We performed further QM/MM simulations, of mutants and also enforcing specific conformations, to test the effects on the reaction barrier. It has been previously suggested that the carbapenem 6α-1R-hydroxyethyl group can retard the deacylation rate by forming H-bonds with the DW that weaken its nucleophilicity . Our simulations indicate that the 6α-1R-hydroxyethyl group being oriented in positions I or III (seen in carbapenem-inhibited enzymes) leads to a higher deacylation barrier. These positions somewhat displace the DW, preventing efficient nucleophilic attack upon the acyl-enzyme carbonyl carbon. Of the three distinct rotamers for the carbapenem 6α-1R-hydroxyethyl group, only position II allows the polar component (the hydroxyl group) to point out of the binding site, leading to lower reaction barriers. Position II is observed, both in Xray structures and during our QM/MM MD calculations, only for carbapenem complexes of carbapenem-hydrolyzing enzymes .