Antioxidants Activity of Selected Synthesized Compounds

Antioxidants are chemical compounds or mixture of compounds which when present in low concentrations are used to prevent the oxidation of lipids, sugars, proteins and DNA that can generate aldehydes, ketones, esters and other products that can be harmful to living systems. Antioxidants can be synthetic or natural. Synthetic antioxidants include Butylated Hydroxyl Anisole (BHA), Tert-Butyl Hydroquinone (TBHQ) and Propyl Gallate (PG), etc [1]. The most common oxidants in biological systems are free radicals. Free radicals are atoms, molecules or ion that possess an unpair electron, seeking electrons from other atoms, molecules or ions in order to stabilize themselves. An initial attack causes the odd electron to be paired. However, this can result in the generation of another free radical and promoting a chain reaction [2]. In the biological systems, free radicals are derived from oxygen, nitrogen and sulphur molecules. They occur as parts of groups of molecules called Reactive Oxygen Species (ROS), Reactive Nitrogen Species (RNS) and Reactive Sulphur Species (RSS). ROS includes free radicals such as superoxide anion (O2-.), per hydroxyl radical, hydroxyl radical, nitric oxide, hydrogen peroxide, singlet oxygen, hypochlorous acid and peroxynitrite [3].

Free radicals are formed via three (3) major pathways. These include:
a. Homolytic cleavage of covalent bond of a normal molecule, with each fragment, retaining one of the paired electrons.
b. By loss of a single electron from normal molecule.
c. By addition of a single electron to an atom, molecule.

They are usually formed in vivo [4] via:
a) Normal aerobic respiration in which the mitochondria consume O₂, reduces it by sequential steps to produce O₂, H₂O₂ and OH- as by product.
b) Bacteria or virus infected cells get destroyed by phagocytosis, with an oxidative burst of Nitric Oxide (NO), O₂, H₂O₂ and OCl.
c) Peroxisomes producing H₂O₂ as a product of fatty acid and other lipid molecular degradation. Its further degraded by catalase. Some of the peroxide may be released into other cell compartments, leading to increasing oxidative stress and DNA damage.
d) Animal Cytochrome P₄₅₀ which are the primary defence systems that protect against toxic chemicals may generate oxidative by products that will damage DNA.

Antioxidants have many uses. These include their medicinal uses whereby antioxidants retard ageing, promote longevity and the prevention of other diseases such as cancer and cardiovascular diseases that result from oxidative stress. They are also used as preservatives for foods [5].

Figure 1:Chalcone derivatives.

This paper is a mini review of the antioxidant activities of selected compounds. In one study, the synthesis, characterisation and evaluation of the antioxidant activities of some novel chalcones derivatives (1a-2d) as shown in Figure 1 have been reported [6]. In this report, a novel series of chalcones esters have been synthesized via the reaction of the requisite chalcones with either palmitic or stearic acid. Another class of chalcones comprising 2,3-disubstituted chalcones and 2-amino-6-(substituted-phenyl)- 4-substitutedphenyl-nicotinonitrile derivatives have also been prepared by electrophilic and Michael addition reactions with the respective chalcones. The structural integrity of compounds was established via 1HNMR, 13CNMR, IR and mass spectroscopy. The radical scavenging activities were investigated using 1,1-biphenyl- 2-picrylhydrazyl as the free radical scavenging agent. These compounds were selected on the basis that the above chalcones would quench the 1,1-biphenyl-2-picrylhydrazyl radical scavenging activities. Results indicated that one of the compounds (68.58% at C=2ug/ml) was more effective as an antioxidant agent than the reference antioxidant, ascorbic acid. The synthesis of new indazole derivatives, Figure 2, as potential antioxidants agents have been cited [7]. These derivatives were synthesized by reacting hydrazine hydrate and its derivatives with cyclohexanone derivatives. The latter were prepared from the respective chalcones via refluxing with ethyl acetoacetate in ethanol in the presence of sodium hydroxide. These compounds were characterized by 1HNMR, 13CNMR, IR and mass spectroscopy. All indazole derivatives exhibited radical scavenging activity against DPPH, reducing power capacity and total antioxidant capacity in comparison with respective standard (Figure 3).

Figure 2:Novel Indazole derivatives.

Figure 3:Novel Pyrimidine derivatives.

