Presentation Title

Theoretical Study of the Iron Complexes with Lipoic and Dihydrolipoic Acids: Exploring Secondary Antioxidant Activity

Format of Presentation

15-minute lecture to be presented the Saturday of the conference

Abstract

Following a recent theoretical study of the secondary antioxidant activity of the deprotonated forms of lipoic and dihydrolipoic (DHLA) acids through their formation of complexes with copper,1 the present work focuses on the complexes that these species can form with iron. To this effect, the thermodynamics of the formation equilibria of twenty Fe(III) complexes was studied at the M06/6-31++G(d, p) level of theory combined with the SMD continuum solvation model in water under physiological pH conditions at 298.15K. Fifteen of these complexes were selected to explore the kinetics of their reduction to Fe(II) complexes with , the first step of the Haber-Weiss cycle (see eq (1)). The second step of this cycle (called the Fenton reaction, see eq (2)), leads to the formation of biochemically harmful ·OH radicals. A compound is found to have secondary antioxidant activity if its complex with Fe(III) (or with Cu(II)) is able to slow down the first step of the Haber-Weiss cycle.

(1) [Fe(H2O)6]3+ + O2·- ---> [Fe(H2O)6]2+ + O2

(2) [Fe(H2O)6]2+ + H2O2 --> [Fe(H2O)6]3+ + OH- + ·OH

The three most thermodynamically stable complexes calculated contain doubly deprotonated DHLA coordinating to Fe(III) with one or both carbonyl oxygen atoms and the deprotonated sulfur. Two of these complexes were found to have antioxidant activity as they seem to be able to reduce by up to 30 times the rate constant of the first step of the Haber-Weiss cycle. Similar Cu(II) complexes lead to a much larger rate constant reduction, of up to 217 times, at the same level of theory.

1 R. Castañeda-Arriaga, J. R. Alvarez-Idaboy, N. Mora-Diez, RCS Adv., 2016, 6, 107924-107932.

Department

Chemistry

Faculty Advisor

Nelaine Mora-Diez

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Theoretical Study of the Iron Complexes with Lipoic and Dihydrolipoic Acids: Exploring Secondary Antioxidant Activity

Following a recent theoretical study of the secondary antioxidant activity of the deprotonated forms of lipoic and dihydrolipoic (DHLA) acids through their formation of complexes with copper,1 the present work focuses on the complexes that these species can form with iron. To this effect, the thermodynamics of the formation equilibria of twenty Fe(III) complexes was studied at the M06/6-31++G(d, p) level of theory combined with the SMD continuum solvation model in water under physiological pH conditions at 298.15K. Fifteen of these complexes were selected to explore the kinetics of their reduction to Fe(II) complexes with , the first step of the Haber-Weiss cycle (see eq (1)). The second step of this cycle (called the Fenton reaction, see eq (2)), leads to the formation of biochemically harmful ·OH radicals. A compound is found to have secondary antioxidant activity if its complex with Fe(III) (or with Cu(II)) is able to slow down the first step of the Haber-Weiss cycle.

(1) [Fe(H2O)6]3+ + O2·- ---> [Fe(H2O)6]2+ + O2

(2) [Fe(H2O)6]2+ + H2O2 --> [Fe(H2O)6]3+ + OH- + ·OH

The three most thermodynamically stable complexes calculated contain doubly deprotonated DHLA coordinating to Fe(III) with one or both carbonyl oxygen atoms and the deprotonated sulfur. Two of these complexes were found to have antioxidant activity as they seem to be able to reduce by up to 30 times the rate constant of the first step of the Haber-Weiss cycle. Similar Cu(II) complexes lead to a much larger rate constant reduction, of up to 217 times, at the same level of theory.

1 R. Castañeda-Arriaga, J. R. Alvarez-Idaboy, N. Mora-Diez, RCS Adv., 2016, 6, 107924-107932.