The 34-kDa periplasmic iron-transport protein (FBP) from Neisseria gonorrhoeae (nFBP) contains Fe(III) and (hydrogen)phosphate (synergistic anion). It has a characteristic ligand-to-metal charge-transfer absorption band at 481 nm. Phosphate can be displaced by (bi)carbonate to give Fe.CO(3).nFBP (lambda(max) 459 nm). The local structures of native Fe-PO(4)-nFBP and Fe.CO(3).nFBP were determined by EXAFS at the FeK edge using full multiple scattering analysis. The EXAFS analysis reveals that both phosphate and carbonate ligands bind to FBP in monodentate mode in contrast to transferrins, which bind carbonate in bidentate mode. The EXAFS analysis also suggests an alternative to the crystallographically determined position of the Glu ligand, and this in turn suggests that an H-bonding network may help to stabilize monodentate binding of the synergistic anion. The anions oxalate, pyrophosphate, and nitrilotriacetate also appear to serve as synergistic anions but not sulfate or perchlorate. The oxidation of Fe(II) in the presence of nFBP led to a weak Fe(III).nFBP complex (lambda(max) 471 nm). Iron and phosphate can be removed from FBP at low pH (pH 4.5) in the presence of a large excess of citrate. Apo-FBP is less soluble and less stable than Fe.nFBP and binds relatively weakly to Ga(III) and Bi(III) but not to Co(III) ions, all of which bind strongly to apo-human serum transferrin.