pH- and phase-dependent antioxidant behavior of γ- and α-terpinene in bulk and structured lipid systems

Pro-aromatic terpenes such as γ-terpinene and α-terpinene, natural components of essential oils, are unconventional inhibitors of lipid peroxidation. We investigated how environmental factors modulate their antioxidant performance in lipid systems representative of food matrices, including bulk oils, emulsified systems, model membranes, and hepatic cells. Both terpenes suppressed peroxidation in heterogeneous systems at 0.2-1 wt% relative to oxidizable lipids, while showing concentration-dependent inhibition-promotion behavior in homogeneous phases. Mechanistically, the terpinenes react with lipid peroxyl radicals to form p-cymene and HOO^• radicals. The antioxidant outcome is environmentally controlled: in bulk organic media and at acidic pH, HOO^• can sustain propagation, whereas in micelles and liposomes its partitioning and pH-dependent conversion to superoxide limit chain propagation. α-Terpinene was less effective due to competing peroxyl-addition pathways. Both terpenes reduced RSL3-induced lipid peroxidation in HepG2 cells without cytotoxicity. These results highlight radical speciation and compartmentalization as key determinants of antioxidant efficiency of pro-aromatic terpenes in structured lipid systems.