New Substrates for the Oscillating Briggs−Rauscher Reaction

Höner

Furrow

et al. 2004

Helvetica Chimica Acta

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A method to determine the relative antioxidant capacity of radical scavengers based on the inhibition of the oscillations of the BriggsÀRauscher (BR) oscillating reaction was previously reported. A semiquantitative mechanistic interpretation of the inhibitory effects required two steps to obtain simulated inhibition times in very good agreement with the experimental ones. The first step is inhibitory, involving H-atom transfer from antioxidant to the HOO . radical; the second step is a first-order degradation of the antioxidant to unspecified products. Since the degradation may be due to oxidation and/or iodination of the antioxidant, we studied the kinetics of the subsystems IO À 3 (H ) antioxidant and I 2 (H ) antioxidant. We used 2,5-and 2,6-dihydroxybenzoic acids, caffeic acid ( 3-(3,4-dihydroxyphenyl)prop-2-enoic acid), ferulic acid ( 3-(4-hydroxy-3-methoxyphenyl)prop-2-enoic acid), pyrocatechol ( benzene-1,2-diol), and hydroquinone ( benzene-1,4-diol) as antioxidants. Spectra in the wavelength range 500 ± 250 nm were repeated at given time intervals to follow the peaks of the iodine and oxidation products, which were mainly quinones. For the iodination of the above diphenols ( benzenediol derivatives) the substitution and/or addition reactions with I 2 or HOI were found to be relatively slow compared to oxidation by IO À 3 . Approximate rate constants for oxidation were obtained on the basis of a reasonable kinetic model by using a suitable numerical integration program. Although these complexities can arise also in the completely inhibited BR oscillator, we believe that the inhibitory effects are due to the HOO . scavenging action by diphenols or by quinones since HOO . radicals are also potential reducing agents. We propose two steps that could maintain a small reservoir of diphenol, while both quinone and diphenol deplete HOO . radicals. In short, the complexities do not affect the method for monitoring the relative activity of antioxidants based on the BR oscillating reaction. The effects of temperature on the inverse of the oscillatory time in the BR-uninhibited system, on the inverse of inhibition times, and on the time length of the resumed oscillations for four antioxidants were also investigated. Apparent average activation energies were obtained.

“…oscillations has been recognized in previous work [6] [7]. We have ascribed inhibitory effects to the antioxidants× scavenging action against HOO .…”

mentioning

confidence: 64%

An Experimental and Mechanistic Investigation of the Complexities Arising During Inhibition of the BriggsRauscher Reaction by Antioxidants

Höner

Furrow

et al. 2004

Helvetica Chimica Acta

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“…In 2002, Furrow et al [29] reported a 13-step new mechanism (named FCA model) for the Briggs -Rauscher reaction that takes into account the important role played by HOO . radicals in its oscillatory behavior.…”

mentioning

confidence: 99%

New Polyphenolic β‐Lactams with Antioxidant Activity

Cainelli

Angeloni

Chemistry & Biodiversity

et al. 2008

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Novel 4-alkylidene-beta-lactams with a polyphenolic side chain were synthesized and evaluated as radical scavengers. We have undertaken a detailed study of the antioxidant activity in vitro with chemical and biological testing of the new beta-lactams and of the corresponding methyl polyhydroxy benzoates. Antioxidant activity of beta-lactams and methyl benzoates was measured with the Briggs-Rauscher oscillating reaction, the TEAC (Trolox( Equivalent Antioxidant Capacity) assay, and as ability to inhibit ROS (=Reactive Oxygen Species) production on myoblast H9c2 cells. The results were discussed with regard to mechanism and correlated with structural parameters.

“…This mixture was allowed to stir overnight until it was clear. In both Steps D1, M, D2, D3, I5R, O2, and I3 were taken from the FCA (Furrow, Cervellati, Amadori) mechanism that was found effective in simulating the BR oscillator with malonic acid and its C(2) substituted derivatives, as well as with other substrates that iodinate in a different way [16]. These steps represent the oxyiodine intermediate reactions and the formation and consumption of radicals.…”

Section: Results -Behavior Of the Completely Unperturbed And Perturbmentioning

confidence: 99%

Perturbations of an Acetone‐Based Briggs–Rauscher System by α‐Tocopherol (Vitamin E)

Furrow

Greco

et al. 2010

Helvetica Chimica Acta

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The malonic acid (MA)-based oscillating Briggs -Rauscher reaction (BR) in batch mode has been shown to be sensitive to various hydrophilic polyphenol antioxidants. Several of these have been shown to cause cessation of oscillations for a period of time before a restart occurs. The length of time before oscillations restart is related to the type of antioxidant and its concentration. Procedures have been devised to use this method as a tool for measuring antioxidant activity from pure compounds and from extracts of natural sources. The antioxidant activity has been related to the reaction of the antioxidants with HOO. radicals present in the oscillating system. Vitamin E (a-tocopherol), a typical highly lipophilic antioxidant containing an phenolic OH group, is soluble in acetone that also is a suitable substrate for the BR reaction. Perturbations of a highly concentrated acetone-based BR oscillator by acetonic solutions of vitamin E were studied. The inhibitory effects were found similar to those provoked by hydrophilic polyphenols in the MA-based oscillator, but to obtain reasonable inhibition times, the concentration of vitamin E must be at the mm level instead mm. However, there is a region of concentrations where there is a nearly linear relation between concentration and inhibition time. A comparison with a hydrophilic diphenol (2,6-dihydroxybenzoic acid) in the acetone-based oscillator showed that the inhibitory reaction is much slower in this system than in the MA one. We were able to model the perturbations by vitamin E assuming its reaction with HOO . radicals by using the FCA mechanism previously reported with some little modifications.