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 O2•- 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 O2•-. Relative to the Cu(II)/Cu(I) reduction with ascorbate, the effects of DHLA are moderate/high, and with O2•- they are minor. LA has minor to negligible inhibition effects in all the cases considered.

中文翻译：

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铁与硫辛酸和二氢硫辛酸络合物的理论研究：探索次级抗氧化活性。

M06(SMD)/6-31+ 研究了二十九种 Fe(III) 与各种去质子化形式的硫辛酸 (LA) 和二氢硫辛酸 (DHLA) 配合物（配位数为 4、5 和 6）的热力学稳定性+G(d,p) 水中的理论水平，生理 pH 条件为 298.15 K。尽管与 LA ^{- 的}配合物比与 DHLA ^-的配合物更稳定，但热力学最稳定的 Fe(III) 配合物涉及 DHLA ^{2 -}。这二十四种放能复合物用于评估 DHLA 和 LA 相对于 O2•- 和抗坏血酸还原 Fe(III)/Fe(II) 的次级抗氧化活性。计算单电子转移 (SET) 反应的速率常数。Fe(III) 配合物的热力学稳定性与其 SET 反应的速率常数并不完全相关，但放能较多的配合物通常表现出较小的 SET 速率常数。一些Cu(II)配合物及其还原成Cu(I)的过程也在相同的理论水平上进行了研究以进行比较。Fe(III) 配合物似乎比 Cu(II) 配合物更稳定。相对于抗坏血酸的Fe(III)/Fe(II)还原，DHLA可以完全抑制^• OH自由基的形成，但不能通过与O2•-反应。相对于抗坏血酸的 Cu(II)/Cu(I) 还原作用，DHLA 的效果为中度/高度，而对于 O2•- 的效果较小。在所有考虑的情况下，LA 的抑制作用都很小，甚至可以忽略不计。