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 = 0.15 mol L⁠−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 pL⁠0.5 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, pL⁠0.5 = 7.19 and 6.67, respectively, at physiological pH, I = 0.15 mol L⁠−1 and t = 25 °C were obtained.