Potentiometric, UV and 1H NMR study on the interaction of penicillin derivatives with Zn(II) in aqueous solution.

The interaction of Zn(II) with ampicillin [(2S,5R,6R)-6-([(2R)-2-amino-2-phenylacetyl]amino)-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2- carboxylic acid] and amoxicillin [(2S,5R,6R)-6-{[(2R)-2-amino-2-(4-hydroxyphenyl)-acetyl]amino}-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-24-carboxylic acid] in NaCl aqueous solution at different ionic strengths and at t=25°C was investigated by potentiometric, UV and 1H NMR techniques. Fairly similar speciation models were obtained for the two systems. At I=0.15molL-1, two different sets of measurements, at low and high concentrations, were carried out. For the Zn2+-amoxicillin system, the Zn2L2(OH)2 species was obtained in the set of measurements at high concentration. The spectrophotometric and 1H NMR results thus obtained are fully consistent with the speciation models found from potentiometric investigations, confirming the formation as well as the relative stability of the complex species. The dependence of the stability constants on the ionic strength was modeled by means of the Debye-Hückel and SIT (Specific ion Interaction Theory) approaches, and the parameter that accounts for the variation of the stability constants with the ionic strength and the specific ion interaction parameters were determined for all the ionic species. The sequestering ability of the ligands towards Zn2+ was evaluated by determining the pL0.5 parameter at different ionic strengths. It resulted that the sequestering ability of ampicillin is higher of ~0.5 order of magnitude with respect to amoxicillin.