The hydration effect around Lys73:OQ2 indicated here has an apparently smaller effect on the calculated barriers than that of hydration around Lys73:OQ1, since the presence of an additional water molecule hydrogen bonded to OQ1 raised the barrier for ceftazidime deacylation by approximately 5 kcal/mol. Orientation II (corresponding to a dihedral angle of between 147°-192° depending on the structure and protein chain) is observed in most OXA-48:carbapenem AC crystal structures. In this orientation, no part of the 6α-hydroxyethyl moiety interacts with either the DW or with Lys73, so the antibiotic may possibly not interfere with the reactive atoms. However, calculated deacylation barriers are increased by 2.1 kcal/mol for imipenem, and by 2.4 kcal/mol for meropenem, when comparing orientation II against I (in which only one water molecule is hydrogen bonded to OQ2).