Thiosemicarbazone Ligands and their Transition Metal Complexes as Antioxidants: A Review

Antioxidant molecules prevent the harmful reaction of oxidants inside the body by providing a single electron or hydrogen atom based on their mechanism. Different forms of oxidants such as reactive oxygen species (ROS) or reactive nitrogen species (RNS) could harm the proteins, lipids, and DNA. Due to the chain reactions of oxidants in the medium that exist, unwanted consequences could happen inside the body. The antioxidants by reacting with oxidants prevent the damages caused by the chain reaction of oxidants. Natural antioxidants are a well-known group of matters such as vitamins, fruits, vegetables, and nuts. The antioxidant capacity/activity of natural antioxidants is studied comprehensively and reported in the literature. Synthetic antioxidants such as thiosemicarbazones either in the form of ligand or their metal complexes, could exhibit potent antioxidant capacity/activity comparable or sometimes more potent compared to the natural antioxidant. The existence of different electron donor groups in the structure of thiosemicarbazones such as hydroxyl and amine groups, could appear in the form of antioxidant capacity or activity. In the literature, various methods of determining the antioxidant capacity/activity of compounds such as thiosemicarbazones are reported. Among them, the three methods such as CUPRAC (CUPric Reducing Antioxidant Capacity), DPPH (2,2-di(4-tert-octyl phenyl)-1-picrylhydrazyl) and ABTS (2,2’-azino-bis (3-ethylbenzthiazoline-6-acid) are well-known methods for the evaluating of antioxidant capacity/activity of thiosemicarbazones. In this review, the antioxidant capacity/activity of thiosemicarbazones by the last three methods is studied and the relationship between the structure of thiosemicarbazone ligands and their corresponding metal complexes with their antioxidant capacity/activity is analyzed.