Penicillin degradation catalysed by Zn(II) ions in methanol

Abstract The rates of degradation, catalysed by Zn 2+ , of four classical penicillins—amoxicillin, ampicillin and penicillins G and V—were followed at 20 °C in methanol by spectrophotometric assays. Kinetic schemes of the reactions of degradation catalysed by Zn 2+ ions were analogous to those given previously for the reaction catalysed by Cd 2+ ions. The methanolysis of penicillin V occurs with the formation of a single intermediate substrate–metal complex (SM), whereas the degradations of amoxicillin, ampicillin and penicillin G occur with the initial formation of two complexes with different stoichiometry, SM and S 2 M, both in equilibrium. In all cases, the degradation reaction is of the first order with respect to SM, with velocity constants at 20 °C of 0.0093, 0.0288, 0.0304 and 0.0349 min −1 , for amoxicillin, ampicillin, penicillin V and penicillin G, respectively. The compound S 2 M degraded at a much lower rate than SM and constitutes a zero-order process. The catalytic effect of the ion Zn 2+ in the degradation of the penicillins was much weaker than that of the ion Cd 2+ , owing to the lesser ionic radius of the former and the fact that in the case of the reaction catalysed by Zn 2+ , the compound S 2 M occurred in a much greater amount than the SM. At the end of the degradation reaction, the corresponding penamaldic derivative of the antibiotic was produced, established by the coordination of the Zn 2+ ion, forming a single complex 2:1 (derivative penamaldic–metal) in the case of amoxicillin and ampicillin; and two complexes, 1:1 and 2:1, for the other antibiotics. Finally, the molar absorption coefficients of the products of reaction at the wavelength of maximum absorption at 20 °C were calculated.