UV-ABC screens of luteolin derivatives compared to edelweiss extract

Pure luteolin is a remarkably heat (200 °C/6 days) and UV stable UV-A screen, however, native luteolin enriched to 37% in an edelweiss extract lost its UV-A screen properties upon UV irradiation (∼4 MJ m−2). This contrasting behavior led to the examination of a series of purified luteolin derivatives as UV screen candidates. 3′,4′,5,7-Tetralipoyloxyflavones were synthesized from luteolin (3′,4′,5,7-tetrahydroxyflavone) and fatty acid chlorides. These acylated semi-biomolecules show a hypsochromic shift in UV–Vis spectra of about ΔλA→B = 58 nm and absorbed in the centre of the harmful UV-B band (λmax = 295 nm). Luteolin was also hydroxyethylated with Br(CH2)2OH. This substitution has no effect on the λmax = 330 nm absorption of luteolin (UV-A band). Finally the natural 4′-O-β-glucosyl-3′,5,7-trihydroxyflavone was extracted from edelweiss and used as a purified natural benchmark. Glycosylated and hydroxyethylated luteolin are both UV stable. Fully acylated luteolin derivatives degrade upon UV exposure to a stable UV-C screen with a hypsochroic shift ΔλB→C = 35 nm. All in all, three molecular structures based on luteolin with sunscreen properties were found, distinguishable in: UV-A, UV-B, and UV-C filters. The natural product based UV-absorbers show promise as alternatives to synthetic molecules and nanoparticles in sunscreen products.