Oxidation And Antioxidants In Organic Chemistry And Biology Pdf

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Free radicals and other oxidants have gained importance in the field of biology due to their central role in various physiological conditions as well as their implication in a diverse range of diseases. The free radicals, both the reactive oxygen species ROS and reactive nitrogen species RNS , are derived from both endogenous sources mitochondria, peroxisomes, endoplasmic reticulum, phagocytic cells etc.

Beneficial renal effects of some medications, such as chelation therapy depend at least partially on the ability to alleviate oxidative stress.

Antioxidant Compounds and Their Antioxidant Mechanism

The normal biochemical reactions in our body, increased exposure to the environment, and higher levels of dietary xenobiotic's result in the generation of reactive oxygen species ROS and reactive nitrogen species RNS. The reported chemical evidence suggests that dietary antioxidants help in disease prevention. Therefore, it is very important to understand the reaction mechanism of antioxidants with the free radicals. This review elaborates the mechanism of action of the natural antioxidant compounds and assays for the evaluation of their antioxidant activities.

The reaction mechanisms of the antioxidant assays are briefly discussed references. Practical applications: understanding the reaction mechanisms can help in evaluating the antioxidant activity of various antioxidant compounds as well as in the development of novel antioxidants. Normal biochemical reactions, increased exposure to the environment, and higher levels of dietary xenobiotics result in the generation of reactive oxygen species ROS and reactive nitrogen species RNS.

The oxidative stress can be effectively neutralized by enhancing cellular defenses in the form of antioxidants. Antioxidants can be categorized in multiple ways. Based on their activity, they can be categorized as enzymatic and non-enzymatic antioxidants. Enzymatic antioxidants work by breaking down and removing free radicals.

The antioxidant enzymes convert dangerous oxidative products to hydrogen peroxide H 2 O 2 and then to water, in a multi-step process in presence of cofactors such as copper, zinc, manganese, and iron. Non-enzymatic antioxidants work by interrupting free radical chain reactions. Few examples of the non-enzymatic antioxidants are vitamin C, vitamin E, plant polyphenol, carotenoids, and glutathione. The other way of categorizing the antioxidants is based on their solubility in the water or lipids.

The antioxidants can be categorized as water-soluble and lipid-soluble antioxidants. The water-soluble antioxidants e. The lipid-soluble antioxidants e. The antioxidants can also be categorized according to their size, the small-molecule antioxidants and large-molecule antioxidants.

The small-molecule antioxidants neutralize the ROS in a process called radical scavenging and carry them away. To understand the mechanism of action of antioxidants, it is necessary to understand the generation of free radicals and their damaging reactions.

This review elaborates the generation and damages that free radicals create, mechanism of action of the natural antioxidant compounds and assays for the evaluation of their antioxidant properties. The reaction mechanisms of the antioxidant assays are discussed.

The scope of this article is limited to the natural antioxidants and the in vitro assays for evaluation of their antioxidant properties. These ROS can act by either of the two oxygen dependent mechanisms resulting in the destruction of the microorganism or other foreign matter. The reactive species can also be generated by the myeloperoxidase—halide—H 2 O 2 system.

The enzyme myeloperoxidase MPO is present in the neutrophil cytoplasmic granules. In presence of the chloride ion, which is ubiquitous, H 2 O 2 is converted to hypochlorous HOCl, eqn 3 , a potent oxidant and antimicrobial agent.

Peroxynitrite reacts with the aromatic amino acid residues in the enzyme resulting in the nitration of the aromatic amino acids. Such a change in the aminoacid residue can result in the enzyme inactivation. However, nitric oxide is an important cytotoxic effector molecule in the defense against tumor cells, various protozoa, fungi, helminthes, and mycobacteria.

The peroxyl radicals are the carriers of the chain reactions. The peroxyl radicals can further oxidize PUFA molecules and initiate new chain reactions, producing lipid hydroperoxides LOOH eqn 10 and 11 that can break down to yet more radical species.

Lipid hydroperoxides always break down to aldehydes. Many of these aldehydes are biologically active compounds, which can diffuse from the original site of attack and spread the attack to the other parts of the cell. This type of oxidative damage to DNA is highly correlated to the physiological conditions such as mutagenesis, carcinogenesis, and aging.

However, the Chydroxy-adduct radical of guanine is converted to the 8-hydroxyguanine upon oxidation reaction. The oxidation reaction of these adduct radicals with water followed by deprotonation results in the formation of the cytosine glycol and thymine glycol, respectively.

The reactions of carbon-centered sugar radicals result in the DNA strand breaks and base-free sites by a variety of mechanisms. The amino acid's lysine, proline, histidine, and arginine have been found to be the most sensitive to oxidative damage. Recent studies indicate that, a wide range of residue modifications can occur including formation of peroxides, 27,28 and carbonyls.

