Neutral and alkaline hydrolyses of model β-lactam antibiotics.: A ab initio study of water catalysis

The effect of an ancillary water molecule on the neutral and alkaline hydrolysis mechanisms of a simple beta-lactam molecule (N-methylazetidinone) has been studied at the Hartree-Fock and MP2 levels using the 6-31G* and 6-31+G* basis sets. The results have been compared with a nonassisted study carried out previously. Solvent effects have been also considered bq means of a polarizable continuum model. In the neutral hydrolysis, the additional water molecule diminishes the free-energy barriers only when correlation energy is taken into account, Concerted and stepwise mechanisms have been described. The corresponding barriers are close, and the actual mechanism could be conditioned by the molecular environment, solution, protein, etc. Using the results of a molecular dynamics simulation of N-methylazetidinone in aqueous solution, it has been shown that the stepwise process is more likely to occur in such conditions. In the alkaline hydrolysis, the first reaction step consists of the formation of a tetrahedral intermediate which requires a desolvation of the hydroxyl anion difficult to reproduce by calculation, Afterward, the hydrolysis reaction proceeds through either concerted or stepwise mechanisms for ring opening and proton transfer. The concerted channel presents a very low energy barrier, and the species involved are dependent on the calculation level. The stepwise mechanism is virtually the same as that previously reported for the nonassisted hydrolysis, the relative energy of all the points along the path being diminished and the energy barriers remaining essentially unaltered.