Structures for the reactants, transition states, and products, along with selected distances, are shown in Fig. (additional distances are provided in Table in the SI). As can be seen from this Figure, structural changes during reaction II are similar for TaAA9 ), which increases by 0.09-0.10 Å, consistent with the protonation of the oxyl. We also note that for TaAA9 we were able to obtain an isomer of 3, here denoted as 3 ′ (shown in Fig. ). This isomer differs from 3 in Fig. in that the OH-group of the tyrosine points towards the C ϵ1 in the imidazole ring of His1. In 3, the OH group instead forms a hydrogen bond with the oxygen in Gln173. Since the energy difference between the isomers is rather small (∆E = 12 -19 kJ/mol in their triplet state depending on the functional), it is likely that they both exist in solution. The structure of the 3 ′ conformer seems optimal for the transfer of the H-atom of the tyrosine OH-group to the histidyl, according to reaction III in Fig. as recently suggested. We investigate this possibility in the next subsection.