Physicochemical properties of tire-derived para-phenylenediamine quinones - A comparison of experimental and computational approaches

Para-phenylenediamine (PPD) compounds are added to tire rubber at percent levels to sacrificially react with oxidants for prolonged service life. Recently, the PPD transformation product N-(1,3-dimethylbutyl)-N’-phenyl-p-phenylenediamine quinone (6PPDQ) has been identified in roadway runoff as a potent toxicant for coho salmon (Oncorhynchus kisutch). As 6PPD may be phased out in favour of alternative PPDs, understanding the physicochemical properties of their corresponding quinones is important for predicting their environmental fate, distribution, and toxicity. Here, we present an experimentally determined log KOW for 6PPDQ (4.0 ± 0.2) as well as water solubility values for 6PPDQ and five structural analogues (3.2 – 170 µg/L). The water solubilities were several orders of magnitude lower than those predicted by EPI Suite and OPERA, popular Quantitative Structure Activity Relationship (QSAR) programs. We also report octanol-water and air-water partition ratios for PPDQs using Density Functional Theory (DFT) and QSAR approaches. Both methods provided similar rank ordering of compounds. We found that DFT tends to underestimate log KOW values, while QSAR models provide a better agreement with experimental results. Conversely, QSAR models provided poorer predictions of log KAW values than DFT. We discuss the strengths and limitations of both computational approaches, the need for more experimentally derived values, and caution researchers interpreting predicted physicochemical properties, particularly for emerging contaminants for which QSARs may be insufficiently parameterized.