Why gamma- and delta- are less active than beta-lactams? An ab initio study

Ab initio calculations (HF/6-31G** and MP2/6-31G**) were performed to inves tigate the intramolecular hydrogen bonding in the model beta-, gamma-, and delta -lactam molecules. It was found that the intramolecular (C=O)O-H . . .O=C hydrogen bond stabilizes much more the gamma- and delta -lactam fused ring systems than the beta -lactam penicillin and cephalosporin-like system s. This observation suggests that gamma- and delta -lactams block themselve s by the intramolecular hydrogen bond and therefore are less active toward receptor active site than beta -lactams, It is also likely that this factor can discriminate the beta -lactamase inhibitors.