Bacterial iron transport: coordination properties of azotobactin, the highly fluorescent siderophore of Azotobacter vinelandii.

Azotobacter vinelandii, a nitrogen-fixing soil bacterium, secretes in iron deficiency azotobactin δ, a highly fluorescent pyoverdin-like chromopeptidic hexadentate siderophore. The chromophore, derived from 2,3-diamino-6,7 dihydroxyquinoline, is bound to a peptide chain of 10 amino acids:  (l)-Asp-(d)-Ser-(l)-Hse-Gly-(d)-β-threo-HOAsp-(l)-Ser-(d)-Cit-(l)-Hse-(l)-Hse lactone-(d)-Nδ-Acetyl, Nδ-HOOrn. Azotobactin δ has three different iron(III) binding sites which are one hydroxamate group at the C-terminal end of the peptidic chain (Nδ-Acetyl, Nδ-HOOrn), one α-hydroxycarboxylic function in the middle of the chain (β-threo-hydroxyaspartic acid), and one catechol group on the chromophore. The coordination properties of its iron(III) and iron(II) complexes were measured by spectrophotometry, potentiometry, and voltammetry after the determination of the acid−base functions of the uncomplexed free siderophore. Strongly negatively charged ferric species were observed at neutral p[H]'s corresponding to a predomina...