Thus, the oxidative damage to tissue results in the increased amount of oxidized protein. A detailed review by Cooke et al. Low levels of antioxidants have been associated with the heart disease and cancer. The other disorders to which antioxidants provide protection are cataract, cerebral ischemia, diabetes mellitus, eczema, gastrointestinal inflammatory diseases, genetic disorders. The different expression profiles, subcellular locations, and substrates of the antioxidant enzymes reveal the complex nature of the ROS biology.

Clearly, the antioxidant enzymes play a major role in the prevention of oxidative damage. The enzymatic antioxidants and their mechanism of antioxidant activity has been explained in details in several review articles.

The nonenzymatic antioxidants are of two types, the natural antioxidants and the synthetic antioxidants. However, the scope of this article is limited to the natural antioxidants; hence the synthetic antioxidants will not be considered for the discussion.

The resultant tocopheroxyl radical is relatively stable and in normal circumstances, insufficiently reactive to initiate lipid peroxidation itself, which is an essential criterion of a good antioxidant. Vitamin C or ascorbic acid 2 , is a water-soluble free radical scavenger.

Moreover, it regenerates vitamin E in cell membranes in combination with GSH or compounds capable of donating reducing equivalents. The pairs of ascorbate radicals react rapidly to produce one molecule of ascorbate and one molecule of dehydroascorbate. The dehydroascorbate does not have any antioxidant capacity.

Hence, dehydroascorbate is converted back into the ascorbate by the addition of two electrons. The last stage of the addition of two electrons to the dehydroascorbate has been proposed to be carried out by oxidoreductase.

Antioxidant potential of vitamin A 3 was first described by Monaghan and Schmitt, 52 who reported that vitamin A can protect lipids against rancidity. Several reviews have appeared to outline the basic structural and metabolic characteristics of vitamin A and information about its potential as antioxidants in relation to the heart diseases.

It has been reported that the bioflavonoids have a protective effect on the DNA damage induced by the hydroxyl radicals. The flavonoids complexed with the copper or iron prevent the generation of the ROS. Therefore, it is very important to consider the concentration of the chelating metal ions, such as copper or iron while evaluating the protective or degenerative effects of quercetin and other bioflavonoids.

Anthocynidine, a class of flavonoids are potential antioxidants and their effectiveness in the inhibition of the lipid oxidation is related to their metal ion-chelating activity Scheme 8 and free-radical scavenging activity Scheme 9.

Three structural groups are important determinants of the radical-scavenging activity of anthocynidines 18— Second, the 2,3 double bond in conjugation. Third, the 4-oxofunction in the C-ring. Flavonoids form complexes with the metal ions by using the 3- or 5-hydroxyl and 4-ketosubstituents or hydroxyl groups in ortho position in the B-ring. As shown in the Scheme 9 , the anthocynidins cynidin 19 can donate an electron accompanied by a hydrogen nucleus to a free radical from —OH groups attached to the phenolic rings.

In this process, the polyphenolic reducing agent changes to an aroxyl radical, which is comparatively more stable due to resonance than the free radical that it has reduced. The overall result is the termination of damaging oxidative chain reactions. Carotenoids are among the most common lipid soluble phytonutrients. Carotenoids are well known to scavenge the peroxyl radicals more efficiently as compared to any other ROS. The peroxyl radicals generated in the process of lipid peroxidation can damage the lipids in the cell wall.

Scavenging of peroxyl radicals can disrupt the reaction sequence and prevent the damage to cellular lipids. The long unsaturated alkyl chains in carotenoids make them highly lipophilic. Carotenoids are known to play an important role in the protection of cellular membranes and lipoproteins against the ROS due to their peroxyl radical scavenging activity. Lycopene 24 , is the most potent antioxidant naturally present in many fruits and vegetables. The high number of conjugated double bonds in lycopen endows it the singlet oxygen quenching ability.

It is widely accepted that, the dietary antioxidants that protect LDL from oxidation can prevent the atherosclerosis and coronary heart disease. Hydroxycinnamic acids 30—33 and their conjugates prevent oxidative damage to the LDL.

The presence of the o -dihydroxy group in the phenolic ring as in caffeic acid enhances the antioxidant activity of hydroxycinnamic acids toward human LDL oxidation in vitro.

