex in the reaction mechanism.

chrysobactin

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