Photo-protective effects of selected furocoumarins on β-pinene, R-(+)-limonene and γ-terpinene upon UV-A irradiation

Abstract

The effect of furocoumarins on terpene photo-oxidation under UV-A light was investigated. For this purpose, four furocoumarins (8-methoxypsoralen, bergapten, bergaptol, bergamottin) each at a level of 5%, was added to solutions of the terpenes β-pinene, R-(+)-limonene and γ-terpinene in ethanol followed by UV irradiation at 366 nm. Bergaptol and bergamottin were synthesized and fully elucidated by NMR spectroscopy. UV-induced transformation of all terpenes was substantially reduced in the presence of furocoumarins. Best photo-protection was observed for γ-terpinene with the addition of bergaptol, i.e. degradation was reduced by 73% compared to the neat substance over a time period of ten days. Bergamottin (50.2%), bergapten (39.8%) and 8-methoxypsoralen (39.6%) also reduced degradation substantially. The protective effect of bergaptol was even noticeable at low concentrations of 0.1%. The main oxidation product of γ-terpinene was p-cymene. Limonene and β-pinene showed a predominated hydroperoxide formation under UV light which could almost completely prevented in the presence of furocoumarins. The protective effect of furocoumarins was presumably due to energy dissipation as a result of the conversion of high energetic radiation (UV-A) into visible light via fluorescence. Phosphorescence or self-quenching within the triplet state were further mechanisms avoiding reactions of excited furocoumarins with the terpenes under investigation. Hence, the photo-stability of the furocoumarins themselves correlated with the extent of their protective effect on all three terpenes.