Bacterial siderophores: the solution stoichiometry and coordination of the Fe(III) complexes of pyochelin and related compounds

Pyochelin, its analog 3''-nor-NH-pyochelin, and the related methyl hydroxamate, 2-(2'-hydroxyphenyl)-4,5-dihydrothiazol-4-carboxylic acid methoxymethyl amide, have been prepared together with their Fe(III) complexes. The solution stoichiometry and the coordination of the three Fe(III) complexes in methanol or buffered (pH approximately 2) 50:50 (v/v) methanol-water mixtures were determined using various spectroscopic methods: UV-vis absorption, X-ray absorption, extended X-ray absorption fine structure and electron paramagnetic resonance. All three systems showed both a 1:1 and 2:1 ligand-Fe(III) stoichiometry, but presented different coordination properties. Conditional formation constants (pH approximately 2) were determined for both the 1:1 and 2:1 complexes in all three systems. Computation of the coordination-conformational energies by semiempirical methods indicated that the coordination in the case of the 2:1 complexes of pyochelin-Fe(III) and 3''-nor-NH-pyochelin-Fe(III) was asymmetrical, with one molecule of pyochelin (or 3''-nor-NH-pyochelin) tetradentately coordinated (O1, N1, N2 and O3) to the Fe(III), and the second molecule bound bidentately (O1, N1 or N2, O3), to complete the octahedral geometry. In contrast, two molecules of the methyl hydroxamate each provided a set of tridentate ligand atoms in the formation of the 2:1 ligand-Fe(III) complex. These results are consistent with the role of pyochelin in the uptake of iron by the FptA receptor in the outer membrane of Pseudomonas aeruginosa and in several gram-negative bacteria.