High-level DFT/MM calculations and electronic structure analysis. Consistent with the DFTB3/MM enhanced sampling simulations of the nucleophilic attack reaction, similar geometries and reaction energetics were obtained using a static DFT/MM approach. The QM region was treated at the B3LYP-D3BJ/def2-TZVP level of theory. Geometry optimisations were performed at the B3LYP-D3BJ/6-31G(d):MM level. The QM/MM calculations yielded an energy barrier of 10.6 kcal mol -1 for the formation of a stable TI in the case of oxytocin, with the TI lying 6.1 kcal mol -1 above the reactant state. For angiotensin IV, the formation of the TI was less favourable, with an energy barrier of 16.0 kcal mol -1 and a reaction energy of 15.2 kcal mol -1 (see potential energy profiles in Figure , SI).