Alternatively, since the following second proton transfer has a low barrier, and since the following dissociation of H 2 O 2 from Cu(II) is also more favorable than dissociating other oxygen species, this is also a viable pathway. Therefore, we propose that the reaction proceeds from the superoxide [Cu-OO] + 2 via PCET to 3 red , followed by a second proton transfer and the dissociation of H 2 O 2 from Cu(II). This is different from previous proposals for the generation of H 2 O 2 . Caldararu et al. investigate an JdAA10 LPMO with a quite different active-site architecture (using glutamate instead of histidine as proton donor) and conclude that the overall reaction proceeds as a proton transfer 2 --→ 3, followed by reduction 3 --→ 3 red , and then another proton transfer. They find that H 2 O 2 dissociation from Cu(II) was not possible, but after reduction it is thermoneutral. Thus, their proposed reaction ends in Cu(I) and H 2 O 2 . An alternative possibility was investigated by Wang et al., who used ascorbate both as reductant and proton donor (HAA), also ending in Cu(I) and H 2 O 2 . These authors investigated a different LPMO family (LsAA9), but one with a second-coordination sphere that more closely resembles the Tf AA10A investigated here. Thus, the LsAA9 also has a secondsphere histidine but Wang et al. argue against it as a proton donor. It is tempting to speculate that the different results may be due to the fact that different mechanisms are active for different LPMO families. However, seeing that we obtain similar results to the study on JdAA10 LPMO for the first proton transfer (2 --→ 3) with the TPSS functional, we believe it is too early for such conclusions. Analysis of the resulting electronic states indicates that the TPSS functional obtains a wrong spin state and thereby underestimates the reaction energy and barrier for this particular reaction. Additional investigations would be required to elucidate if the mechanism in JdAA10 is the same as found here, but we find it likely that this is the case. Compared to the mechanism for LsAA9 , we cannot compare directly without including ascorbate. We are aiming to perform such studies in future investigations.