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 Cu I Cu I Cu I (μ 2 -OH 2 ) ( FR ) from native intermediate Cu II Cu II Cu II (μ 3 -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 ([Cu 3 (μ 3 -O)] L H , [Cu 3 (μ 3 -OH)] L , [Cu 3 (μ 3 -OH)] L H , and [Cu 3 (μ 3 -OH 2 )] 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 Cu II Cu II Cu II (μ 3 -O) LH to Cu II Cu II Cu I (μ 3 -OH) L , while a stepwise PT-ET mechanism is determined for the reduction of Cu II Cu I Cu I (μ 3 -OH) LH to Cu I Cu I Cu I (μ 2 -OH 2 ) 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.