Structurally Selective\nOzonolysis of <i>p</i>‑Phenylenediamines and Toxicity\nin Coho Salmon and Rainbow\nTrout

The tire-rubber-derived ozonation product of <i>N</i>-(1,3-dimethylbutyl)-<i>N</i>′-phenyl-<i>p</i>-phenylenediamine (6PPD), <i>N</i>-(1,3-dimethylbutyl)-<i>N</i>′-phenyl-<i>p</i>-phenylenediamine-quinone\n(6PPD-Q), was recently discovered to cause acute mortality in coho\nsalmon (Oncorhynchus kisutch). <i>para</i>-Phenylenediamines (PPDs) with variable side chains\ndistinct from 6PPD have been identified as potential replacement antioxidants,\nbut their toxicities remain unclear under environmentally relevant\nozone conditions. We herein tested the multiphase gas-surface ozone\nreactivity of four select PPDs [6PPD, <i>N</i>-isopropyl-<i>N</i>′-phenyl-<i>p</i>-phenylenediamine (IPPD), <i>N</i>,<i>N</i>′-diphenyl-<i>p</i>-phenylenediamine (DPPD), and <i>N</i>-phenyl-<i>N</i>′-cyclohexyl-<i>p</i>-phenylenediamine (CPPD)] and\nevaluated the toxicity of their reaction mixtures in coho salmon,\nrainbow trout (Oncorhynchus mykiss),\nand fathead minnow (Pimephales promelas). 6PPD and IPPD were found to rapidly react with ozone, while no\nsignificant multiphase ozone reactivity was observed for DPPD or CPPD.\nThe viability of coho salmon CSE-119 cells was strongly affected by\nthe ozonolysis products of 6PPD but not by those of the other three\nPPDs. Acute mortality was only observed in juvenile rainbow trout\nthat were exposed to oxidized 6PPD, suggesting a common mechanism\nof toxic action in the two salmonid fish species. This study reports\nthe structurally selective ozone reactivity of PPDs and the unique\ntoxicity of 6PPD ozonolysis mixtures, which demonstrates that other\nPPDs are potential alternative antioxidants.