Abstract
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 -30mV versus NHE, corresponding to a ligand-based reduction, and at -385mV versus NHE, corresponding to an irreversible Fe(3+)/Fe(2+) reduction of the Fe(L(PF)) (3) (6-) complex.
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