Novel derivatives of 4-R-5-phenethyl-4H-1,2,4-triazole- 3-thiols were synthesized. The antioxidant activity of the synthesized compounds was evaluated in vitro by the method of the non-enzymatic initiation of BOD with salts of iron (II). These compounds were found to be more potent than some of the existing antioxidants [8]. Synthesis and antioxidant activity of some Novel Pyrimidine Derivatives have been cited [9,10]. The percentage yield of the synthesized compounds ranges from 52-61%. All synthesized compounds were characterized via 1HNMR, 13CNMR and IR spectroscopy. Antioxidant activities were investigated via the DPPH scavenging method. 0.002% DPPH in methanol was used as the free radical. The absorbance was measured at 517nm using UV-Visible spectroscopy. The absorbance of the DPPH control was also noted. The scavenging activity of the compounds versus the stable DPPH was calculated using the equation: Scavenging activity (%)=(A-B)/AX100, where ‘A’ was the absorbance of DPPH solution and ‘B’ was the absorbance of DPPH solution with selected compounds. It was found that the Scavenging activity (%) of different concentrations (ug/mL), ranging from 25 to 400ug/mL, increased with increasing concentration of the compound, with 400ug/mL exhibiting the largest. At that concentration, the scavenging activity range from 64.23% to 96.11 %. The Schiff base ligand: (2-{(E)- [(4-chlorophenyl) amino] methyl} phenol), Figure 4 and its M(II) complexes (M=Cu, Ni, Zn and Co) have been synthesized and their antioxidant properties evaluated via the FRAP, DPPH, FIC (Ferrous Ion Chelation) and phosphomolybdenum method by hydroxyl free radical (•OH) scavenging activity.

Figure 4:(2-{(E)-[(4-chlorophenyl) imino] methyl} phenol).

The phosphomolybdenum assay involves the reduction of Mo (VI) to Mo(V) by the selected antioxidant compound, resulting in the formation of a green precipitate-Mo(V) complex in acidic solution. It was observed that the synthesized compounds show significant antioxidant properties as compared to conventional antioxidants such as Vitamin-C and EDTA [11]. Fulvic Acid (FA) is a class of compound including humic substances together with humic acid and humin. The structure of Fulvic acid is shown in Figure 5. It is formed via the degradation of organic substances by chemical and biological process. FA consists of a mixture of closely related complex aromatic polymers with the presence of aromatic rings, phenolic hydroxyl, ketone carbonyl, quinone carbonyl, carboxyl and alkoxyl groups. For the first time, the scavenging activity of biosynthesized fulvic acid in comparison with reference compounds has been noted. FA displays a scavenging activity compared with the reference compounds although it was less efficient than Nordihydroguaiaretic Acid (NDGA), ascorbic acid, pyruvate, Di Methyl Thio Urea (DMTU), penicillamine and Glutathione (GSH) for O2. Thus, FA is a good candidate to be used in pharmaceutical or food industries as an accessible synthetic and natural antioxidants [12]. Oxidative stress is the source for numerous diseases, such as diabetes, atherosclerosis, cancer, neurodegenerative and cardiovascular diseases. Therapeutic antioxidants are promising candidates for preventing and treating oxidative stress and thus prevent the above diseases. In response to this, the design, synthesis and antioxidant activity of six compounds containing the 2-methoxyphenol moiety has been reported [13]. These compounds were synthesized in yields of 48-95%. The synthesized derivatives were characterized using ¹H NMR, ¹³C NMR, Fourier- Transform Infrared (FT-IR) spectroscopy and elemental analyses. The antioxidant properties of the compounds were evaluated using the 2,2-Diphenyl-1-Picryl-Hydrazyl-Hydrate (DPPH), 2,2’-Azino- Bis (3-Ethylbenzothiazoline-6-Sulfonic Acid (ABTS), and Oxygen Radical Absorbance Capacity (ORAC) assay. All six (6) compounds showed strong antioxidant activity using the three assays above.

Figure 5:The structure of fulvic acid.

Anthocyanidins are coloured dyes that are responsible for the bright colour of fruits and flowers. They belong to the flavonoid family and the structure comprise of three rings: A, B and C. Aromatic ring (A) is fused with heterocyclic ring, containing and oxygen (C), which is bonded to a third aromatic ring (B). In one report, Anthocyanidins (1) to (3) were synthesized in excellent yield (97-98%) to study the effect of methoxy substitution on the B ring with respect to their antioxidant properties (Figure 6). Comparative FRAP studies show 2′- and 4′-methoxy substituents have higher antioxidant activities, which may be attributed to both resonance and inductive effects [14]. In recent years, developing phenolic antioxidants that are potent is a very active area of research. However, the use of phenolic compounds has also been limited by poor antioxidant activity in several in vivo research. Polymeric phenols have received much attention owing to their potent antioxidant properties and increased stability in aqueous systems. One approach to synthesise polyphenolic antioxidants via enzyme catalysed synthesis of polymeric phenols [15].

Figure 6:

The design and synthesis of compounds with antioxidant activities has been increasing with the view to find potent antioxidants with least side effects. Synthetic antioxidants are not environmentally safe, coupled with their arduous synthesis and should be superseded with herbal antioxidants. Some of the synthesised antioxidants reported to date are: Butylated Hydroxyl Anisole (BHA), Tert Butyl Hydroxyquinone (TBHQ), Propyl Gallate (PG), Chalcones, indazole derivatives, triazole derivatives, pyrimidine derivatives, Schiff base derivatives, fulvic acid etc. They have shown variation in antioxidant potency compared to reference standard such as ascorbic acid. In some cases, lesser and in others greater. Amongst the methods used to determine radical scavenging activity include the FRAP, DPPH, FIC (Ferrous Ion Chelation) and phosphomolybdenum method.

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