The Electrochemical Approach towards Proton Coupled Electron Transfer Pathways for Oxidation of Thymine in Water (original) (raw)
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Talanta, 1969
In order to evaluate the striking discrepancy between the experimental ease of pol~o~phic reduction of adenine and cytosine, and that predicted by molecular orbital calculation, the el~tr~hemical oxidation-reduction behaviour of pyrimidine, cytosine, purine, adenine and related compounds was investigated at both mercury and graphite electrodes. Information was obtained on the specific adsorption of reactant and product species on the electrode, the reversibility of the energy-controlling electron-transfer step, and accompanying chemical reactions. Triangular sweep voltammetry, a.c. and d.c. polarography, and electrocapillary data, in particular, were utilized. The fist three techniques were critically examined for their potential analytical utility. The results were compared with previously obtained electrochemical data and the sequence of electron-transfer and various nonelectron transfer steps was more Crmly established. It became clear that in order validly to correlate quantum mechanically calculated data for the energy required to add or remove an electron to or from the outermost electron level of each molecule (in the gas phase), with electrochemical redox potentials (in solution), the effects of adsorption, electron-transfer reversibility and solvation energy must be considered.
A biopolymer-based voltammetric sensor for thymine: Elucidation of electrochemical kinetics
Ionics, 2017
In the work accounted here, the electro-oxidation of one of the pyrimidine bases thymine is studied on a glassy carbon electrode modified with the biopolymer, poly (lglutamic acid), as the probe. On the probe, the overpotential for thymine oxidation in alkaline medium has been significantly decreased attesting the electrocatalytic nature of the biopolymer film. The experimental parameters to obtain the lowest oxidation potential have been optimised and the probe calibrated for the determination of thymine. In the square wave mode, using 0.1 M NaOH as the supporting electrolyte, the oxidation peak current was found to be linear in the range from 30 to 1000 μM, with a detection limit of 9.2 μM. The diffusion coefficient of thymine in 0.1 M sodium hydroxide has been determined using chronoamperometric studies. To study the mechanistic aspects of the electro-oxidation process, variation of the oxidation peak parameters with scan rate has been studied in the linear sweep mode. Electrochemical kinetic parameters, namely charge transfer coefficient α and the standard heterogeneous rate constant k s for the electrooxidation, have also been determined. The determination of thymine in spiked synthetic blood serum and urine has been conducted to demonstrate the application of the sensor for thymine determination in real samples.
Elucidation of the biological redox chemistry of purines using electrochemical techniques
Journal of Chemical Education, 1983
Modern electrochemical and related analytical methods provide powerful techniques to investigate the complex redox chemistry of naturally occurring organic compounds. Electrochemical studies cannot only provide mechanistic information of fundamental interest hut also can give valuable insights into the chemical aspects of enzymatic and in vivo redox reactions of such compounds (1,2). This may he illustrated by recent work concerned with the electrochemical oxidation of the purine, uric acid. This discussion will he limited to information obtained at pH 2 7. Detailed information over a wider pH range has been presented elsewhere (3). Electrochemical Studies Uric acid (I, Fig. 7) gives a single voltammetric oxidation peak (I,) at a pyrolytic graphite electrode. At a sweep rate of 5 mV s-' the peak potential, E,, shifts toward more negative potentials according to eqn. (1) (4). Cyclic voltammetry (CV) of uric acid (Fig. 1) shows that having scanned oxidation peak I,, two reduction peaks (I, and 11,) appear on the reverse sweep (5-12). The peak potentials for peaks I, and I, are separated by more than 56 mV so the species responsible for these peaks are said to form a quasireversible couple (13). With increasing sweep rate, the height of peak I, grows relative to peak I, and, correspondingly, the height of peak 11, decreases. Controlled potential coulometry of uric acid at potentials slightly positive of peak I, shows that for each molecule oxidized two electrons are transferred. The quasi-reversihle nature of the peaks I, and I,, the dE,ld(pH) slope of-55 mV per pH unit, and the coulometric n-value of 2 all support the conclusion that the reaction for peak I, is a 2e-2HC process. The primary product of this process is responsihle for reduction peak I, in CV. However, in view of the fact that peak I, is smaller than peak I,, except at very fast sweep rates, indicates that this product must he very unstable, i.e., it undergoes a rapid chemical follow-up reaction. The nature and kinetics of this follow-up reaction may he elucidated using double potential-step chronoamperometry. This technique may he understood by reference to Figure 1. Thus, an initial potential, El, of 0.0 V is applied to a stationary micrographite working electrode dipping into the quiet (i.e., unstirred) uric acid solution. At E,, of course, no faradaic process (i.e., electrooxidation) occurs and only a small capacitive current flows for a very short time to charge the electrode. After a short equilibration time (e.g., 5 s), the potential is pulsed to 0.6V, E2. The value E 2 is >ZOO mV positive of E, for peak I, so that uric acid is oxidized at a rate controlled only by its rate of mass transport to the electrode surface, i.e., the oxidation current is dependent upon its rate of diffusion. After the potential is held at Ez for a predetermined time (TF, sec) it is pulsed hack to El (0.0 V). At E 2 the
421 — Electrochemical and enzymic oxidation of biological purines
Bioelectrochemistry and Bioenergetics, 1981
The electrochemicai and enzymic oxidations of uric acid, various N-methylated uric acids and of guanine, 8oxyguanine and uric acid-g-riboside have been studied. The voltammekic, spectral and kinetic evidence supports the conclusion that the electrochemical and enzymic (peroxidase) reactions procee d by similar, if not identical, chemical mechanisms.
