Thermodynamics of Proton-Coupled Electron Transfer at Tricopper μ-Oxo/Hydroxo/Aqua Complexes

Multicopper oxidases (MCOs) utilize a tricopper active site to reduce dioxygen to water through 4H⁺ 4e^- proton-coupled electron transfer (PCET). Understanding the thermodynamics of PCET at a tricopper cluster is essential for elucidating how MCOs harness the oxidative power of O₂ while mitigating oxidative damage. In this study, we determined the O-H bond dissociation free energies (BDFEs) and pK_a values of a series of tricopper hydroxo and tricopper aqua complexes as synthetic models of the tricopper site in MCOs. Tricopper intermediates on the path of alternating electron and proton transfer (ET-PT-ET-PT-ET) have modest BDFE(O-H) values in the range of 53.0-57.1 kcal/mol. In contrast, those not on the path of ET-PT-ET-PT-ET display much higher (78.1 kcal/mol) or lower (44.7 kcal/mol) BDFE(O-H) values. Additionally, the pK_a of bridging OH and OH₂ motifs increase by 8-16 pK_a units per oxidation state. The same oxidation state changes have a lesser impact on the pK_a of N-H motif in the secondary coordination sphere, with an increase of ca. 5 pK_a units per oxidation state. The steeper pK_a increase of the tricopper center promotes proton transfer from the secondary coordination sphere. Overall, our study shed light on the PCET pathway least prone to decomposition, elucidating why tricopper centers are an optimal choice for promoting efficient oxygen reduction reaction.