Antioxidant activity of chlorophylls and their derivatives

The antioxidant activity of six natural isolated chlorophyll derivatives and Cu-chlorophyllin was investigated by measuring their protective action against lipid oxidation. For this, the β-carotene bleaching method and the stable radical 2,2-diphenyl-1-picryldrazyl (DPPH) scavenging assay were employed. The results obtained by the β-carotene bleaching method showed that all chlorophyll derivatives presented a dose-dependent response. Pheophorbide b and pheophytin b were the strongest natural antioxidant compounds, whose activities were comparable to BHT. The high antioxidant activity found for pheophorbide b, in comparison to pheophorbide a, demonstrated the importance of the aldehyde group for functionality. On the other hand, by the DPPH assay, all natural pigments showed low antioxidant activity when compared to Trolox. Cu-chlorophyllin, tested by both methods, presented a higher antioxidant activity than that of natural chlorophylls, showing the importance of the nature of the chelated metal in the porphyrin ring. The mechanism of antioxidant activity displayed by the natural chlorophyll derivatives does not seem to be based on the ability to donate hydrogen but maybe, on the protection of linoleic acid against oxidation and/or preventing decomposition of hydroperoxides.