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Coumarins are phenolic-type compounds with efficient antioxidant activity due to their ability to scavenge reactive oxygen species. Nevertheless, their ability to behave as photosensitizers capable of generating reactive oxygen species, such as singlet oxygen, has been less studied. In this work, the photosensitizing ability of seven hydroxycoumarins was evaluated through the photooxidation of ergosterol by quantifying the conversion of ergosterol into ergosterol peroxide. In our experimental conditions, we found that almost every tested antioxidant coumarin promotes the peroxidation of ergosterol. The results suggest that the hydroxycoumarins exhibit potential photosensitizing activity by promoting singlet oxygen generation by a Type II photochemical mechanism. Density functional theory (DFT) calculations were also performed to obtain further insight into the chemical reactivity of tested compounds; the observed tendency in the group of antioxidant coumarins to promote the reaction was their hardness due to the principle of maximum hardness. To evaluate our conclusion, we performed the reaction using a highly polarizable coumarin as a photosensitizer, which resulted in an increased photosensitizing capacity supported with DFT calculations, which reinforces our analysis. Finally, we found that hydroxycoumarins can be potentially pro-oxidants since some of them can act as photosensitizers and generate singlet oxygen in the presence of UV–Vis light, a characteristic that must be considered when these compounds are used as antioxidants.

Photodynamic therapy (PDT) is a potentially non-invasive therapeutic strategy that involves the use of a photosensitizing agent molecule capable to absorb light at a specific wavelength and to transfer the absorbed energy to generate singlet oxygen. This oxidizing agent reacts with double bonds of membrane phospholipids and sterols present in biological membranes causing damages and/or cell permeability. With the aim to analyze and to determine the effectiveness of possible photosensitizing agents, in this work we evaluate the in vitro amoebicidal effect of nine cosmetic dyes, 1–9, belonging to the xanthene, chlorophyll, aminoanthraquinone, and pyrene group of chromophores, against Acanthamoeba castellanii Neff. Red 22 (2, eosin Y) and red 28 (4) were the most effective photosensitizers to inhibit cell growth, with 2 showing the highest growth inhibition when it is activated by white light. The study of their mechanisms of action reveals that red 22 (2) increases membrane permeability and induces chromatin condensation of parasites, and could be used in combined therapies with currently used drugs.

Photosensitizer molecules capable of absorbing UV radiation and visible light through photochemical reactions can generate singlet oxygen (1O2). 1O2 is an oxidizing agent that can react with biomolecules causing oxidative damage. Cosmetic dyes possess chromophore groups capable of performing the absorption process of electromagnetic radiation thereby they could act as photosensitizers. The aim of this research was to evaluate the photosensitizing ability of 25 dyes used as cosmetic ingredients through photo-oxidation of ergosterol as an indirect detection method of 1O2. The presence of 1O2 was indirectly detected by ergosterol conversion into ergosterol peroxide through 1H-NMR mixture analysis. Our results showed that 9 cosmetic dyes belonging to pyrene, chlorophyll, xanthene and aminoanthraquinone groups were able to act as photosensitizers in the generation of 1O2. The photosensitizing ability of these dyes could be contradictory if they are used as cosmetic ingredients because of pro-oxidants effects caused by 1O2 and its implications in skin photoaging. On the other hand, in regard to the photosensitizing ability of the remaining dyes tested belonging to the azo, triarylmethane, hydroxyanthraquinone, quinoline and indigoid groups, these were found not capable of photosensitizing 1O2 generation, consequently their use as cosmetic ingredients wouldn't suppose an oxidation risk caused by 1O2.