Penicillin degradation catalysed by Zn(II) ions in methanol

The rates of degradation, catalysed by Zn2+, of four classical penicillins-amoxicillin, ampicillin and penicillins G and V-were followed at 20 degreesC in methanol by spectrophotometric assays. Kinetic schemes of the reactions of degradation catalysed by Zn2+ ions were analogous to those given previously for the reaction catalysed by Cd2+ 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 S2M, both in equilibrium. In all cases, the degradation reaction is of the first order with respect to SM, with velocity constants at 20 degreesC of 0.0093, 0.0288, 0.0304 and 0.0349 min(-1), for amoxicillin, ampicillin, penicillin V and penicillin G, respectively. The compound S2M degraded at a much lower rate than SM and constitutes a zero-order process. The catalytic effect of the ion Zn2+ in the degradation of the penicillins was much weaker than that of the ion Cd2+, owing to the lesser ionic radius of the former and the fact that in the case of the reaction catalysed by Zn2+, the compound S2M 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 Zn2+ 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 20degreesC were calculated. (C) 2003 Elsevier B.V. All rights reserved.