Proton-Coupled Electron Transfer of Coenzyme Q in Unbuffered Solution by Pore Confined In Situ Liquid ToF-SIMS

Proton-coupled electron transfer (PCET) process of coenzyme Q in buffered solutions, which is a well-defined overall 2 e − , 2 H + process, has been systematically studied, while that in unbuffered aqueous solutions is still too complicated to be fully understood, primarily due to the uncontrolled local proton concentration at the electrode-electrolyte interface. Herein, time-of-flight secondary ion mass spectrometry (ToF-SIMS) coupled with a microfluidic electrochemical reactor, namely pore confined in situ liquid ToF-SIMS analysis, was adopted to monitor the PCET process of coenzyme Q 0 (CoQ 0 ) at the electrode-electrolyte interface in unbuffered aqueous solution. Evolutions of CoQ 0 and related intermediates during the electrochemical reaction were measured in real-time, which provided direct molecular evidences for the PCET process. The direct observation of CoQ 0 H 2 and hydrated CoQ 0 dianion implied that the reduction of CoQ 0 in unbuffered electrolyte was not a simple overall 2 e − , 2 H + procedure. Moreover, the identification of the CoQ 0 H 2 dimer and the quinone-hydroquinone complex demonstrated the further transformation of CoQ 0 and CoQ 0 H 2 by hydrogen bonding interaction or π -interaction. These results provided a full picture of the mechanism for the PCET process of CoQ 0 in unbuffered aqueous solution, which could contribute to the comprehensive understanding of the electrochemical reactions of coenzyme Q.