An ab initio study of the β-lactam structure

Abstract

Model compounds representing the p-lactam antibiotics have been studied by ab initio quantum chemical methods. The peptide bond length is optimized at the STO-3G level in formamide as a function of the angles at carbon and nitrogen. All ring bonds and angles in β-lactam and the peptide bond in vinyl β-lactam are optimized as a function of the external angles at nitrogen. The optimized peptide bond lengths in these model compounds compare well with the observed bond lengths in crystal structures and with their biological activities. The peptide bond is found to be sensitive to the angles at nitrogen in β-lactam and to the presence of an adjacent double bond. The energies of p-lactam conformers are studied at the 4-31G level as a function of the external angles at nitrogen; the planar form is predicted to be the most stable structure.