Synthesis and iron-binding properties of quinolobactin,a siderophore from a pyoverdine-deficient Pseudomonas fluorescens

Quinolobactin is a new siderophore produced by a pyoverdine deficient mutant of Pseudomonas fluorescens. A simple and efficient synthesis of quinolobactin is described, starting from xanthurenic acid. The protonation constants of quinolobactin were determined by potentiometric titrations as pK_a2=5.50 ± 0.07, pK_a1=10.30 ± 0.05. The equilibria of the metal complexes were studied by means of spectrophotometric and potentiometric titrations. The overall stability constants of the quinolobactin-Fe^III complexes was found to be log ₁₁₁=18.60 ± 0.10, log ₁₂₁=32.60 ± 0.20, log ₁₂₀=28.20 ± 0.25 resulting in a pFe^III value of 18.2 at pH 7.4. The UV-visible spectral parameters of the [FeL₂] are in agreement with a complex containing two ligands coordinated to one Fe³⁺ cation through the oxygen and nitrogen quinoline atoms.