We next considered the possibility of the first proton transfer taking place after reduction of the superoxide (reaction 2 red --→ 3 red , see Fig. ). All attempts to obtain intermediate 2 HIP led to a spontaneous proton transfer from HIP to the distant oxygen in [Cu-OO] 0 , resulting in the formation of 3 red (see Table ). We were able to perform a linear transit calculations with a smaller QM region (see Supporting Information †), the reaction is only downhill with a reaction energy of -118.6 kJ/mol (-100.1 kJ/mol with TPSS). For the intermediate lowest in energy (3l red , calculated with the large QM region), the spin density (see Table ) is 0.55 on Cu and 0.19 on O p in OOH • . The remaining spin density is distributed among the nitrogen atoms of the histidine brace, corresponding to a [Cu(II)-OOH • ] + species.