The Effects of pH on the Kinetics of the Follow-up Reaction of Dopamine o-quinone

The electrochemical studies of dopamine have been conducted for many years due to dopamine’s importance as a neurotransmitter and its function in daily life for individuals. Dopamine undergoes electrochemical redox reactions in which the different products can be detected by cyclic voltammetry.3 The focus of the experiments was to determine the pH range at which dopamine o-quinone would undergo cyclization as well as the relationship between the rate of cyclization as pH in the physiological range. The measurements were taken using a gold working electrode vs. a Ag/AgCl (1 M KCl) reference electrode. It was observed that the peaks corresponding to the redox reactions for aminochrome and leukoaminochrome formed at pH 5.77 but not pH 5.60. From this data, the pH range at which cyclization of dopamine o-quinone occurred was between pH 5.60 and 5.77. Pre-peaks (appearing as shoulders) were noticed before the oxidation peak pertaining to the oxidation of dopamine when using dopamine solution that had darkened from being exposed to light. Measurements were taken at a pH range from 7.20 to 7.85 to determine the relationship between rate of cyclization and pH. A linear relationship was established such that the rate of cyclization was dependent upon the pH, and that as the pH increased, the rate of cyclization also increased. However, the data points for pH’s 7.50 and 7.70 did not fit into the linearity due to the measurements taken using dirty electrodes as well as unknown factors, hence were determined to be outliers. Despite this, it was observed that at pH 7.85, the rate of cyclization is 0.0332 s-1, which was the largest rate observed when looking at the linear relationship between the rate of cyclization and pH. Overall, from the experiments performed, we were able to determine both the threshold at which cyclization of dopamine o- quinone began as well as the relationship between the rate of cyclization and pH values within the physiological range