Structure of putrebactin, a new dihydroxamate siderophore produced by Shewanella putrefaciens

Shewanella putrefaciens is a bacterium implicated in oil pipeline corrosion and fish spoilage, and is one of very few isolated microorganisms able to use iron(III) as an electron acceptor. S. putrefaciens strain 200 produced a novel cyclic dihydroxamate siderophore, putrebactin, during aerobic growth. Putrebactin was determined to be 1,11-dihydroxy-1,6,11,16-tetraazacycloeicosane-2,5,12,15-tetrone, a cyclic dimer of succinyl-(N-hydroxyputrescine), by H-1 and C-13 NMR spectroscopy, fast atom bombardment and chemical ionization mass spectrometry, and X-ray crystallography. The protonation constants of putrebactin were determined to be 8.82 and 9.71. Potentiometric titration of the ferric complex revealed a sharp equivalence point at 3.0 equivalents of base per mole of Fe(III), consistent with loss of 3 protons per equivalent of bound ferric ion, while Job's method of continuous variation supported a shift from 1:1 to 3:2 complex stoichiometry as a function of pH. Putrebactin is similar in structure to two other siderophores, bisucaberin and alcaligin, produced by unrelated bacteria.