Transition state structures of a dipeptide related to the mode of action of beta-lactam antibiotics.

The tetrahedral adducts formed during nucleophilic attack by a hydroxyl ion on the carbonyl carbon of a model dipeptide, glycylglycine, were studied by modified-intermediate-neglect-of-differential-overlap molecular orbital calculations. This dipeptide is taken to represent the D-alanyl-D-alanine terminus of the polypeptides involved in the cross-linking transpeptidation reaction of peptidoglycan in bacterial cell walls. It was found that nucleophilic attack on one face of the carbonyl carbon leads to a transition intermediate species structurally similar to that afforded by the bicyclic nucleus of penicillins and cephalosporin antibiotics. The results support the concept that the beta-lactam antibiotics, which are known to inhibit various bacterial cell wall enzymes, may act as transition state analogs. Also, the structure formed from nucleophilic attack on the so-called alpha face of the dipeptide is more similar to the antibiotic structures than is that from attack on the opposite face. In agreement with other types of experiments, the results suggest that the alpha face may be the one approached by a nucleophile in the receptor site(s) of the appropriate cell wall enzymes.