Generally, the rate of antioxidant reactions (activity) seem unrelated to total antioxidant effect (capacity); the latter is perhaps governed by thermodynamic considerations (pls. cf. redox reactions, Nernst equation etc.) and the complicated mess of real-world antioxidant reactions [1]. This is one justification cited for combining both rate and end-point measurements in order to get more representative antioxidant evaluations. Consistent with this approach, there is the general chemistry principle which states: the kinetics and thermodynamic properties of reaction are unrelated [2]. But why not? Some research (now quite old) shows that the rate of autoxidation reactions vary linearly as function of the net changes redox potential [ref. 3]. Marcus theory shows kinetic and thermodynamic "cross-overs" occur for single electron transfer reactions and (perhaps) other linear free energy relations [ref. 2]. Professor R.A. Marcus won the 1992 chemistry Nobel prize for the research completed initially in 1956; this model follows-on from Eyring's absolute reaction rate theory (cf. Activation free energy narrative) but in addition delta-G# is shown to be a function of Gibbs free energy (cf. delta-Go = n F Eo) plus a term for the solvent reorganization energy (L) – Marcus in brief!
Questions
Q1. Why is Marcus' theory not finding itself into standard chemistry texts?
Q2. Is there evidence for kinetics and thermodynamic cross-relations for single electron transfer reactions involving real-world antioxidants?.
Acknowledgement.
this discussion was prompted by this post on the Marcus theory.
https://bionerdnotes.wordpress.com/2022/03/26/marcus-theory-and-the-energetics-of-electron-transfer-processes/?blogsub=confirming#subscribe-blog
References
[1]. Campos, A., Duran, N., Lopez-Alarcon, C., Lissi, E., 2012. Kinetic and stoichiometric evaluation of free radicals scavengers activities based on diphenyl-picryl hydroxyl (DPPH) consumption. Journal of the Chilean Chemical Society 57, 1381–1384. http://dx.doi.org/10.4067/S0717-97072012000400010
[2]. Silverstein, T.P., 2012. Marcus theory: thermodynamics CAN control the kinetics of electron transfer reactions. Journal of Chemical Education 89, 1159–1167.
https://www.researchgate.net/profile/Todd-Silverstein/publication/237153713_Marcus_Theory_Thermodynamics_CAN_Control_the_Kinetics_of_Electron_Transfer_Reactions/links/6310b3ec1ddd447021287a9f/Marcus-Theory-Thermodynamics-CAN-Control-the-Kinetics-of-Electron-Transfer-Reactions.pdf
[3]. Kurimura, Y., Ochiai, R., Matsuura, N., 1968. Oxygen oxidation of ferrous Ions induced by chelation. Bulletin of the Chemical Society of Japan 41, 2234–2239. https://doi.org/10.1246/bcsj.41.2234
Dear Richard K Owusu-Apenten , good question.
First: "the rate of antioxidant reactions (activity) seem unrelated to total antioxidant effect (capacity)."
When we saying antioxidant activity, we should clarify what Reactive Nitrogen or Oxygen Species (RNOS) is scavenged by antioxidant. The activity of a given antioxidant molecule is simply the rate constant of the reaction of this molecule with a given RNOS. You can apply whatever theory to link rate constants with thermodynamics. Commonly, this is the reaction of hydrogen atom abstraction, which belongs to the family of coupled electron and proton transfer. The Marcus theory is applicable for this type of reactions. For the mixture of antioxidants, the activity is the apparent reaction rate constant. The activity can be measured in units relative to an activity of a standard antioxidant such as ascorbic acid or Trolox.
Your Q1. Where is no "standard" chem text.
Q2. Tons of literature. This is the excellent book of the late Prof. Denisov
Book Handbook of Antioxidants
The total antioxidant effect (capacity) or TAC is the total number of radicals which can be scavenged by a single antioxidant or by the unknown mixture of antioxidants measured relatively to TAC of a standard antioxidant. TAC depends on assay (on RNOS used by an assay). There is no a single standard protocol to measure TAC. I believe, that the TAC based on ABTS radical is the mostly related to physiological conditions.
I discussed these question in my publications. I don't provide refs to avoid self advertisement.
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