Metal binding by pyridine-2,6-bis(monothiocarboxylic acid), a biochelator produced by Pseudomonas stutzeri and Pseudomonas putida

Pyridine-2,6-bis(monothiocarboxylic acid) (pdtc),a natural metal chelator produced by Pseudomonas stutzeri and Pseudomonas putidathat promotes the degradation of carbon tetrachloride, was synthesized and studiedby potentiometric and spectrophotometric techniques. The first two stepwise protonationconstants (pK) for successive proton addition to pdtc were found to be 5.48 and2.58. The third stepwise protonation constant was estimated to be 1.3. The stability (affinity)constants for iron(III), nickel(II), and cobalt(III) were determined by potentiometric orspectrophotometric titration. The results show that pdtc has strong affinity for Fe(III)and comparable affinities for various other metals. The stability constants (log K) are 33.93 for Co(pdtc)₂ ^1-; 33.36 for Fe(pdtc)₂ ^1-; and 33.28 for Ni(pdtc)₂ ^2-. These protonationconstants and high affinity constants show that over a physiological pH range theferric pdtc complex has one of the highest effective stability constants for ironbinding among known bacterial chelators.