Mohamed A. Abdallah

Fluorescent siderophore-based chemosensors: iron(III) quantitative determinations.

Abstract A highly sensitive and selective method is described for a rapid and easy determination of iron(III). This procedure is based on fluorimetric detection combined with the attractive properties of siderophores and biomimetic ligands, which are strong and selective ferric chelators. Azotobactin δ, a bacterial fluorescent siderophore, three fluorescent derivatives of desferriferrioxamine B with a linear structure (NBD-, MA-, NCP-desferriferrioxamine B) and one tripodal biomimetic ligand of desferriferrichrome carrying an anthracenyl fluorescent probe were examined. A very efficient static quenching mechanism by iron was observed for all the ligands considered in this work. Our results identify azotobactin δ as the most promising chemosensor of ferric traces in water, more sensitive than the NBD-desferriferrioxamine B fluorescent ligand. Under more lipophilic conditions, the anthryl-desferriferrichrome biomimetic analogue showed similar analytical potential and was found to be more sensitive than the lipophilic MA- and NCP-desferriferrioxamine B. Their detection limits were respectively 0.5 ng mL–1 for azotobactin δ and 0.6 ng mL–1 for the anthryl tripodal chelator. The calibration curves were linear over the range 0–95 ng mL–1 and 0–180 ng mL–1. Various foreign cations have been examined and only copper(II) and aluminium(III) were shown to interfere when present in similar concentrations as iron(III). The developed procedure using fluorescent siderophores or biomimetic ligands of iron(III) may be applied (1) to monitor iron(III)-dependent biological systems and (2) to determine iron(III) quantitatively in natural waters and in biological systems.

An Improved Stereocontrolled Synthesis of Pyochelin, Siderophore of Pseudomonas aeruginosa and Burkholderia cepacia

Abstract A considerably improved stereocontrolled synthesis of pyochelin, a hydroxyphenylthiazolinylthiazolidine type of siderophore common to most strains of Pseudomonas aeruginosa and Burkholderia cepacia is described. 2′-(2-Hydroxyphenyl)-2′-thiazoline-4′-carboxaldehyde, a key molecule involved in this synthesis has been prepared by reduction of 2′-(2-hydroxyphenyl)-2′-thiazoline-4′-( N -methoxy, N -methyl) carboxamide with lithium aluminium hydride. The aldehyde was further coupled with ( R )- N -methylcysteine to yield pyochelin. Under the conditions reported, epimerization at the C-4′ center was considerably diminished.

Multiple conformations of the metal-bound pyoverdine PvdI, a siderophore of Pseudomonas aeruginosa: a nuclear magnetic resonance study.

Under iron-deficient conditions, the Gram-negative bacterium Pseudomonas aeruginosa ATCC 15692 secretes a peptidic siderophore, pyoverdine PvdI, composed of an aromatic chromophore derived from 2,3-diamino-6,7-dihydroxyquinoline and a partially cyclized octapeptide, d-Ser- l-Arg- d-Ser- l-FoOHOrn-( l-Lys- l-FoOHOrn- l-Thr- l-Thr), in which the C-terminal carboxyl group forms a peptidic bond with the primary amine of the l-Lys side chain. In aqueous solution at room temperature, the (1)H NMR spectrum of pyoverdine PvdI-Ga(III) showed clear evidence of exchange broadening. At 253 K, two distinct conformations were observed and the measurement of structural constraints was possible. The three-dimensional structures of the two PvdI-Ga(III) conformers were determined, and analysis of the structures indicates that the observed conformational exchange involves a stereoisomerization of the metal binding coordination accompanied by a change in the global shape of the siderophore. This conformational transition was further characterized by heteronuclear relaxation experiments. The possible implications of this dynamic behavior for siderophore recognition by the receptor FpvAI are discussed.

A stereocontrolled synthesis of a new class of 3,4,5,6-tetrahydropyrimidine-based chiral amino acids

Abstract The stereocontrolled synthesis of seven 2-substituted-4-carboxy-3,4,5,6-tetrahydropyrimidines bearing either one chiral center at C-4 or two chiral centers at C-4 and C-8 was performed by condensation of (S)- or (R)-2,4-diaminobutyric acid (Daba) with iminoethers derived from glycine, (S)- and (R serine, (S)- and (R)-tyrosine. Under the conditions reported, epimerization was always completely prevented at the C-4 center, whereas at the C-8 center, it was completely avoided in the case of tyrosine derivatives and considerably diminished for the serine derivatives.

Crystallization and preliminary X-ray analysis of the outer membrane pyoverdine receptor FpvA from Pseudomonas aeruginosa

FpvA, the pyoverdine outer-membrane receptor from Pseudomonas aeruginosa, is involved in iron uptake when bacteria grow under iron limitation. Crystals of the in vivo pyoverdine-loaded FpvA were obtained under several crystallization conditions using different detergents. A native data set was collected at 3.6 A resolution and a three-wavelength MAD data set was collected at 3.6 A resolution using crystals of selenomethionine-substituted protein. The crystals grew under similar conditions and both belong to space group C2, but have different unit-cell parameters.

Fur mutants of Pseudomonas aeruginosa PAO1: phenotypic characterization in terms of iron-dependent pattern of deregulation of pyoverdin synthesis

Fur mutants (Mnr) of Pseudomonas aeruginosaFe10 (FF13 and FF28) and PAO1 (FPA12) were evaluated for the pattern of deregulation of pyoverdin synthesis in iron-replete medium with Casamino acids [CAM(Fe)] or succinate [SM(Fe)]. With respect to siderophore synthesis, we found in CAM(Fe) medium two Fur phenotypes: FurA (full deregulation, FF13) and FurB (partial deregulation, FF28 and FPA12). Fur mutants compared to parental strains grew with slower specific growth rates on SM(0) and CAM(0) media in a stirred bioreactor. Fur mutants grew in SM(0) with μ about half of that in CAM(0).

Overproduction of pyoverdins by Fur mutants of Pseudomonas aeruginosa strains PAO1 and Fe10 in stirred bioreactors

Fur mutants FPA12 and FF13 of strains Pseudomonas aeruginosa PAO1 and Fe10, respectively, were prepared and their production of pyoverdin evaluated. The strains were cultivated in stirred bioreactor in iron-deficient and iron-supplemented medium containing Casamino acids (CA) or succinate as a source of carbon and energy. When the pyoverdin production rate reached its maximum, the demand of iron-depleted cultures for O2 was decreased. Mutant FF13 overproduced pyoverdin in both iron-depleted (862 mg l−1) and iron-supplemented (428 mg l−1) CA medium and could also be used to produce pyoverdin when grown in a conventional stirred tank fermenter.

Synthesis and activity of p-azidobenzoyloxyferricrocin, a photoactivatable analog of ferrichrome

Abstractp-azidobenzoyloxy desferriferricrocin (AF) 2, a photoactivatable analog of ferrichrome, was prepared by selective acylation of the serine group of ferricrocin 1 in two steps: transesterification of ferricrocin followed by demetallation. A model compound, (L) 2-benzyloxycarbonylamino-3-p-azidobenzoyloxy N-isopropyl propionamide 8, was separately synthesized in order to set up optimal transesterification conditions to avoid α, β-elimination or epimerization of serine. Binding of iron-loaded AF (FeAF) to the FhuA outer membrane receptor protein of Escherichia coli AB2847 was demonstrated by inhibition of ferrichrome transport, interference with the infection by the bacteriophage φ80 and with killing of cells by albomycin and colicin M. FeAF transported iron only weakly which indicates that the photoaffinity moiety is incompatible with transport or intracellular iron release from the siderophore.