Recognition of antioxidants and photosensitizers in Dyssodia pinnata by EPR spectroscopy

Abstract

Introduction Previous studies report the isolation mainly of terthiophene derivatives and flavonoids from Dyssodia species. Terthiophenes are known as photosensitizers by their capacity to generate singlet oxygen (1O2), and flavonoid antioxidant activity is recognized. These opposite properties could represent interesting options in photodynamic therapy. Objective To determine the antioxidant and photosensitizer activities of extracts and isolates of Dyssodia pinnata by electron paramagnetic resonance (EPR). Methodology Extracts and isolates were evaluated as antioxidants by the interactions with copper ion (Cu2+) observed in EPR, and by the 2,2‐diphenyl‐1‐picrylhydrazyl radical (DPPH) and the thiobarbituric reactive substances (TBARS) methods. Their abilities as 1O2 producers were also estimated by EPR. Results Terthiophenes were obtained from hexane (DPH) and acetone (DPA) extracts, and flavonoids from DPA and methanol (DPM) extracts. The interaction with Cu2+ of extracts and isolated compounds of Dyssodia pinnata showed two effects in EPR: reduction and chelation; flavonoids exhibited both effects, while terthiophenes showed only reduction. DPA, DPM, and flavonoids were active in DPPH and TBARS assays. Quercetagetin‐7‐O‐β‐glucoside showed the highest antioxidant and chelating activities, 3‐glycosidated flavonoids were less active. Upon irradiation extracts and terthiophenes induced 1O2 formation. Conclusion Flavonoid reducing activity on Cu2+ and free radical scavenging capacity were related to the number of hydroxy groups and to the conjugation between the B and C rings. All tested flavonols showed a major complex with Cu2+, with the most probable site of chelation between the 5‐hydroxy and 4‐oxo groups. Extracts and terthiophene derivatives showed photosensitizer activity. Thus, EPR is useful to evaluate free radical scavenging and pro‐oxidant properties.