Voltammetric kinetic discrimination of two sequential proton-coupled electron transfers in serotonin oxidation: Electrochemical interrogation of a serotonin intermediate

Abstract

An understanding of the electrochemical mechanism involved in serotonin oxidation is critical to the development of accurate sensing applications. Electrochemical signals of a reactive serotonin intermediate can reveal detailed mechanistic information. Here, oxidation of a serotonin radical intermediate, SH∙, is electrochemically interrogated in a buffered system at a neutral pH by voltammetric kinetic discrimination (VKD). At a low scan rate of 0.02 V/s, the voltammogram showed one oxidation peak that can be attributed to electro-oxidation of serotonin with two proton-coupled electron transfers (PCETs) in the absence of voltammetric resolution. This corresponds to the inherent thermodynamic instability of a reactive serotonin intermediate, leading to potential inversion in two sequential PCETs. However, at a scan rate of 1 V/s, the single voltammetric peak gradually resolved into two, each of which are associated with sequential electro-oxidation of serotonin via 1e−-1H+. The peak resolution originated from kinetic discrimination between the first and second PCET in serotonin oxidation; the first PCET step is Nernstian, while the second is electrochemically irreversible. The observed VKD in electro-oxidation of serotonin can be explained by voltammetric simulations.