BackgroundPhenolic compounds are widely distributed in plant kingdom and constitute one of the most important classes of natural and synthetic antioxidants. In the present study fifty one natural and synthetic structurally variant phenolic, enolic and anilinic compounds were examined as antioxidants and radical scavengers against DPPH, hydroxyl and peroxyl radicals. The structural diversity of the used phenolic compounds includes monophenols with substituents frequently present in natural phenols e.g. alkyl, alkoxy, ester and carboxyl groups, besides many other electron donating and withdrawing groups, in addition to polyphenols with 1–3 hydroxyl groups and aminophenols. Some common groups e.g. alkyl, carboxyl, amino and second OH groups were incorporated in ortho, meta and para positions.ResultsSAR study indicates that the most important structural feature of phenolic compounds required to possess good antiradical and antioxidant activities is the presence of a second hydroxyl or an amino group in o- or p-position because of their strong electron donating effect in these positions and the formation of a stable quinone-like products upon two hydrogen-atom transfer process; otherwise, the presence of a number of alkoxy (in o or p-position) and /or alkyl groups (in o, m or p-position) should be present to stabilize the resulted phenoxyl radical and reach good activity. Anilines showed also similar structural feature requirements as phenols to achieve good activities, except o-diamines which gave low activity because of the high energy of the resulted 1,2-dimine product upon the 2H-transfer process. Enols with ene-1,2-diol structure undergo the same process and give good activity. Good correlations were obtained between DPPH inhibition and inhibition of both OH and peroxyl radicals. In addition, good correlations were obtained between DPPH inhibition and antioxidant activities in sunflower oil and liver homogenate systems.ConclusionsIn conclusion, the structures of good anti radical and antioxidant phenols and anilines are defined. The obtained good correlations imply that measuring anti DPPH activity can be used as a simple predictive test for the anti hydroxyl and peroxyl radical, and antioxidant activities. Kinetic measurements showed that strong antioxidants with high activity have also high reaction rates indicating that factors stabilizing the phenoxyl radicals lower also the activation energy of the hydrogen transfer process.

Background: Moringa oleifera is rich in various active phyto-constituents (tannins, sterols, terpenoids, flavonoids, saponins, anthraquinones, alkaloids, and vitamins) in addition to different minerals in its leaves and seeds. Presence of these constituents is responsible for the antioxidant activity and the ability to protect against oxidative damage. Based on measurements of the cytotoxic activities, M. oleifera leaves were found to be more effective than the plant seeds. Therefore, the plant leaves were selected for preparation of silver plant nano-extract during the present study. Methods: The silver nanoparticles (Ag-NPs) were synthesized then characterized by transmission electron microscope (TEM), ultraviolet-visible (UV-VIS) spectroscopy and dynamic light scattering (DLS) measurements. Moreover, the in vitro antioxidants were assayed in M. oleifera leaves extract before and after incorporating Ag-NPs through measurement of total polyphenolic compounds and scavenging activities against free radicals in addition to the cytotoxic activity against growth of human colon carcinoma. Furthermore, the phenolic compounds were detected by gas chromatography coupled with a mass spectrometer (GC/MS) and fourier transform infrared (FT-IR) spectroscopy. Also, the median lethal dose (LD 50) of the extract and nano-extract was evaluated. Results: It was showed that incorporation of Ag-NPs into the M. oleifera leaves extract enhanced the total antioxidant capacity, concentration of total polyphenolic compounds, reducing power and scavenging activity against attack of free radicals in addition to increasing the cytotoxicity against growth of colon cancer cells. This might be related to increasing the phenolic compounds as a result of incorporation of Ag-NPs and detected by the GC/MS and FT-IR analysis. It was found that there was no wide gap in the LD 50 between M. oleifera leaves extract and silver nano-extract. The LD 50 values of the M. oleifera leaves extract and silver nano-extract were about 14,250 and 13,750 mg/Kg, respectively. Conclusion: The study revealed that incorporation of Ag-NPs into the M. oleifera extract enhanced the in vitro antioxidative efficiency and might be related to increasing the phenolic compounds.

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