Structural dependence of Mn complexation by siderophores: Donor group dependence on complex stability and reactivity

Siderophores traditionally have been viewed as solely being involved in the biogeochemical cycling of Fe(III). This paradigm, however, ignores the diverse roles siderophores may play in the cycling of other trace metals, such as Mn, Co, Mo, and V. Recent work has shown that siderophores form complexes with high stability constants with Mn(III), which are in some cases higher than that of the corresponding Fe(III) complex. Herein, we report on a structural analysis of the dissolved Fe(III)- and Mn(III)-siderophore complexes of rhizoferrin and two pyoverdin-type siderophores using X-ray spectroscopic techniques. Additionally, the stability constants of the Mn(III)-pyoverdinPaA and Mn(III)-rhizoferrin complexes have been quantified as log β111 = 47.5 ± 0.3 and log β110 = 29.8 ± 0.3, respectively. Comparisons of thermodynamic stability and solution structures of Fe(III)- and Mn(III)-complexes with a variety of siderophores demonstrate the relationship between donor group identity, siderophore structure, and strength of complex formation. Rhizoferrin and two mixed-moiety pyoverdins bind with a higher affinity for Mn(III) than Fe(III), possibly because of binding moiety composition which makes them better able to accommodate Jahn-Teller distortion. In contrast, Fe(III) forms complexes of higher relative stability with siderophores that contain hydroxamate and catecholate moieties, more rigid donor groups that form five-membered chelate rings.