Coordinative interaction of beta-lactam antibiotics with Cu(II) in aqueous solution

Introduction: Coordination of the drug with the enzyme-bound metal ions as well as interaction with any metal ions found in the living cells has stimulated a great deal of investigation. In spite of the significant number of data concerning the protolytic and copper(II) complexing properties of ampicillin and amoxicillin, it must be stated that only few of them, obtained by the more sophisticated fitting procedures, were to some extent consistent with our results. Material and methods: Ampicillin sodium salt (Sigma) and amoxicillin (Fluka) were pure, copper(II) nitrate(V) (POCh Gliwice), potassium(V) nitrate (J.T. Baker) were p.a. Potentiometric equilibria studies were carried out by means of a Molspin automatic titration kit with combined microelectrode OSH 10-10. Overall concentration formation constants: betamlh = [MmLlHh]/[M]m[L]l[H]h were calculated by SUPERQUAD. The UV/Vis absorption spectra were recorded on a Cary 50 Bio spectrophotometer, slit width 1.5 nm, in cells of path length 1 cm. Results and discussion: It was found that both ampicillin and amoxicillin are characterized by three protonation constants in contrast with the dipeptide analogs. The difference is due to the amide function which is not deprotonated for dipeptides at pH<14. Our protonation constants K2 and K3 of ampicillin are in fairly good agreement with the results of one of the references although those authors report only a calculated (estimated) value of K1 = beta011. On contrary, two references concerning amoxicillin confirm all the three protonation equilibria. The potentiometric and spectrophotometric measurements made possible to fit the experimental results to the theoretical model which comprised the accepted protonation constants and the assumed complexing equilibria. Conclusions: The formation of three complexes in solution with ampicillin: CuAmp, Cu(Amp)2 and Cu(Amp)3 but two complexes with amoxicillin: CuAmx and Cu(Amx)2 was proved. Nevertheless, the values of corresponding formation constants are difficult to be compared with the references due to differing protonation models. It seems advisable to attribute such a divergence to a considerable steric hindrance effect connected with the presence of rigid thiazolidyl and beta-lactam rings between the amide bond and the C-teminus.

Keywords: Ampicillin, Amoxicillin, Copper