The iron complexation of a fluorescent green pyoverdin siderophore produced by the environmental bacterium Pseudomonas putida was characterized by solution thermodynamic methods. Pyoverdin binds iron through three bidentate chelate groups, a catecholate, a hydroxamate, and an alpha-hydroxycarboxylic acid. The deprotonation constants of the free pyoverdin and Fe(III)-pyoverdin complex were determined through a series of potentiometric and spectrophotometric experiments. The ferric complex of pyoverdin forms at very low pH (pH < 2), but full iron coordination does not occur until neutral pH. The calculated pM value of 25.13 is slightly lower than that for pyoverdin PaA (pM = 27), which coordinates iron by a catecholate and two hydroxamate groups. The redox potential of Fe-pyoverdin was found to be very pH sensitive. At high pH (similar to pH 9-11) where pyoverdin coordinates Fe in a hexadentate mode the redox potential is -0.480 V (NHE); however, at neutral pH where full Fe coordination is incomplete, the redox potential is more positive (E-1/2 = -0.395 V). The positive shift in the redox potential and the partial dissociation of the Fe-pyoverdin complex with pH decrease provides a path toward in vivo iron release.