Conductometric Determination of the Stability Constants of the Quinolone Antibacterial Agents Levofloxacin and Sparfloxacin with Divalent Metal Ions

The stability of the metal ion complexes of the fluoroquinolone antibiotics sparfloxacinand levofloxacin with the divalent magnesium, calcium, barium and nickel perchlorates has been investigated by electrical conductivity in methanol at 25°C. Under the experimental conditions used, it was found that the metal ions interact with ligands in the zwitterionic state leading to the formation of positively charged complexes. The complexes of levofloxacin with the four metal ions are slightly more stable than those of sparfloxacin due to the differences in the substitution pattern between the two ligands. The stability of the complexes formed decreases in the order Mg^2+ Ca^2+ Ni^2+ Ba^2+. The observed order of decrease of the complex stability is attributed to the variation in ionic radii between metal ions and to the nature of the interaction between the ligands and the metal ions.