The o -dihydroxy substituents also allow the metal ion chelation similar to that of flavanoids. Theaflavin 34 and theaflavingallate 35 possesses in vitro antioxidative properties against lipid peroxidation in the erythrocyte membranes and microsomes. They also suppress the mutagenic effects induced by H 2 O 2. Apart from the aromatic hydroxyl groups of theaflavins, the gallic acid moiety is essential for their antioxidant activity. The theaflavingallate 35 is a stronger antioxidant than that of theaflavin Moreover, the digallate derivatives of theaflavin demonstrate the increased antioxidant activity.

Allicin diallyl thiosulfinate 36 is the biologically active compound mainly found in the garlic extracts. Allicin is known to possess various biological activities including the antibacterial, antifungal, and inhibition of cancer promotion. The S—S bond in the thiosulfinate is much weaker than the S—C bond in a sulfoxide.

Hence, this process can occur at room temperature. Allicin is known to undergo Cope elimination at room temperature to give 2-propenesulfenic acid and thioacrolein as shown in the Scheme The Scheme 12a demonstrate the mechanism of the radical-scavenging activity of the allicin.

The radical-scavenging activity of allicin involves H-atom transfer to a peroxyl radical from the methylene of the allyl group on the divalent sulfur. Scheme 12b demonstrate an alternative mechanism, where the radical-scavenging activity of allicin can be accounted for 2-propenesulfenic acid, which is produced from allicin by Cope elimination. Piperine 1-piperoylpiperidine 37 , is an alkaloid present in fruits of black pepper Piper nigrum , long pepper Piper longum , and other piper species family: Piperaceae.

Piperine possesses many pharmacological activities, including anti-inflammatory and analgesic effect, anti-ulcer activities, antidepressant effect, cognitive enhancing effect, cytoprotective effect, and antioxidant activity. Whereas, in low concentrations piperine acts as an antioxidant. The free radical can undergo electron transfer or abstract H-atom from either of these two sites.

Antioxidant Compounds and Their Antioxidant Mechanism

Fat-Soluble Vitamins pp Cite as. Vitamin E is the family name given to a group of tocopherols and tocotrienols that function as the principal lipid-soluble chain-breaking antioxidants in biological membranes and lipoproteins. Unable to display preview. Download preview PDF. Skip to main content. This service is more advanced with JavaScript available. Advertisement Hide.

An antioxidant is a substance that at low concentrations delays or prevents oxidation of a substrate. Antioxidant compounds act through several chemical mechanisms: hydrogen atom transfer HAT , single electron transfer SET , and the ability to chelate transition metals. The importance of antioxidant mechanisms is to understand the biological meaning of antioxidants, their possible uses, their production by organic synthesis or biotechnological methods, or for the standardization of the determination of antioxidant activity. In general, antioxidant molecules can react either by multiple mechanisms or by a predominant mechanism. The chemical structure of the antioxidant substance allows understanding of the antioxidant reaction mechanism. This chapter reviews the in vitro antioxidant reaction mechanisms of organic compounds polyphenols, carotenoids, and vitamins C against free radicals FR and prooxidant compounds under diverse conditions, as well as the most commonly used methods to evaluate the antioxidant activity of these compounds according to the mechanism involved in the reaction with free radicals and the methods of in vitro antioxidant evaluation that are used frequently depending on the reaction mechanism of the antioxidant.


Oxidation and Antioxidants in Organic Chemistry and Biology book cover. Enlarge Download. Oxidation After elucidating the chemistry and kinetics of antioxidant action, the book covers oxidative processes that occur in biological systems.


Current Organic Chemistry

The normal biochemical reactions in our body, increased exposure to the environment, and higher levels of dietary xenobiotic's result in the generation of reactive oxygen species ROS and reactive nitrogen species RNS. The reported chemical evidence suggests that dietary antioxidants help in disease prevention. Therefore, it is very important to understand the reaction mechanism of antioxidants with the free radicals. This review elaborates the mechanism of action of the natural antioxidant compounds and assays for the evaluation of their antioxidant activities.

In the current pandemic scenario, publication process of all articles related to Coronavirus will be expedited, for timely publication of much needed research. Search Advanced Search. Toggle navigation. Impact Factor: 1.

In chemistry , a radical more precisely, a free radical is an atom , molecule , or ion that has unpaired valence electrons or an open electron shell , and therefore may be seen as having one or more "dangling" covalent bonds. With some exceptions, these "dangling" bonds make free radicals highly chemically reactive towards other substances, or even towards themselves: their molecules will often spontaneously dimerize or polymerize if they come in contact with each other. Most radicals are reasonably stable only at very low concentrations in inert media or in a vacuum. Free radicals may be created in a number of ways, including synthesis with very dilute or rarefied reagents, reactions at very low temperatures, or breakup of larger molecules. The latter can be affected by any process that puts enough energy into the parent molecule, such as ionizing radiation , heat, electrical discharges, electrolysis , and chemical reactions.

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