β-Lactam degradation catalysed by Cd ion in methanol

Kinetic schemes are established for degradation catalysed by Cd²⁺ ions in methanolic medium for penicillin G, penicillin V and cephalothin, a cephalosporin. Methanolysis of penicillin V and cephalothin occurs with the formation of a single substrate-metal ion intermediate complex, SM, while degradation of penicillin G occurs with the initial formation of two complexes with different stoichiometry, SM and S₂M. In each case, degradation is of first order with respect to SM with rate constant values equal to 0.079 min⁻¹, 0.120 min⁻¹ and 0.166 min⁻¹at 20, 25 and 30°C, respectively, for penicillin G; 0.061 min⁻¹ at 20°C for penicillin V; and 2.0×10⁻³ min⁻¹ at 20°C for cephalothin. Activation energy for the decomposition process of the SM intermediate for penicillin G was calculated to be about 5.5×10⁴ J/mol. Equilibrium constant values between SM compound and S₂M at 20°C (77.1 l/mol), 25°C (45.3 l/mol) and at 30°C (25.7 l/mol) were also calculated as well as the normal enthalpy of this equilibrium. With respect to the reaction products there is evidence that Cd²⁺ becomes part of their structure, forming complexes between Cd²⁺ and the product resulting from antibiotic methanolysis (L). Some characteristics of these complexes are discussed.