Investigation of antioxidant effect of Urtica dioica (L.) leaf and Alcea setosa (L.) flower extracts on fatty acids photooxidation

Light energy, especially in combination with oxygen by producing singlet oxygen (1O2), can react with the double bonds of unsaturated fatty acids and reduce the quality of food and fats. In this study, the effect of singlet oxygen on photooxidation of fatty acids was investigated. The generation of singlet oxygen and peroxide products in the presence of meso-tetraphenylporphyrin (H2TPP) as a photocatalyst and light was proved by nuclear magnetic resonance spectroscopy (1H NMR), visible-ultraviolet spectroscopy (UV-Vis) and iodometric titration. The rate of fatty acid peroxidation determined immediately after photooxidation using meq/kg unit. Effect of hydroalcoholic extracts of Urtica dioica (L.) leaf and Alcea setosa (L.) flower was compared with synthetic antioxidants and well-known singlet oxygen scavengers. The antioxidant activities of these plants showed that hydroethanolic extracts of Urtica dioica (L.) leaf and Alcea setosa (L.) flower, respectively diminished conversion of oleic acid to peroxide products 79.45 and 81.05% after 120 minutes photooxidation. While these value for vitamin E (as a fat-soluble chemical antioxidant), sodium azide (as a very strong inhibitor of singlet oxygen) and dimethyl sulfoxide (as a strong solvent in reducing the lifetime of singlet oxygen) were 83.83%, 91.65% and 93.25%, respectively. Also, the hydroalcoholic extracts of Urtica dioica (L) leaf,  Alcea setosa (L) reduced the conversion of linoleic acid (as a oxidizable fatty acid with high degree of unsaturation) to peroxide products by 56.43 and 59.06%, respectively. These results declare high antioxidant efficiency of Urtica dioica (L) leaf and Alcea setosa (L), in preventing of photooxidation of fatty acids. In this study, the effects of solvent, photocatalyst, light and oxygen in fatty acid photooxidation were also investigated.