The conformational dynamics of the hydroxyethyl group over time were explored during repeated independent 1 ns unrestrained QM/MM MD simulations of each of the eight meropenem acyl-enzymes. Those simulations showed that the hydroxyethyl group occupies different positions in carbapenem-inhibited enzymes (TEM-1, SHV-1, CTX-M-16 and BlaC) from that in carbapenemases (SME-1, SFC-1, KPC-2 and NMC-A), i.e., Positions I and II, respectively (Figure ). Position III occurs for both carbapenemases and inhibited enzymes. Although Position II was also observed in one of the simulations of CTX-M-16, structural analysis showed that the methyl component of the 6α-1R-hydroxyethyl group replaces DW in the active site (moving DW >5 Å from its original position). This trajectory, due to the resulting absence of the nucleophile in the active site, is therefore a hydrolytically inactive complex, and was consequently not included in Figure . The influence of the hydroxyethyl group on activation free energy was also investigated, which for some systems required restraining the dihedral to positions not observed in X-ray structures or QM/MM MD simulations. Three enzymes were chosen as representatives of slightly different class A lactamase active site arrangements: SFC-1 (carbapenemase), TEM-1 (carbapenem-inhibited)