We continue by calculating the formation of H 2 O 2 at the activesite, using again the second-coordination-sphere histidine as proton donor, assuming that the histidine can be protonated a second time. However, before investigating the proton transfer, we investigated if internal transfer of the proton from the distant oxygen O d to the proximal one O p in 3a or 3a red is feasible. Since the reaction barriers are 180.9 kJ/mol for the reaction 3a --→ 3 int in the open-shell singlet state and 158.1 kJ/mol for the reaction 3i red --→ 3 red int , internal proton transfers are unfeasible (structures and energies are given in Fig. ).