Dietary isothiocyanates as redox-active bioactives: A comprehensive review of chemistry

Isothiocyanates (ITCs, R-NCS), abundant in cruciferous vegetables, are bioactive compounds showing notable reduced cancer risk at the dietary and population levels, primarily attributed to their antioxidant activity. Although numerous studies reported that ITCs can act as radical scavengers or redox agents in various antioxidant tests, it is not known which products are formed at the end of these reactions, the reaction mechanisms, and which groups or atoms are directly involved in the reaction. Additionally, some ITC derivatives can be converted into disulfide, thiourea, or thio-oxide derivatives after reacting with radicals. The cellular effects of these products may be very different from those of the original ITCs. Therefore, a chemical evidence chain identifying the reaction products should be clarified. This work explored various <i>in vitro</i> methods available for measuring antioxidant activity of ITCs, with a particular focus on the underlying chemistry mechanisms of these assays. ITCs exerted their antioxidant activity through mechanisms involving electron-transfer (ET) and/or hydrogen atom transfer (HAT)-based reactions, which occurred on either the -NCS group and/or the R group depending on the structure of ITCs. The reaction pathways and corresponding reaction products were summarized/proposed. Overall, this review provided a theoretical foundation for future research into the health-promoting effects of ITCs.