Penicillin degradation catalysed by Zn(II) ions in methanol.

The rates of degradation, catalysed by Zn(2+), of four classical penicillins-amoxicillin, ampicillin and penicillins G and V-were followed at 20 degrees 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 degrees 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 degrees C were calculated.