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

A potentiometric, UV and ¹H NMR study on Cu²⁺-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] systems in NaCl aqueous solution at I=0.15molL^-1 and t=25°C is reported. On the basis of potentiometric results two speciation models were proposed for each system. It was found that spectrophotometric and ¹H 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 Cu²⁺-ampicillin, also MLH species. The stability constants obtained by UV and ¹H 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=0.15molL^-1 and t=25°C were obtained.