Voltammetric Sensing of Thiols. The Electrocatalytic Oxidation of 4-Acetamidophenol in the Presence of Cysteine:  a Mechanistic Rotating Disk Electrode Study

Abstract

A numerical method is illustrated for the modeling of an important electrocatalytic reaction at the rotating disk electrode (RDE). This reaction, labeled EC2XE, occurs via the following route: A − 2e- ⇌ B, B + X → B−X (k 2), B−X − 2e- ⇌ [B−X]2+. The numerical method is based on a finite difference formulation of coupled mass transport and kinetic equations and makes use of a Hale coordinate transformation. Iterative solutions are performed using a Gauss−Newton scheme to solve the nonlinear differential equations. Numerical solutions are calculated to generate working surfaces in the effective number of electrons transferred, N eff. Cyclic voltammetry is reported in hydrostatic conditions with a 4-acetamidophenol (paracetamol)/cysteine system, which is shown to proceed via the EC2XE reaction pathway. The resulting data is modeled using the commercial package DIGISIM, and a result of k 2 = 1.25 ± 0.25 × 107 mol-1 cm3 s-1 is obtained. RDE studies of the same system using steady-state linear sweep hydrodynamic voltammetry is next performed to collect limiting current data at rotation speeds of 2, 4, 8, 16, and 25 Hz. Working surface interpolation of the resulting data results in a mean value of 1.6 ± 0.35 × 107 mol-1 cm3 s-1 for the rate constant k 2.

Mechanistic Aspects of On‐Line Electrochemical Tagging of Free L‐Cysteine Residues during Electrospray Ionisation for Mass Spectrometry in Protein Analysis

“…However, such reactions will be neglected in the following treatment since hydroquinone and L ‐cysteine were always present in comparable amounts in the experiments. It should be noted that the suggested mechanism emphasizes the possibilities of using hydroquinone to detect and quantify cysteine electrochemically 15–20. The potential efficiency of the electrochemical detection is easily realised as up to four cysteine units may react with the electrochemically generated quinone giving rise to an overall ten electron oxidation per hydroquinone molecule.…”

“…More recently, the use of catechol derivatives as precursors for electrogenerated electrophiles in 1,4‐Michael additions has been described 10–14. A similar reaction was first exploited by Compton and co‐workers to detect and quantify L ‐cysteine 15–20…”

Generation of mass tags by the inherent electrochemistry of electrospray for protein mass spectrometry

“…This model was also used to correlate the on-line electrochemical tagging within the microspray experiments with the kinetics obtained by electrochemical studies [68]. The problem was described according to the literature [84,85] by Scheme 2, where A, B, X, C, E represent, respectively, hydroquinones 1, benzoquinones 2, labeling targets containing cysteine residues 3, the formed adducts 4, and the reoxidised adduct forms 5. This corresponds to an EC 2X E mechanism where the homogeneous reaction (2) is characterized by the kinetic constant k. In this model, based on a microband electrode, the electrochemical reactions were assumed to be rapid (only limited by diffusion) due to the high overpotentials induced by the galvanostatic mode used for the experiments.…”

On-line cysteine modification for protein analysis: new probes for electrochemical tagging nanospray mass spectrometry

“…The electrochemically induced tagging of L L -cysteine has previously been studied using non-substituted hydroquinone [17] as well as other thiols and experimental conditions [23][24][25][26][27][28][29]. The overall mechanism can be described as in Scheme 1 for the experimental conditions used in the present work.…”