Spectrofluorimetric study of the degradation of α-amino β-lactam antibiotics catalysed by metal ions in methanol†

Abstract

The α-aminopenicillins ampicillin and amoxicillin and a cephalosporin, cephalothin, give rise to a fluorescent product when their methanolic solutions are incubated for prolonged time periods. The process also occurs in the presence of the metal ions Cd²⁺, Co²⁺ and Zn²⁺. The effects of the different ions on the emission and excitation wavelengths and the appearance rate of the fluorophore were studied. The appearance of the fluorescent product was zero order for ampicillin and amoxicillin in metal ion-free solution and solutions with Cd²⁺ and Zn²⁺, whereas in the presence of Co²⁺ ion it was first order under the experimental conditions used; for cephalothin it was first order in all cases. Apparent fluorescent compound formation rates were calculated in the zero-order reactions and rate constants in the first-order reactions. The activation energy of the formation reaction of the fluorescent products of amoxicillin and ampicillin was calculated from a study of the reactions at four temperatures; all the values recorded were between 34 and 118 kJ mol^–1. As a possible mechanism for the formation of these products, cyclization of the penamaldic derivative of the antibiotic, which is formed in the first stage of the methanolytic reaction, is proposed.