Photoprotective compounds from the lichen Diploicia canescens

Some phenol compounds produced by lichens, strongly absorb in both UV-A (320–400nm) and UV-B (280–320nm) ranges and exhibit chemical and photochemical stability [1]. Looking for such lichen metabolites, we focused our studies on lichen species gathered on rocks of the Britanny coast. The crustose lichen Diploicia canescens (Dicks.) A. Massal. was selected according to the UV profiles of extracts screened with a spectrophotodensitometer CAMAG® coupled with HTPLC. This lichen was known to be rich in secondary metabolites especially chlorinated compounds [2], promising candidates as photoprotective agents [3].

A thorough phytochemical study of the lichen using classical extraction processes led to the isolation of 11 secondary metabolites. Spectroscopic data (λ_max (ε) UVA/UVB ratio, λ_c, Sun Protection Factor) were recorded for each compound and compared to 3 commercialised sunscreens. Three of the isolated compounds presented data comparable to the standards. The dichlorinated depsidone scensidin, can be considered as a moderate UVB filter. The major tetrachlorinated metabolite diploicin afforded a good protection against UV-B and UV-A radiations. Secalonic acid B, a bisxanthone, exhibited an excellent UV-A profile. Thus, D. canescens afforded various metabolites which could act in synergy to protect the lichen against damages induced by UV-A and UV-B radiations. Further investigations (antioxidant activity and cytotoxicity) are in progress on these promising molecules.

Acknowledgement: ICSN and Pr. Y. Lallemand and Pr. T. Sevenet for financial support, CRMPO for mass measurement, Dr K. Articus-Lepage for lichen identification.

References: 1. Hidalgo, ME. et al. (1994) Phytochemistry 37:1585–1587. 2. Elix, AJG. (1988) Mycotaxon, 33:457–466. 3. Fernandez, E. et al. (1998)J. Photochem. Photobiol. B. Biol. 42:195–201