Photoprotective capacities of lichen metabolites: a joint theoretical and experimental study.

The adaptative capacity of lichens to UV radiation could be expressed by the production of photo-absorbing secondary metabolites in thalli. A preliminary screening performed on twelve lichen species by high performance thin layer chromatography (HPTLC) revealed five major compounds absorbing in the UVA (315-400nm) or UVB (280-315nm) ranges in Diploicia canescens. After phytochemical investigation of this lichen, twelve metabolites have been isolated and characterized. Those obtained in sufficient quantities were evaluated for their photoprotective capacities and compared to three referent sunscreens. Experimental spectra were compared to theoretical spectra as obtained at the TD-DFT level of theory. Different DFT functionals were tested to accurately reproduce the UV/Vis spectra of five depsidones, one diphenylether and two bisxanthones. Results indicate that absorption wavelengths and molecular extinction coefficients (oscillator strengths) obtained for the bisxanthones were similar to those of the UVA referent sunscreen.