Mechanistic Continuum from Stepwise to Concerted Proton-Coupled Electron Transfer Pathways at a Synthetic Tricopper Cluster

Multicopper oxidases (MCOs) couple the oxidation of metal or organic substrates with the four-electron, four-proton proton-coupled electron transfer (PCET) conversion of O₂ to H₂O. This transformation requires at least four Cu ions: a Type I (T1) Cu site and a trinuclear Cu cluster composed of mononuclear Type II and binuclear Type III Cu centers (T2/T3). While the potential of the T2/T3 cluster remains relatively invariant (0.36-0.40 V vs NHE), that of the T1 sites varies widely among different MCOs (0.34-0.76 V vs NHE). Herein, we employ a synthetic tricopper cluster to elucidate how changes in electron-transfer driving force (Δ<i>G</i>_ET) influence the mechanism of PCET from a Cu(II,II,II)-oxo cluster to a Cu(II,I,I)-hydroxo cluster, which models one of the PCET steps involved in the reductive regeneration of the Cu(I,I,I) state in MCOs. We find that three mechanistic pathways─electron transfer-proton transfer (ET-PT), proton transfer-electron transfer (PT-ET), and concerted proton-electron transfer (CPET)─are all accessible by tuning the electron-transfer driving force and temperature. The well-defined spectroscopic signatures of ET and PT intermediates enable quantitative kinetic analysis that resolves the relative contributions of each pathway as the mechanism evolves from ET-PT to CPET to PT-ET. These results reveal a mechanistic continuum rather than a discrete switch between stepwise and concerted PCET processes.