Complexation of oxoanions and cationic metals by the biscatecholate siderophore azotochelin

Abstract

Azotochelin is a biscatecholate siderophore produced by the nitrogen-fixing soil bacterium Azotobacter vinelandii. The complexation properties of azotochelin with a series of oxoanions [Mo(VI), W(VI) and V(V)] and divalent cations [Cu(II), Zn(II), Co(II) and Mn(II)] were investigated by potentiometry, UV–vis and X-ray spectroscopy. Azotochelin forms a strong 1:1 complex with molybdate (log K =  7.6 ± 0.4) and with tungstate and vanadate; the stability of the complexes increases in the order Mo < V < W (log K ^Mo_app  = 7.3 ± 0.4; log K ^V_app  = 8.8 ± 0.4 and log K ^W_app  = 9.0 ± 0.4 at pH 6.6). The Mo atom in the 1:1 Mo–azotochelin complex is bound to two oxo groups in a cis position and to the two catecholate groups of azotochelin, resulting in a slightly distorted octahedral configuration. Below pH 5, azotochelin appears to form polynuclear complexes with Mo in addition to the 1:1 complex. Azotochelin also forms strong complexes with divalent metals. Of the metals studied, Cu(II) binds most strongly to azotochelin ${(\log \beta_{{{\text{CuLH}}^{{2 -}}}}=-12.9\pm 0.1)}$, followed by Zn(II) ${(\log \beta _{{{\text {ZnL}}^{{3 -}}}} =-24.1\pm 0.14, \log \beta _{{{\text {ZnLH}}^{{2 -}}}} =-17.83\pm 0.09)}$, Mn(II) ${(\log \beta _{{{\text {MnL}}^{{3 -}}}} = -29, \log\beta_{{{\text {MnLH}}^{{2-}}}}=-18.6\pm 0.8, \log \beta _{{{\text {MnLH}}_{2} ^{-}}} =-11.5\pm 0.7)}$ and Co(II) ${(\log \beta _{{{\text {CoLH}}^{{2 -}}}}= -23.0\pm0.3, \log \beta _{{{\text {CoLH}}_{2} ^{-}}}=-13.5\pm 0.2)}$. Since very few organic ligands are known to bind strongly to oxoanions (and particularly molybdate) at circumneutral pH, the unusual properties of azotochelin may be used for the separation and concentration of oxoanions in the laboratory and in the field. In addition, azotochelin may prove useful for the investigation of the biogeochemistry of Mo, W and V in aquatic and terrestrial systems.