Structure-selective metabolic pathways and major metabolites of alkylated polycyclic aromatic hydrocarbons identified in ryegrass and pumpkin.

Alkylated polycyclic aromatic hydrocarbons (alkylated PAHs) are prevalent environmental contaminants, potentially exhibiting toxicity comparable to unsubstituted PAHs. Through systematic plant exposure experiments, this study investigated the transformation of alkylated PAHs, using 5-methylchrysene (CHR-5Me) as a model compound in pumpkin and ryegrass. By leveraging isotope-labeled and unlabeled standards to generate characteristic ion pairs, alongside high-resolution mass spectrometry (HRMS) non-target screening and quantum chemical calculations, we effectively characterized the metabolic pathways of CHR-5Me within the plant systems. An unsubstituted PAH, chrysene (CHR) was studied simultaneously to assess structural effects. For CHR-5Me, three hydroxylated, one carboxylated, and one glucopyranoside-conjugated products were verified in the hydroponic exposure systems, while CHR yielded two detectable hydroxylated and one methoxylated products. Among those metabolites, carboxylation and glucopyranoside conjugation of alkylated PAHs were found for the first time in living plants. CHR-5Me-glucopyranoside was exclusively observed in pumpkin, whereas methoxylation product was only confined to ryegrass rhizosphere solution (which contains root exudates and rhizosphere microorganisms), highlighting interspecies metabolic differences. In addition, CHR-5Me has stronger transformation potential and generated more metabolites than CHR due to the methyl group in CHR-5Me enhances its reaction activity and reduces steric hindrance. This research constructed the transformation pathways of CHR-5Me in plants, demonstrating its strong biotransformation activity, and providing novel insights into the environmental fate of alkylated PAHs.