Oxidation pathways of 2 2'-benzothiazolinone azines. I. Electrochemistry

Revelation of Fe(V)/Fe(IV) Involvement in the Fe(VI)-ABTS System: Kinetic Modeling and Product Analysis

To quantitatively probe iron intermediate species [Fe(V)/Fe(IV)] in Fe(VI) oxidation, this study systematically investigated the reaction kinetics of Fe(VI) oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic)acid (ABTS) at different ratios of [ABTS]₀/[Fe(VI)]₀ (i.e., >1.0, =1.0, and <1.0) in pH 7.0 phosphate (10 mM)-buffered solution. Compared to the literature, a more comprehensive and robust kinetic model for the Fe(VI)-ABTS system including interactions between high-valent iron species [Fe(VI), Fe(V), and Fe(IV)], ABTS, and the ABTS^•+ radical was proposed and validated. The oxidation of ABTS by Fe(VI) (k = (5.96 ± 0.9%) × 10⁵ M^-1 s^-1), Fe(V) (k = (2.04 ± 0.0%) × 10⁵ M^-1 s^-1), or Fe(IV) (k = (4.64 ± 13.0%) × 10⁵ M^-1 s^-1) proceeds via one-electron transfer to generate ABTS^•+, which is subsequently oxidized by Fe(VI) (k = (8.5 ± 0.0%) × 10² M^-1 s^-1), Fe(V) (k = (1.0 ± 40.0%) × 10⁵ M^-1 s^-1), or Fe(IV) (k = (1.9 ± 17.0%) × 10³ M^-1 s^-1), respectively, via two-electron (oxygen atom) transfer to generate colorless ABTS_ox. At [ABTS]₀/[Fe(VI)]₀ > 1.0, experimental data and model simulation both indicated that the reaction stoichiometric ratio of Fe(VI)/ABTS^•+ increased from 1.0:1.0 to 1.0:1.2 as [ABTS]₀ was increased. Furthermore, the Fe(VI)-ABTS-substrate model was developed to successfully determine reactivity of Fe(V) to different substrates (k = (0.7-1.42) × 10⁶ M^-1 s^-1). Overall, the improved Fe(VI)-ABTS kinetic model provides a useful tool to quantitatively probe Fe(V)/Fe(IV) behaviors in Fe(VI) oxidation and gains new fundamental insights.

Catalytic effects of silver plasmonic nanoparticles on the redox reaction leading to ABTS˙+formation studied using UV-visible and Raman spectroscopy

The strong effect of silver plasmonic nanoparticles on ABTS leading to the formation of ABTS radical cation is investigated by UV-visible, Raman and SERS spectroscopy.

Kinetics of oxidation of benzyl alcohols by the dication and radical cation of ABTS. Comparison with laccase–ABTS oxidations: an apparent paradox

Laccase, a blue copper oxidase, in view of its moderate redox potential can oxidise only phenolic compounds by electron-transfer. However, in the presence of ABTS (2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonate) as a redox mediator, laccase reacts with the more difficult to oxidise non-phenolic substrates, such as benzyl alcohols. The role of ABTS in these mediated oxidations is investigated. Redox interaction with laccase could produce in situ two reactive intermediates from ABTS, namely ABTS++ or ABTS*+. These species have been independently generated by oxidation with Ce(iv) or Co(iii) salts, respectively, and their efficiency as monoelectronic oxidants tested in a kinetic study towards a series of non-phenolic substrates; a Marcus treatment is provided in the case of ABTS++. On these grounds, intervention of ABTS++ as a reactive intermediate in laccase-ABTS oxidations appears unlikely, because the experimental conditions under which ABTS++ is unambiguously generated, and survives long enough to serve as a diffusible mediator, are too harsh (2 M H2SO4 solution) and incompatible with the operation of the enzyme. Likewise, ABTS*+ seems an intermediate of limited importance in laccase-ABTS oxidations, because this weaker monoelectronic oxidant is unable to react directly with many of the non-phenolic substrates that laccase-ABTS can oxidise. To solve this paradox, it is alternatively suggested that degradation by-products of either ABTS++ or ABTS*+ are formed in situ by hydrolysis during the laccase-ABTS reactions, and may be responsible for the observed oxidation of non-phenolics.