Ultraviolet–visible phototransformation of dehydroacetic acid – Structural characterization of photoproducts and global ecotoxicity

Rationale: The present work is devoted to the structural elucidation of by-products issued from the direct ultraviolet–visible (UV–vis) irradiation of dehydroacetic acid (DHA) in solution and in cosmetic emulsion. Methods: Analyses were carried out using gas chromatography coupled with ion trap mass spectrometry and by high-performance liquid chromatography coupled with ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (LC/UHRMS). The potential toxicities of by-products were estimated by in silico calculations based on a QSAR (Quantitative Structure–Activity Relationship) approach and by in vitro bioassays conducted on Vibrio fischeri bacteria. Results: Three photoproducts were detected by LC/MS while one photoproduct was detected by GC/MS. The first photoproduct (PP1) corresponds to an isomer of DHA while two isomeric compounds correspond to dimeric structures. The oral rat LD₅₀ of PP1 was evaluated to be 4.5 times lower than that of the parent molecule which classes it in the category ‘moderately toxic’ on the Hodge and Sterne toxicity classification. In vitro assays on Vibrio fischeri bacteria showed that the global ecotoxicity of the DHA solution increases with irradiation time. Conclusions: With the exception of one photoproduct, the structures proposed for the photoproducts on the basis of mass spectral interpretation have not been reported in previous studies. All photoproducts, with the exception of dimers, were detected after irradiation in the cosmetic emulsion. This result shows that personal care products containing DHA must be protected from direct sunlight to prevent photodegradation.