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

Abstract

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 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=0.15molL−1 and t=25°C were obtained.