Before investigating reaction III, we note that we have here only investigated H-abstraction from His1. This was decided based on the structure in TaAA9, where H-abstraction from His1 appeared more plausible compared to His86, given the notably shorter distance between H ϵ1 and O Oxyl (2.48 Å) in contrast to the H ϵ1 of His86 (3.39 Å). This was confirmed by a test calculation (Table ) for the triplet potential energy surface (PES), showing that the reaction barrier is indeed significantly higher compared to the abstraction from His1 (27-28 kJ/mol depending on the functional). Moreover, the product is thermodynamically less stable by 19-21 kJ/mol. It is interesting to note that in LsAA9, the abstraction from His78 (equivalent to His86 in TaAA9) occurs with only minor changes in energy (<8 kJ/mol for the reaction barrier and energy for both functionals), compared to the H-abstraction from His1. The difference between the two LPMOs occurs since the distances of H ϵ1 on His78 and His1 to O Oxyl in 1 is much closer (2.31 Å and 2.70 Å35 ) in LsAA9. This underlines that small differences in the active site architecture can lead to mechanistic differences, and this will become more apparent in the discussion regarding reaction III in the next subsection.