Flavonoid Glycosides and Phenolic Acids from Inula Oculus-Christi Modulate Membrane Organization and Provide Antioxidant Protection.

Oxidative stress induces lipid peroxidation within the membrane bilayer, thereby compromising membrane integrity. Polyphenols (PPs), renowned for their antioxidant properties, have been shown to mitigate oxidative damage. Here, we investigated the structural and antioxidant effects of PPs-specifically flavonoid glycosides (FGs) and phenolic acids (PAs)-extracted from Inula oculus-christi using steady-state fluorescence spectroscopy in both model and cell membranes. Membrane lipid order was evaluated using DPH and Laurdan spectroscopy, while DPH-TEMPO fluorescence quenching was employed to quantify raft-like domain formation in model systems. The antioxidant capacity of the PP extracts was assessed via fluorescence quenching of cis-parinaric acid. Both FGs and PAs conferred approximately 2-fold antioxidant protection, with FGs showing a 1.13-fold greater effect than PAs. In addition, both PP classes promoted lipid raft formation, particularly in cholesterol-rich membranes. PPs increased order in the liquid-disordered (L(d)) phase while inducing disorder in the liquid-ordered (L(o)) phase, depending on the lipid-to-PP ratio. Notably, FGs enhanced membrane fluidity more strongly in A549 than in MDCKII cells, as reflected by a ~5.7-fold decrease in Laurdan GP in A549 (from 0.04 to -0.17) versus a ~1.4-fold decrease in MDCKII at 200 μg/mL. These findings highlight the dual structural and antioxidative roles of FGs and PAs in preserving membrane integrity under oxidative stress.