We continue by comparing the reaction energies and barriers for the two proton transfers from 3 --→ 4 and 3 red --→ 4 red (see Fig. and). In both reactant species (3a HIP and 3 red HIP ), the OOH species is turned so that the unprotonated oxygen O p is closer to H ε2 in HIP208, with O p -H ε2 distances of 3.6 Å in 3a HIP and 2.9 Å in 3 red HIP ). Both reactions proceed in a similar way (see Fig. and) with a turning of HIP208 to a stable intermediate I/I red , followed by proton transfer. The first part of the reaction 3a HIP --→ I has a small barrier of 16.5 kJ/mol and is 9.9 kJ/mol downhill. After the reduction, the reaction 3a red HIP --→ I red has a similar barrier of 15.0 kJ/mol, but the intermediate is 13.3 kJ/mol higher in energy than the reactant. The distance between O p and H ε2 is 1.7 Å in both intermediate structures. The following proton transfers I --→ 4a and I red --→ 4a red have similar barriers of 17.4 kJ/mol and 17.5 kJ/mol and reaction energies of 3.9 kJ/mol and 10.8 kJ/mol, respectively. The big-QM calculations agree overall well with the QM/MM results: for the reaction 3a HIP --→ 4a, we obtain total barriers of 16.5 and 27.6 kJ/mol with QM/MM and big-QM calculations, respectively. After reduction, the total barriers are 30.8 and 22.0 kJ/mol with QM/MM and big-QM calculations, respectively. Thus the second proton transfer is equally feasible both before and after the reduction, regardless of the used functional (see Figures and).