Potentiometric, UV and 1H NMR study on the interaction of Cu2+ with ampicillin and amoxicillin in aqueous solution

A potentiometric, UV and H-1 NMR study on Cu2+-ampicillin [(2S,5R,6R)-6-([(2R)-2-amino-2-phenylacetyl] amino)-3,3-dimethy1-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid] and-amoxicillin [(2S,5R,6R)-[(2R)-2-amino-2-(4-hydroxyphenyl)-acetyl] amino)-3,3-dimethy1-7-oxo-4-thia-l-azabicyclo [3.2.0]heptane24-carboxylic acid] systems in NaCl aqueous solution at I = 0.15 mol L-1 and t = 25 degrees C is reported. On the basis of potentiometric results two speciation models were proposed for each system. It was found that spectrophotometric and H-1 NMR measurements are essential for selecting the most reliable speciation models. They included ML, MLOH and ML(OH)(2) species in both systems and, only for Cult-ampicillin, also MLH species. The stability constants obtained by UV and H-1 NMR titrations were comparable to the ones calculated by potentiometry. The sequestering ability of the ligands under study towards Cu2+ by pL(0.5) empiric parameter (ligand concentration required to sequester 50% of the metal cation present in traces) at several pH values was calculated as well. For ampicillin and amoxicillin, pL(0.5) = 7.19 and 6.67, respectively, at physiological pH, I = 0.15 mol L-1 and t = 25 degrees C were obtained.