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

The thermodynamic stability of twenty-nine Fe(III) complexes with various deprotonated forms of lipoic (LA) and dihydrolipoic (DHLA) acids, with coordination numbers 4, 5 and 6, is studied at the M06(SMD)/6-31++G(d,p) level of theory in water under physiological pH conditions at 298.15 K. Even though the complexes with LA^- are more stable than those with DHLA⁻, the most thermodynamically stable Fe(III) complexes involve DHLA²⁻. The twenty-four exergonic complexes are used to evaluate the secondary antioxidant activity of DHLA and LA relative to the Fe(III)/Fe(II) reduction by <inline-formula><math display="inline"><semantics><mrow><msubsup><mi mathvariant="normal">O</mi><mn>2</mn><mrow><mo>•</mo><mo>−</mo></mrow></msubsup></mrow></semantics></math></inline-formula> and ascorbate. Rate constants for the single-electron transfer (SET) reactions are calculated. The thermodynamic stability of the Fe(III) complexes does not fully correlate with the rate constant of their SET reactions, but more exergonic complexes usually exhibit smaller SET rate constants. Some Cu(II) complexes and their reduction to Cu(I) are also studied at the same level of theory for comparison. The Fe(III) complexes appear to be more stable than their Cu(II) counterparts. Relative to the Fe(III)/Fe(II) reduction with ascorbate, DHLA can fully inhibit the formation of ^•OH radicals, but not by reaction with <inline-formula><math display="inline"><semantics><mrow><msubsup><mi mathvariant="normal">O</mi><mn>2</mn><mrow><mo>•</mo><mo>−</mo></mrow></msubsup></mrow></semantics></math></inline-formula>. Relative to the Cu(II)/Cu(I) reduction with ascorbate, the effects of DHLA are moderate/high, and with <inline-formula><math display="inline"><semantics><mrow><msubsup><mi mathvariant="normal">O</mi><mn>2</mn><mrow><mo>•</mo><mo>−</mo></mrow></msubsup></mrow></semantics></math></inline-formula> they are minor. LA has minor to negligible inhibition effects in all the cases considered.