Mechanistic Insights into Tyrosine Oxidation Revealed by Fast-Scan Cyclic Voltammetry

Abstract

A precise understanding of opioid dynamics in the brain is lacking, largely because few analytical tools are available that are sufficiently sensitive and selective for directly monitoring these important neurochemicals in situ. Tyrosine is a key part of the opioid neuropeptides, and its redox activity can be exploited in their detection. However, a solid understanding of tyrosine electrochemistry on carbon electrodes is first required. It is known that, upon oxidation, tyrosine quickly undergoes chemical transformation to products that foul the electrode, compromising the electrochemical information. This issue can be avoided by using fast-scan cyclic voltammetry (FSCV) which enables collection of well-defined voltammograms on a millisecond timescale before appreciable accumulation of byproducts. In this work, the tyrosine oxidation reaction is characterized and electrode fouling is assessed when tyrosine is incorporated into short synthetic peptides containing other amino acids that are part of the opioid neuropeptides: Gly, Phe, Met, Leu, and Arg. Voltammetric data were obtained in phosphate buffered solution on carbon microelectrodes at scan rates up to 1 kV/s. The results demonstrate the dependence of the peak oxidation potential on solution pH, and identify possible reaction pathways and byproducts that foul the electrode. Peak oxidation potential, peak shape, and ultimately, sensitivity to tyrosine are dependent on the peptide sequence. NMR spectroscopy is used to relate the proximity of proton acceptor/donor functional groups to the electrochemical results in the context of proton-coupled electron transfer (PCET) theory. These fundamental studies are important for the electrochemical detection of important neuropeptides in the complex in vivo environment. Furthermore, a better understanding of the tunability of PCET reaction rates in electrochemical systems has broad implications for technology in a range of areas including artificial photosynthesis, solar energy conversion, and energy storage.