Redox Cycling Behavior of Catecholamines and Their Mixtures at Different Diffusion Distances: Steps Toward Quantitative Speciation

Catecholamines (dopamine, DA; norepinephrine, NE and epinephrine, EP) are neurotransmitters of great importance in the mammalian brain. The electrochemical oxidation of these catecholamines has been studied before and can undergo a disproportionation reaction.^1,2 During electrochemical oxidation, a catecholamine undergoes a reversible 2‑electron transfer process and is oxidized to its o‑quinone form. At physiological pH, the o-quinone deprotonates and undergoes homogeneous intramolecular cyclization to form the leucoaminochrome. The leucoaminochrome can oxidize to form the aminochrome which is not electroactive by reacting with the o-quinone form and re-generating the starting catecholamine species. The degree of cyclization reaction will change the reversibility of the total reaction. Different catecholamines have different apparent rate constants of the cyclization reaction (k_DA = 0.13 ± 0.05 s^-1, k_NE = 0.98 ± 0.52 s^-1, k_EP = 87 ± 10 s^-1, in phosphate buffer at pH 7.4³), which provide the possibility for distinguishing the three catecholamines from each other in an electrochemical analysis.

The electrochemical method of redox cycling has the ability of recycling electrochemically reversible species, thus providing amplified signals and the selectivity of reversible species over irreversible ones.⁴ In the redox cycling process of catecholamines, the oxidation and subsequent reduction reactions take place at different electrodes, namely the generator and collector electrodes, respectively. The time between the two reactions is controlled by the time the species diffuse across the gap between those electrodes. The extent of reversibility of the heterogeneous electron transfer reaction during redox cycling is then determined by the relationship of diffusion time and cyclization reaction time.⁵

We will report the electrochemical redox cycling behaviors of individual and binary mixtures of DA, NE, and EP using microfabricated gold microelectrode arrays with various gap widths between the generators and collectors. The catecholamines showed decreasing collector signals as gap width increased with increasing cyclization rates, which is a reflection of how much o-quinone survives diffusion time from the generator. With increasing gap width, the collector signals of the three catecholamines all decreased because of decreasing collection efficiencies, as expected.  However, the signals decreased at different extents, depending on the cyclization rates.  The collector signal for EP became undetectable at a smaller gap width than that for NE, while the collector signal for DA was still measurable.   In the binary mixture of catecholamines, o-quinone and leucoaminochrome from different catecholamines can also react, thus make the collector signal decrease further, compared to solutions containing a single kind of catecholamine.

Acknowledgments

Research was supported partially through the National Science Foundation (CBET-1336853) and the Arkansas Biosciences Institute, the major research component of the Arkansas Tobacco Settlement Proceeds Act of 2000.

References

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