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 pKa2 = 5.50+/-0.07, pKa1 = 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-FeIII complexes was found to be log beta111 = 18.60+/-0.10, log beta121 = 32.60+/-0.20, log beta120 = 28.20+/-0.25 resulting in a pFeIII 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.