A Fast Kinetic Study of Formation and Decay of 2,2′-Azinobis(3-ethylbenzothiazole-6-sulfonate) Radical Cation in Aqueous Solution

Abstract

The kinetics of oxidation of 2,2′-azinobis(3-ethylbenzothiazole-6-sulfonate) (ABTS) to the radical cation, ABTS·+ by S2O82−, HSO5−, MnO4−, Cr2O72−, Ce(IV), and molecular Cl2, Br2, and I2 and further oxidation to the dication, ABTS2+ by the above oxidants except HSO5−, Br2, and I2 have been carried out by the stoppedflow spectrophotometric technique. The first-stage oxidation was followed by monitoring the absorption of ABTS·+ at 417 nm and the second-stage oxidation, by following the disappearance of ABTS·+ or the appearance of ABTS2+ at 513 nm. Both the reactions, formation and decay of ABTS·+ were found to obey a total second-order kinetics, first-order each with respect to [ABTS] or [ABTS·+] and [oxidant]. With I2, ABTS underwent reversible reaction, the stoichiometry of which being, I2+2ABTS\ightleftarrows2ABTS·++2I− with K=142.8, whereasBr_2generatedonlytheradicalcationandCl_2gaveadecomposedproductviaradicalcationanddicationintermediates.ABTS^2+formedinpresenceofexcessMnO_4^-,Cr_2O_7^2-,andCe(IV)decayedasinthecaseofCl_2reactionbutinpresenceofS_2O_8^2-thedicationwasstableformorethanthreedays.IntheabsenceofexcessS_2O_8^2-,ABTS^2+wentbacktoABTS^·+,possiblyviaoxidationofwater.TheradicalcationwasconvertedbacktotheparentcompoundABTSbyreducingagentssuchasdithioniteandsulfiteions.

Reappraising a Controversy: Formation and Role of the Azodication (ABTS2+) in the Laccase-ABTS Catalyzed Breakdown of Lignin

“…The absorbance peaks of the transformation products appear at wavelength positions within λ = 513-573 nm and exceed the narrow span of 518-520 nm reported for the electrochemically [30,41] and wet-chemically [44,45] generated ABTS 2+ . Testing several inorganic peroxides yielded a further ABTS oxidation product that showed absorbance peaks at around A 420 and A 530 in scans [45] that resemble those given for the red ABTS derivatives in the Figures 3 and 5.…”

Formation and decay of the ABTS derived radical cation: A comparison of different preparation procedures

“…In spite of the extensive use of such methods, no studies have been devoted to the comparison of the behavior of radicals produced by different methodologies. Furthermore, the kinetics of the radical formation and decay has been little investigated [11][12][13]. In the present work, we report these type of results obtained by procedures that employ PDS, manganese dioxide, or AAPH to promote the ABTS oxidation.…”

ABTS/PP Decolorization Assay of Antioxidant Capacity Reaction Pathways

“…The ABTS •+ radical cation can undergo further oxidation to form the dication ABTS 2+ with λ max at 513-520 nm, ε 36 000 L mol −1 cm −1 (see Figure 2a) [42]). The ABTS 2+ formation kinetics from ABTS •+ , when an excess of S 2 O 8 2− is applied, is approximately three times slower than the radical cation ABTS •+ formation from ABTS [47,48]. Apparently, the ABTS 2+ dication does not influence ABTS/PP assay measurements as it has rather low solubility in water and is relatively stable only in the excess of S 2 O 8 2− anions or in a very acidic medium, with any aqueous dilution resulting in a shift of the reaction balance back to ABTS •+ [41,[47][48][49][50][51].…”

“…The ABTS 2+ formation kinetics from ABTS •+ , when an excess of S 2 O 8 2− is applied, is approximately three times slower than the radical cation ABTS •+ formation from ABTS [47,48]. Apparently, the ABTS 2+ dication does not influence ABTS/PP assay measurements as it has rather low solubility in water and is relatively stable only in the excess of S 2 O 8 2− anions or in a very acidic medium, with any aqueous dilution resulting in a shift of the reaction balance back to ABTS •+ [41,[47][48][49][50][51]. Generally speaking, both "cation" and "dication" terms for compounds 3 and 4 are questionable as the total charge implies that they are an anion and neutral compound, as well as a dianion for ABTS itself (1).…”