Multiple Proton-Coupled Electron Transfers at a Tricopper Cluster: Modeling the Reductive Regeneration Process in Multicopper Oxidases.

Metal clusters in enzymes carry out the life-sustaining reactions by accumulating multiple redox equivalents in a narrow potential range. This redox potential leveling effect commonly observed in Nature has yet to be reproduced with synthetic metal clusters. Herein, we employ a fully encapsulated synthetic tricopper complex to model the three-electron two-proton reductive regeneration of fully reduced trinuclear copper cluster CuICuICuI(µ2-OH2) (FR) from native intermediate CuIICuIICuII(µ3-O) (NI) in multicopper oxidases (MCOs). The tricopper cluster can access four oxidation states (I,I,I to II,II,II) and four protonation states ([Cu3(µ3-O)]LH, [Cu3(µ3-OH)]L, [Cu3(µ3-OH)]LH, and [Cu3(µ3-OH2)]L, where LH denotes the protonated ligand), allowing mechanistic investigation of proton-coupled electron transfer (PCET) relevant to MCOs. Seven tricopper complexes with discrete oxidation and protonation states were characterized with spectroscopy or X-ray single-crystal diffraction. A stepwise electron transfer-proton transfer (ET-PT) mechanism is established for the reduction of CuIICuIICuII(µ3-O)LH to CuIICuIICuI(µ3-OH)L, while a stepwise PT-ET mechanism is determined for the reduction of CuIICuICuI(µ3-OH)LH to CuICuICuI(µ2-OH2)L. The switch-over from ET-PT to PT-ET mechanism showcases that the tricopper complex can adopt different PCET mechanisms to circumvent high-barrier proton transfer steps. Overall, three-electron two-proton reduction occurs within a narrow potential range of 170 mV, exemplifying the redox potential leveling effect of secondary proton relays in delivering multiple redox equivalents at metal clusters.