Structure of ferric pseudobactin, a siderophore from a plant growth promoting Pseudomonas.

Both plant growth promoting Pseudomonas B10 and its yellow-green, fluorescent iron transfer agent (siderophore) pseudobactin enhance the growth of the potato and control certain phytopathogenic microorganisms. The structure of the little compound has been determined by single-crystal X-ray diffraction methods using counter data. The structure consisted of a linear hexapeptide, L-Lys-D-threo-beta-OH-Asp-L-Ala-D-allo-Thr-L-Ala-D-N delta-OH-Orn, in which the N delta-OH nitrogen of the ornithine was cyclized with the C-terminal carboxyl group, and the N epsilon-amino group of the lysine was linked via an amide bond to a fluorescent quinoline derivative. The iron-chelating groups consisted of a hydroxamate group derived from N delta-hydroxyornithine, an alpha-hydroxy acid derived from beta-hydroxyaspartic acid, and an o-dihydroxy aromatic group derived from the quinoline moiety. The combination of metal-chelating ligands and the alternating L- and D-amino acids was unusual. The little compound crystallized as a single coordination isomer with the lambda absolute configuration. The present study is the first structural determination of a fluorescent siderophore. In the crystal structure, ferric pseudobactin formed a dimer, which constituted the asymmetric unit. The asymmetric unit also contained 26 water molecules. The molecules in the dimer were related by a pseudo-2-fold symmetry axis. Red-brown crystals of ferric pseudobactin (C42H57N12O16Fe . 13H2O), obtained from pyridine-acetic acid buffer solution equilibrated with water, conformed to space group I2 with a = 29.006 (23) A, b = 14.511 (13) A, c = 28.791 (21) A, and beta = 96.06 (5) degrees at -135 (2) degrees C. For eight molecules per unit cell, the calculated density was 1.38 g/cm3; the observed density was 1.40 g/cm3. The structure was refined by least-squares methods with anisotropic thermal parameters for all nonhydrogen atoms to a final R factor of 0.08 (8989 observed reflections).