The stability constants of the fluorescent siderophore pseudobactin St3 (PSB 3 ) produced by Pseudomonas putida strain 3 with Fe 3 , Cu 2+ , Mn 2+ , Zn 2+ , and Fe 2+ were determined using fluorescence spectroscopy and potentiometric titrations. Stability constants for PSB 3 , with Fe 3+ , Fe 2+ , Mn 2+ , and Zn 2+ were determined based on titration to saturation of a PSB 3 ligand solution with either of the metals, then back‐titrating the solution with an EDTA solution along with simultaneous measurements of fluorescence and visible spectra. A nonlinear curve fitting between the fluorescence intensity ( Y ) and the concentration ( C ) of the metal or EDTA was used for the determination of the concentrations of the free and complexed ligands. The overall stability constant (log K ML ) obtained for Fe 3+ by this method was the same as that obtained by visible spectrosopy. The log K ML . values obtained by fluorescence spectroscopy for Mn 2+ and Zn 2+ (17.5 and 19.4, respectively) were in good agreement with log K ML values obtained by potentiometric titration (17.3 and 19.8, respectively). It was found that two chromatography bands obtained from PSB 3 solutions isolated and purified from cultures grown under different conditions chelated Fe 3+ with practically the same affinity. The log K ML of PSB 3 with Fe 3+ , Zn 2+ , Cu 2+ , Mn 2+ , and Fe 2+ were calculated using the protonation constants determined by potentiometry (p K a1 = 11.28, p K a2 = 7.74, p K a3 = 5.35) and were found to be 29.6, 19.8, 22.3, 17.3, and 8.3, respectively.
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Chen et al. (1994) studied this question.