Siderophore−Manganese(III) Interactions. I. Air-Oxidation of Manganese(II) Promoted by Desferrioxamine B

Abstract

Recent studies suggest that aqueous Mn(ll) complexes, particularly those with non-carboxylated ligands such as microbial siderophores, may be stable in soil and aquatic environments. In this paper, we determine the stability constants for Mn(ll) and Mn(lll) complexes with the common trihydroxamate siderophore, desferrioxamine B (DFOB). Base and redox titrations were conducted to determine DFOB conditional protonation constants and conditional stability constants for 1:1 DFOB complexes with Mn(ll) and Mn(lll). The conditional protonation constants agree well with literature values. We determined stability constants for three Mn(ll)-DFOB species and one Mn(lll)-DFOB species at 25 degrees C in 0.1 M NaCl. The Mn(lll) HDFOB+ complex can be formed readily by air-oxidation of Mn(ll)-DFOB. This reaction exhibits pseudo first-order kinetics with a rate coefficient that can be characterized as the product of oxygen concentration with a linear combination of the concentrations of the three Mn(ll)-DFOB complexes. The second-order rate coefficients appearing in this linear combination are 1 to 2 orders of magnitude smaller than that associated with oxidation of the hydrolytic species Mn(OH)(0)2. The Mn(lll)HDFOB+ complex is stable for pH in the range of 7.0-11.3; but, at pH < 7.0 it decomposes by internal electron transfer, yielding oxidized DFOB products and Mn(ll). For p[H+] > 11.3, the complex degrades by disproportionation, yielding Mn(ll) and solid MnO2. This range of pH stability supports the hypothesis that aqueous Mn(lll) may play a vital role in the biogeochemical cycling of not only manganese, but also other elements, such as carbon, sulfur, nitrogen, oxygen, and redox-active metals.