Kinetics and Theoretical Study of 3,3′‐Diindolylmethane as a Hydroxyl Radical Scavenger

ABSTRACT We examined the hydroxyl radical (·OH) scavenging activity of 3,3′‐diindolylmethane (DIM), which is the final product formed through the metabolism of indole‐3‐carbinol which commonly presents in cruciferous vegetables, for example cabbage and broccoli, induced by Fenton reaction (Fe 2+ + H 2 O 2 → ·OH + OH − + Fe 3+ ). The mixture of Fe 2+ and H 2 O 2 in the presence of plasmid DNA caused DNA strand breaks induced by ·OH. Adding DIM to this reaction mixture reduced the strand breaks of plasmid DNA depending on concentration. We examined the reactivity of DIM with ·OH using an electron spin resonance (ESR) spin trapping technique. The reactivity of DIM with ·OH exhibited almost the same reactivity as dimethyl sulfoxide, the most efficient ·OH scavenger, and its reactive site is one of the two indole rings in DIM. In addition, quantum chemical calculations clearly indicate that the highest occupied molecular orbital (HOMO) of DIM is likely to interact with the single occupied molecular orbitals (SOMO) of ·OH. This is supported by the strong correlation observed between log second order rate constant ( k 2 ) and the energy of HOMO of DIM. Moreover, indole derivatives containing DIM with higher HOMO energies show higher scavenging activities with ·OH. Based on these results, DIM may attribute to the protection of oxidative stress induced by ·OH and may also act as an antioxidant.