AB-INITIO MOLECULAR-ORBITAL CALCULATIONS ON THE NEUTRAL HYDROLYSIS AND METHANOLYSIS OF AZETIDINONES, INCLUDING CATALYSIS BY WATER - RELATIONSHIP TO THE MECHANISM OF ACTION OF BETA-LACTAM ANTIBIOTICS

The docking of penicillins to a computer model of the active site of t he penicillin-binding protein of Streptomyces R61 produces structures that exhibit a four-centred interaction between the O-H of a serine re sidue and the (O)C-N of the beta-lactam ring. If such a structure is a stationary point on the reaction coordinate for the acylation of the serine OH by the beta-lactam carbonyl group, the acylation mechanism w ould appear to consist of O-C and H-N bond formation, concerted with t he cleavage of the C-N bond. The existence of this ''N-protonation'' m echanism, and the energetics of this mechanism in comparison to the us ual ''O-protonation'' pathway in which proton transfer to oxygen and a ddition to the carbonyl group precede C-N bond fission, have been exam ined by ab initio MO calculations on the neutral hydrolysis and methan olysis of N-methylazetidinone, penam, and penam-3 alpha-carboxylate. T he geometries of reactants and transition structures have been optimiz ed fully at the 3-21G or 3-21G levels, the existence of transition st ructures has been confirmed by vibrational analysis, thermochemical da ta have been computed, and the one-point energies of all structures ha ve been determined at the MP2/6-31G level. The N-protonation mechanis m is found to exist with all substrates, and to be preferred over the O-protonation mechanism by over 5 kcal/mol. The activation energies ar e 5 kcal/mol lower in the bicyclic penam than in the monocyclic N-meth ylazetidinone, and attack from the convex face is preferred over attac k from the concave face. The introduction of a 3 alpha-carboxylate gro up, as in penicillin itself, results in an additional 5 kcal/mol decre ase in activation energy. The origins of these trends are discussed. S ince the active sites of some penicillin-recognizing enzymes contain a t least one water molecule, catalysis of the foregoing reactions by on e water molecule has also been examined. The catalysis amounts to over 10 kcal/mol in both the N- and O-protonation mechanisms, and the pref erence for the N-protonation mechanism is maintained. It is concluded that penicillin complexes to its receptor in such a manner as to allow acylation of the active site serine residue to proceed via the energe tically most favourable mechanism.