Polymerized Luteolin Nanoparticles: Synthesis, Structure Elucidation, and Anti-Inflammatory Activity

Luteolin is an anti-inflammatory flavonoid commonly found in many edible plants. The compound is popularly consumed as a supplement regardless of its poor water solubility (27.8 μg/mL at 25 °C) and low bioavailability. Here, mild one-pot polymerization of luteolin into water-dispersible nanospheres, with an average dry size of 234.8 ± 101.6 nm, an aqueous size distribution of 379.1 ± 220.5 nm (PDI = 0.338), an average ζ-potential of −36.2 ± 0.2 mV, and an 89.3 ± 4.8% yield, is described. The nanospheres consist of polymerized luteolin (polyluteolin) with a weight-average molecular mass of around 410000 Da. The chemical structure of polyluteolin is identified through 1H–1H correlated spectroscopy (COSY), 1H–13C heteronuclear single-quantum coherence (HSQC), and 1H–13C heteronuclear multiple-bond correlation (HMBC) NMR spectroscopic analyses of the oligomers, and a polymerization mechanism is proposed. Unlike luteolin that showed both dose-dependent anti-inflammatory activity and cytotoxicity when tested in lipopolysaccharide-stimulated macrophages, the polyluteolin nanoparticles possess dose-dependent anti-inflammatory activity without causing cell death even at high concentrations.