| Summary: | Marine bioactive compounds have been found in very different sources and exert a very vast array of activities. Squid skin, normally considered a discard, is a source of bioactive compounds such as pigments. Recovering these compounds is a potential means of valorizing seafood byproducts. Until now, the structure and molecular properties of the bioactive pigments in jumbo squid skin (JSS) have not been established. In this study, methanol−HCl (1%) pigment extracts from JSS were fractionated by open column chromatography and grouped by thin-layer chromatography in order to isolate antioxidant pigments. Antioxidant activity was determined by the 2,2-diphenyl-1-picrylhydrazyl (DPPH<sup>●</sup>) and 2,2′-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS<sup>●+</sup>) radical scavenging assays and ferric reducing power (FRAP) assay. Fractions 11−34 were separated and grouped according to flow rate values (F1−F8). Fractions F1, F3, and F7 had the lowest IC<sub>50</sub> against ABTS<sup>●+</sup> per milligram, and fractions F3 and F7 showed the lowest IC<sub>50</sub> in the FRAP assay. Finally, fraction F7 had the highest DPPH<sup>●</sup> scavenging activity. The chemical structure of the F7 fraction was characterized by infrared spectroscopy, <sup>1</sup>H nuclear magnetic resonance, and electrospray ionization−mass spectrometry. One of the compounds identified in the fraction was xanthommatin (11-(3-amino-3-carboxypropanoyl)-1-hydroxy-5-oxo-5H-pyrido[3,2-a]phenoxazine-3-carboxylic acid) and their derivatives (hydro- and dihydroxanthommatin). The results show that JSS pigments contain ommochrome molecules like xanthommatin, to which the antioxidant activity can be attributed.
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