Abstract

The antioxidant properties of echinatin (Ech), isolated from liquorice, have recently been reported. It is well known that the free radical species can be deactivated by phenolic antioxidants via different mechanistic pathways. In this work, the scavenging of eighteen different reactive oxygen species (ROS) has been considered, focussing on three main working mechanisms, namely hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET). The investigations have been performed in different dielectric media, viz. gas phase, benzene, methanol and aqueous solution, using density functional theory (DFT) calculations at the M05-2X/6-311++G** level. Various molecular descriptors have been elucidated for Ech as well as the ROS and compared with the reference antioxidant molecule, trolox. In addition, the redox potentials and the equilibrium constants have been calculated to discuss the feasibility of the overall scavenging process. The results demonstrate that the 4-OH group is the first site for H-atom donation, followed by 4’-OH. Further, it has been found that HAT would be the most favourable mechanism in the gas phase. The SPLET mechanism is thermodynamically favoured in polar media like water and methanol, while in the case of non-polar solvents like benzene, the two mechanisms are observed to be competitive.

摘要

最近报道了从甘草中分离出的紫锥菊素 (Ech) 的抗氧化特性。众所周知，酚类抗氧化剂可以通过不同的机制途径使自由基失活。在这项工作中，考虑了 18 种不同活性氧 (ROS) 的清除，重点关注三种主要工作机制，即氢原子转移 (HAT)、单电子转移后质子转移 (SET-PT) 和连续质子丢失电子转移 (SPLET)。研究是在不同的介电介质中进行的，即。气相、苯、甲醇和水溶液，在 M05-2X/6-311++G** 级别使用密度泛函理论 (DFT) 计算。已经阐明了 Ech 和 ROS 的各种分子描述符，并与参考抗氧化剂分子 trolox 进行了比较。此外，还计算了氧化还原电位和平衡常数，以讨论整个清除过程的可行性。结果表明，4-OH 基团是 H 原子捐赠的第一个位点，其次是 4'-OH。此外，已经发现HAT将是气相中最有利的机制。SPLET 机制在水和甲醇等极性介质中在热力学上是有利的，而在苯等非极性溶剂的情况下，观察到这两种机制具有竞争力。