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

Abstract

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 Q0 (CoQ0) at the electrode-electrolyte interface in unbuffered aqueous solution. Evolutions of CoQ0 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 CoQ0H2 and hydrated CoQ0 dianion implied that the reduction of CoQ0 in unbuffered electrolyte was not a simple overall 2 e−, 2 H+ procedure. Moreover, the identification of the CoQ0H2 dimer and the quinone-hydroquinone complex demonstrated the further transformation of CoQ0 and CoQ0H2 by hydrogen bonding interaction or π-interaction. These results provided a full picture of the mechanism for the PCET process of CoQ0 in unbuffered aqueous solution, which could contribute to the comprehensive understanding of the electrochemical reactions of coenzyme Q.