THEORETICAL CALCULATIONS OF BETA-LACTAM ANTIBIOTICS .4. AM1, MNDO, AND MINDO/3 CALCULATIONS OF HYDROLYSIS OF BICYCLIC SYSTEM OF PENICILLINS

We carried out a comprehensive theoretical study on the alkaline hydrolysis of the bicyclic system of penicillins (a four-member ring fused to a thiazolidine ring) on the basis of a B(AC2) mechanism. We assayed the MINDO/3, MNDO, and, AM1 semi-empirical calculation methods in order to determine their suitability for studying beta-lactam rings. Both the geometric and the energetic results obtained for the different intermediate states were compared with literature values - chiefly those determined by ab initio methods - with which they proved to be very consistent. The conformation of the carboxyl group at position 3 was found to be rather significant to the determination of the energy of the different reaction maxima and minima.