Electrochemical studies on β-carbolines alkaloids: Kinetics of irreversible oxidation processes

The comprehension of the kinetic processes involved in redox reactions with alkaloids such as & beta;-carbolines (& beta;Cs) is crucial to unveil differences in the reactivity of these substances. To this end, electrochemical studies on a screen-printed carbon electrode were conducted, revealing the irreversible nature of the process associated with the first oxidation step of norharmane (nHo), harmine (Ha), harmaline (Hlina), harmol (Hol), and harmalol (Hlol). Results from chronoamperometric (CA) studies indicated that in all cases, the oxidation follows a first order reaction. Additionally, cyclic voltammetry (CV) enabled the estimation of the number of electrons transferred (n) as well as the charge transfer coefficient (& alpha;a). Finally, square wave voltammetry (SWV) allowed the investigation of the irreversible nature of the oxidative transformations, determining the oxidation potential (EOx) values, and elucidating of the dependency of the frequency-normalized peak current (ip/f) with the frequency (f). For the first time, the rate constant (k) for the selected & beta;Cs was determined from the maximum observed in the ip/f vs. f plot. By correlating Ln(k) with EOx values, a linear free energy relationship that show the relevance of molecular structure in reaction pathways was revealed, which constitutes the novelty of this work. These findings are discussed in the context of other redox processes involving & beta;C alkaloids.