Characterization of the aqueous iron(III) chelation chemistry of a potential Trojan Horse antimicrobial agent: chelate structure, stability and pH dependent speciation.

The aqueous solution equilibria of a β-lactam antimicrobial agent containing a 3-hydroxy, 4-pyridinone group (L (PF)) binding to Fe(III) in aqueous solution has been characterized through spectrophotometric and potentiometric titrations. The metal-free ligand has four observable protonation constants, pK(a1) = 2.6, pK(a2) = 3.43, pK(a3) = 6.43, and pK(a4) = 9.62. L (PF) forms a 3:1 ligand:Fe(III) complex in aqueous solution through coordinate-covalent bond formation exclusively involving the bidentate hydroxypyridinone moiety. This 3:1 L (PF):Fe complex was found to have a stability constant of log β(130) = 33.46. A speciation diagram for the L (PF) system demonstrates that in the region of physiological pH the tris-(L (PF))Fe(III) complex, Fe(L(PF)) (3) (6-) , predominates. This complex exhibits two irreversible reduction waves in solution at -30 mV versus NHE, corresponding to a ligand-based reduction, and at -385 mV versus NHE, corresponding to an irreversible Fe(3+)/Fe(2+) reduction of the Fe(L(PF)) (3) (6-) complex.