Departamento de Química, División de Ciencias Básicas e Ingeniería, Universidad Autónoma Metropolitana-Iztapalapa, Av San Rafael Atlixco No.186, Col.Vicentina C.P.09340, México DF, Mexico.
Phys Chem Chem Phys. 2011 Apr 21;13(15):7178-88. doi: 10.1039/c0cp02801k. Epub 2011 Mar 15.
The reactions of melatonin (MLT) with hydroxyl and several peroxyl radicals have been studied using the Density Functional Theory, specifically the M05-2X functional. Five mechanisms of reaction have been considered: radical adduct formation (RAF), Hydrogen atom transfer (HAT), single electron transfer (SET), sequential electron proton transfer (SEPT) and proton coupled electron transfer (PCET). It has been found that MLT reacts with OH radicals in a diffusion-limited way, regardless of the polarity of the environment, which indicates that MLT is an excellent OH radical scavenger. The calculated values of the overall rate coefficient of MLT + ˙OH reaction in benzene and water solutions are 2.23 × 10(10) and 1.85 × 10(10) M(-1) s(-1), respectively. MLT is also predicted to be a very good ˙OOCCl(3) scavenger but rather ineffective for scavenging less reactive peroxyl radicals, such as alkenyl peroxyl radicals and ˙OOH. Therefore it is concluded that the protective effect of MLT against lipid peroxidation does not take place by directly trapping peroxyl radicals, but rather by scavenging more reactive species, such as ˙OH, which can initiate the degradation process. Branching ratios for the different channels of reaction are reported for the first time. In aqueous solutions SEPT was found to be the main mechanism for the MLT + ˙OH reaction, accounting for about 44.1% of the overall reactivity of MLT towards this radical. The good agreement between the calculated and the available experimental data, on the studied processes, supports the reliability of the results presented in this work.
使用密度泛函理论（特别是 M05-2X 函数）研究了褪黑素（MLT）与羟基和几种过氧自由基的反应。考虑了五种反应机制：自由基加成物形成（RAF）、氢原子转移（HAT）、单电子转移（SET）、顺序电子质子转移（SEPT）和质子耦合电子转移（PCET）。结果表明，无论环境的极性如何，MLT 均与 OH 自由基以扩散限制的方式反应，这表明 MLT 是一种优秀的 OH 自由基清除剂。在苯和水溶液中 MLT + ˙OH 反应的总速率系数的计算值分别为 2.23×10(10)和 1.85×10(10) M(-1) s(-1)。还预测 MLT 也是非常好的 ˙OOCCl(3)清除剂，但对反应性较低的过氧自由基（如烯基过氧自由基和 ˙OOH）的清除效果较差。因此可以得出结论，MLT 对脂质过氧化的保护作用不是通过直接捕获过氧自由基，而是通过清除更具反应性的物质（如 ˙OH）来实现的，后者可以引发降解过程。首次报道了不同反应通道的分支比。在水溶液中，SEPT 被发现是 MLT + ˙OH 反应的主要机制，约占 MLT 对该自由基总反应性的 44.1%。在研究的过程中，计算结果与现有实验数据之间的良好一致性支持了本工作中提出的结果的可靠性。
