Free-radical-scavenging effect of carbazole derivatives on DPPH and ABTS radicals

The major objective was to measure the trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) equivalent antioxidant capacity (TEAC) of carbazole derivatives (Ar(2)NHs) by means of scavenging 2,2'-diphenyl-1-picrylhydrazyl (DPPH) and the 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate) radical cation (ABTS(+-)). The Ar(2)NHs included phenoxazine (PozNH), phenothiazine (PtzNH), iminostilbene (IsbNH) and diphenylamine (DpaNH), and the TEAC of trolox, alpha-tocopherol (TocH), L-ascorbic acid (VC) and L-ascorbyl-6-laurate (VC-12) were measured as well. The TEAC results revealed that the ability to scavenge DPPH (PozNH > IsbNH similar to PtzNH similar to TroH similar to TocH similar to VC similar to VC12), differed from the ability to scavenge ABTS(+) (PtzNH > IsbNH > PozNH > DpaNH similar to TroH similar to TocH similar to VC similar to VC12). CazNH did not react with DPPH and ABTS(+-). Furthermore, the addition of acetic acid accelerated the reaction rate of Ar2NH to scavenge DPPH, suggesting that a sequential proton loss electron transfer (SPLET) mechanism occurred with amine-type antioxidants during the trapping of DPPH. In contrast, the addition of acetic acid or pyridine reduced the reaction rate of Ar2NH to scavenge ABTS(+-), suggesting that the hydrogen atom transfer (HAT) mechanism is the basis for the reaction that is occurring.