Mechanistic Insight into Tyrosine Oxidation at Carbon-Fiber Microelectrodes Revealed by Fast-Scan Cyclic Voltammetry

A comprehensive understanding of tyrosine (Tyr) redox behavior will allow for improved electroanalytical monitoring and can shed light on complex biochemical processes. However, the study of Tyr at carbon electrodes is impeded by electrode fouling, which obfuscates quantitative analysis and mechanistic interpretation. In this study, fast-scan cyclic voltammetry (FSCV) was used to characterize Tyr redox properties at carbon-fiber microelectrodes. Tyr exhibited a distinct anodic peak indicative of irreversible redox chemistry that was modeled as a single-proton and single-electron transfer reaction combining surface-adsorbed and diffusing species. The adsorption properties and additional reactions were dependent on experimental conditions. Tyr accumulated on the electrode surface when negative holding potentials were employed between voltammetric sweeps, and Tyr oxidation resulted in electrochemical-chemical (EC) reactions that were dependent on the potential waveform employed. An unidentified species was generated when the potential was limited to +0.9 V, and a species that was putatively identified as DOPA/dopaquinone was generated when a 1.4 V switching potential was used, demonstrating a mechanistic role for reactive oxygen species in the reaction mechanism. When incorporated into small peptides, the oxidation potential and electron transfer rates were dependent on the position of Tyr relative to the N- or C-terminus, as well as the hydrophobicity of the nearby residues. Overall, these studies establish a robust framework for interpreting Tyr electroactivity at carbon-fiber microelectrodes, providing critical insight for the quantitative analysis of Tyr and Tyr-bearing biomolecules in physiological systems and guiding the development of antifouling strategies to enhance electrode performance.