The Journal of Physical Chemistry A, 2013
The photo-oxidation of the nucleobase, thymine (Thy), and nucleoside, thymidine (dThy), by dipyridyl (DP) has been investigated in aqueous solution using timeresolved laser flash photolysis. The pH dependence of the oxidation rate constants is measured within a large pH scale. As a consequence, the chemical reactivity of the reactants existing in solution at a certain range of pH is predicted. Bimolecular rate constants of the quenching reactions between triplet dipyridyl and thymine and thymidine are, respectively, k q = 2.4 × 10 7 M −1 s −1 (pH < 5.8) and k q = 1.0 × 10 7 M −1 s −1 (5.8 < pH < 9.8). Cyclic voltammetry was used to measure the potentials of thymine oxidation and dipyridyl reduction in water at pH < 7. Both results give hints for a proton coupled electron-transfer (PCET) reaction from thymine to triplet dipyridyl.
World Journal of Chemical Education, 2019
Electrochemical simulations are presented to introduce students to the capabilities of cyclic voltammetry (CV). The systems chosen involve one and two-electron transfers, and can be delineated with CV as being reversible, quasi-reversible, or irreversible. The rate constants for the electron transfer can be estimated by the theory of Nicholson and Shain. DigiElch, professional ® provides the opportunity of fitting experimental CVs after assuming a reaction scheme. We will demonstrate data fitting for two different mechanisms, electron transfer E and electron transfer followed by a chemical reaction with a subsequent electron transfer of the product ECE.
Biophysical Chemistry, 2008
A new method is introduced to determine the kinetic parameters of electron transfer reactions of biologically important compounds, based on the measurements of the half-peak width (ΔEp/2) of the square-wave voltammograms. A simple surface (diffusionless) redox reaction, and a simple electrode reaction occurring from dissolved state are considered as model systems. In the region of quasireversible electron transfer, the half-peak widths of theoretical square-wave voltammograms are linear functions of the logarithm of the dimensionless kinetic parameter ln(K) that characterizes the rate of the electron transfer reaction. The dimensionless kinetic parameter K is defined as K = ks(fD)− 0.5 for the redox reaction taking place from dissolved state, whereas for the surface redox reaction K is defined as K = ks/f (ks is the standard rate constant of electron transfer, f is the SW frequency, and D is the diffusion coefficient). A set of linear regression equations for the dependences ΔEp/2vs. ln(K) are derived, which can be used for rapid and precise determination of the charge-transfer kinetic parameters. The estimated values for the standard rate constants of various biologically relevant redox systems using this approach are in very good agreement with the experimental values determined by other square-wave voltammetric methods. The square-wave voltammetric half-peak width method can be used as a simple and reliable alternative to other voltammetric methods developed for the kinetic characterization of electron transfer rates.
The Journal of Physical Chemistry A, 2012
The kinetics of triplet state quenching of 3,3′, 4,4′-benzophenone tetracarboxylic acid (BPTC) by DNA bases adenine, adenosine, thymine, and thymidine has been investigated in aqueous solution using time-resolved laser flash photolysis. The observation of the BPTC ketyl radical anion at λ max = 630 nm indicates that one electron transfer is involved in the quenching reactions. The pH-dependence of the quenching rate constants is measured in detail. As a result, the chemical reactivity of the reactants is assigned. The bimolecular rate constants of the quenching reactions between triplet BPTC and adenine, adenosine, thymine, and thymidine are k q = 2.3 × 10 9 (4.7 < pH < 9.9), k q = 4.0 × 10 9 (3.5 < pH < 4.7), k q = 1.0 × 10 9 (4.7 < pH < 9.9), and k q = 4.0 × 10 8 M −1 s −1 (4.7 < pH < 9.8), respectively. Moreover, it reveals that in strong basic medium (pH = 12.0) a keto−enol tautomerism of thymine inhibits its reaction with triplet BPTC. Such a behavior is not possible for thymidine because of its deoxyribose group. In addition, the pH-dependence of the apparent electrochemical standard potential of thymine in aqueous solution was investigated by cyclic voltammetry. The ΔE/ΔpH ≈ −59 mV/pH result is characteristic of proton-coupled electron transfer. This behavior, together with the kinetic analysis, leads to the conclusion that the quenching reactions between triplet BPTC and thymine involve one proton-coupled electron transfer.
Electrochimica Acta, 2012
Protein-film voltammetry (PFV) is a versatile tool designed to provide insight into the enzymes physiological functions by studying the redox properties of various oxido-reductases with suitable voltammetric technique. The determination of the thermodynamic and kinetic parameters relevant to protein's physiological properties is achieved via methodologies established from theoretical considerations of various mechanisms in PFV. So far, the majority of the mathematical models in PFV have been developed for redox proteins undergoing a single-step electron transfer reactions. However, there are many oxido-reductases containing quinone moieties or polyvalent ions of transition metals like Mo, Mn, W, Fe or Co as redox centers, whose redox chemistry can be described only via mathematical models considering successive two-step electron transformation. In this work we consider theoretically the protein-film redox mechanisms of the EE (Electrochemical-Electrochemical), ECE (Electrochemical-Chemical-Electrochemical), and EECat (Electrochemical-Electrochemical-Catalytic) systems under conditions of cyclic staircase voltammetry. We also propose methodologies to determine the kinetics of electron transfer steps by all considered mechanisms. The experimentalists working with PFV can get large benefits from the simulated voltammograms given in this work.
Voltammetry of surface-bound species: Proton-coupled electrochemical reduction
Journal of …, 2010
We consider the one electron, one proton and also the two electron, two proton reductions of surfacebound species. Two mechanisms of reaction are considered: stepwise and concerted. The voltammetry is modelled under three regimes of proton transport: infinitely fast (fully buffered solution), infinitely slow (infinitely high surface coverage of electrode) and the intermediate case of a finite rate of diffusional mass transport to electrode surface. The types of voltammograms observed in each case are presented and discussed.