The first possibility (i) has been investigated previously: in a combined spectroscopic and computational investigation Kjaergaard et al. studied O 2 activation in TaAA9A. Using quantum mechanical (QM) cluster models, they suggested that the re-Fig. The QM and QM/MM systems mainly employed in this study (top). Residue labels correspond to labels in PDB file 5UIZ . Mechanism of H 2 O 2 generation investigated in this study (bottom). Since the crystal structure is photoreduced, we started our calculations from the Cu(I) state 1. In addition to the QM region shown here, we employed a smaller QM region (shown in Fig. ) for the reaction 2 red ---→ 3 red and performed big-QM calculations for the other proton-transfer steps (see Computational Details). lease of superoxide is uphill but feasible with a free-energy of 43.1 kJ/mol, and that O • - 2 abstracts a hydrogen atom from the axial water molecule in the release process. However, in more recent quantum mechanics / molecular mechanics (QM/MM) calculations on JdAA10A, dissociation of O • - 2 from Cu(II) was found to be unfeasible (this reaction corresponds to reaction 2 --→ 2 dis in Fig. ). Instead, the same study showed that the release of OOH • or H 2 O 2 from Cu(I) are favorable (corresponding to reactions 3 red --→ 3 red,dis and dissociation from 4 Cu(I) in Fig. , respectively). This result is supported by an experimental study by Hegnar et al. , who investigated cellulose oxidation and H 2 O 2 generation by GtAA9B and suggest that the superoxide remains bound to Cu(II).