UV-A photodegradation of vanillin in aqueous solutions: Direct photolysis and photosensitized degradation

Phototoxicity of skin due to the application of skincare products or fragrances is increasing from day to day, since a lot of natural extracts, with low or no toxicity in the dark, are being used in the skin exposed to solar radiation. In particular, the use of 4-hydroxy-3-methoxy benzaldehyde, known as vanillin, in the pharmaceutical, food, and cosmetic industries has increased in the last years. This compound is generally considered as safe for humans, and it has been used in a wide range of applications. Significant beneficial properties, such as antioxidant, anticancer, antimicrobial, among others, have been described for vanillin, along with low toxicity. However, although vanillin is used as an ingredient in cosmetic formulations that are applied to the skin, there are few studies on the photochemistry of vanillin degradation. The degradation of vanillin in aqueous solutions exposed to UV-A radiation (365 nm), both by direct absorption and by means of a photosensitized mechanism, was evaluated in different experimental conditions. On the bases of the experimental results, direct photodegradation of vanillin seems unlikely; however, photosensitized degradation of vanillin was observed in the presence of pterin, and vanillin dimers (6,6-dihydroxy-5,5- dimethoxy-[1,1-biphenyl]-3,3-dicarboxaldehyde) were detected as products. The mechanistic analysis indicates that the long-lived pterin triplet excited state is responsible of vanillin degradation, while the contribution of reactive oxygen species, such as superoxide anion, hydrogen peroxide, or singlet oxygen, is negligible. The thermodynamic feasibility of the potential reactions involved in the degradation mechanism was evaluated with the Rehm-Weller equation. Considering both the thermodynamic and kinetic evidence, we proposed a mechanism for the photosensitized degradation of vanillin.