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 (∼4MJm(-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 Δλ(A→B)=58nm and absorbed in the centre of the harmful UV-B band (λ(max)=295nm). Luteolin was also hydroxyethylated with Br(CH(2))(2)OH. This substitution has no effect on the λ(max)=330nm 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 Δλ(B→C)=35nm. 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.