To elucidate the underlying causes of the differences in the PMFs between oxytocin and angiotensin IV, the geometries of the tetrahedral intermediates were analyzed. Visual inspection of snapshots extracted from the ensemble of umbrella sampling windows at a reaction coordinate of 1.5 Å revealed common structural features of the tetrahedral intermediates in both peptides: the scissile NH group forms a hydrogen bond with the Ala429 backbone due to the hybridization of the scissile nitrogen, with the lone pair oriented away from the protonated Glu465. This geometry of the scissile nitrogen was also observed in the oxytocin tetrahedral intermediate, indicating that the lone pair of the scissile nitrogen must adopt an orientation away from the protonated Glu465 following nucleophilic attack to form a stable intermediate (Figure ). The stereochemistry of the TIs in both peptides aligns with Deslongchamps' stereoelectronic theory. This theory posits that a favorable orbital interaction occurs between the nitrogen lone pair and the sigma antibonding orbital of the newly formed C-O bond subsequent to the nucleophilic attack.