LC-HRMS) profiles. In parallel, redox active properties were evaluated by the capacity of the molecules to reduce 2,2-diphenyl-1-picrylhydrazyl (DPPH •) and superoxide (O 2 •−) radicals using UV-Vis and electron spin resonance spectroscopies (ESR), respectively. A spectral similarity network (molecular networking) was used to highlight clusters involved in the observed redox activities. Results Dereplication on Viola alba subsp. dehnhardtii highlighted a reproducible pool of redox active molecules. Polyphenols, particularly O-glycosylated coumarins and C-glycosylated flavonoids, were identified and de novo dereplicated through molecular networking. Confirmatory analyses were undertaken by thin layer chromatography (TLC)-DPPH-MS assays and nuclear magnetic resonance (NMR) spectra of the most active compounds. Conclusion Our dereplication strategy allowed the screening of leaf extracts to highlight new biologically active metabolites in few steps with a limited amount of crude material and reduced time-consuming manipulations.

Oxidized pterins, efficient photosensitizers under UV-A irradiation, accumulate in the skin of patients suffering from vitiligo, a chronic depigmentation disorder. In this work, we have investigated the ability of pterin (Ptr), the parent compound of oxidized pterins, to photosensitize the oxidation of the peptide α-melanocyte-stimulating hormone (α-MSH), which stimulates the production and release of melanin by melanocytes in skin and hair. Our results showed that Ptr is able to photoinduce the degradation of α-MSH upon UV-A irradiation and that the reaction is initiated by an electron transfer from the peptide to the triplet excited state of Ptr. The photosensitized process produces chemical changes in at least two different amino acid residues: tryptophan and tyrosine (Tyr). It was shown that α-MSH undergoes dimerization and oxidation, the former process taking place after the formation of Tyr radicals. The present findings are analyzed in the context of the general behavior of pterins as photosensitizers and the biological implications are discussed.

The tyrosine dimer (Tyr2), a covalent bond between two tyrosines (Tyr), is one of the most important modifications of the oxidative damage of proteins.