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. Free radicals can adversely affect various important classes of biological molecules such as nucleic acids, lipids, and proteins, thereby altering the normal redox status leading to increased oxidative stress.

Beneficial renal effects of some medications, such as chelation therapy depend at least partially on the ability to alleviate oxidative stress. The administration of various natural or synthetic antioxidants has been shown to be of benefit in the prevention and attenuation of metal induced biochemical alterations.

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Antioxidant Chemistry of α-Tocopherol in Biological Systems

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.

Oxidative stress in biological systems is a complex process that is characterized by an imbalance between the production of free radicals FR and the ability of the body to eliminate these reactive species through the use of endogenous and exogenous antioxidants. A biological system in the presence of an excess of ROS can present different pathologies, from cardiovascular diseases to the promotion of cancer.

Biological systems have antioxidant mechanisms to control damage of enzymatic and nonenzymatic natures that allow ROS to be inactivated. The endogenous antioxidants are enzymes, such as superoxide dismutase SOD , catalase CAT , glutathione peroxidase, or non-enzymatic compounds, such as bilirubin and albumin. When an organism is exposed to a high concentration of ROS, the endogenous antioxidant system is compromised and, consequently, it fails to guarantee complete protection of the organism.

To compensate this deficit of antioxidants, the body can use exogenous antioxidants supplied through food, nutritional supplements, or pharmaceuticals. Among the most important exogenous antioxidants are phenolic compounds carotenoids and vitamins C and some minerals such as selenium and zinc. In the study of antioxidant compounds and the mechanisms involved, it is important to distinguish between the concepts of antioxidant activity and capacity.

These terms are often used interchangeably. However, antioxidant activity refers to the rate constant of a reaction between an antioxidant and an oxidant. The antioxidant capacity is a measure of the amount of a certain free radical captured by an antioxidant sample [ 1 ]. Therefore, during the selection of a method, the response parameter must be considered to evaluate the antioxidant properties of a sample, which may be a function of the concentration of the substrate or concentration and the time required to inhibit a defined concentration of the ROS.

The reaction mechanisms of the antioxidant compounds are closely related to the reactivity and chemical structure of FR as well as the environment in which these reactive species are found. Therefore, it is very important to describe the ROS and, to a lesser degree, the reactive nitrogen species RNS , which include both precursors and free radicals. In the literature, there are many in vitro methods to evaluate the effectiveness of antioxidant compounds present in a variety of matrices plant extracts, blood serum, etc.

The in vitro methods can be divided into two main groups: 1 hydrogen atom transfer HAT reactions and 2 transfer reactions of a single electron SET. These methods are widely used because of their high speed and sensitivity. This chapter describes the methods of in vitro antioxidant evaluation that are used frequently depending on the reaction mechanism of the antioxidant.

Oxygen is associated with aerobic life conditions [ 3 ], representing the driving force for the maintenance of cell metabolism and viability and at the same time involving a potential danger due to its paramagnetic characteristics. These characteristics promote the formation of partially oxidized intermediates with a high reactivity. These compounds are known as reactive oxygen species ROS. ROS are free radicals FR or radical precursors. In stable neutral molecules, the electrons are paired in their respective molecular orbitals, known as maximum natural stability.

Therefore, if there are unpaired electrons in an orbital, highly reactive, molecular species are generated that tend to trap an electron from any other molecule to compensate for its electron deficiency.

The oxygen triplet is the main free radical, since it has two unpaired electrons. The reaction rate of triplet oxygen in biological systems is slow. However, it can become highly toxic because it metabolically transforms into one or more highly reactive intermediates that can react with cellular components. This metabolic activation is favored in biological systems, because the reduction of O 2 to H 2 O in the electron transport chain occurs by the transfer of an electron to form FR or ROS [ 4 ].

Free radicals in a biological system can be produced by exogenous factors such as solar radiation, due to the presence of ultraviolet rays. Ultraviolet radiation causes the homolytic breakdown of bonds in molecules. FR also occur during the course of a disease. In a heart attack, for example, when the supply of oxygen and glucose to the heart muscle is suspended, many FR are produced.

Another exogenous factor is chemical intoxication, which promotes the formation of FR. The organism, because it requires the conversion of toxic substances to less dangerous substances, promotes the release of FR. The toxicity of many drugs is actually due to their conversion into free radicals or their effect on the formation of FR.

The presence of contaminants, additives, pesticides, etc. Inflammatory processes are due to endogenous factors that promote the presence of FR in the system. These FR, present inside the cleansing cells of the immune system, have the function of killing pathogenic microorganisms. Tissue damage is caused when FR are excessive during this process. Likewise, it is produced by phagocytic leukocytes as the initial product of the respiratory explosion when consuming O 2.

FR are necessarily present during metabolic processes because many of the chemical reactions involved require these chemical species.

For example, the reactions of polymerization of amino acids to form proteins or the reactions of polymerization of glucose to form glycogen involve the participation of FR. FR are also involved in the catalytic activation of various enzymes of intermediary metabolism, such as hypoxanthine, xanthine oxidase, aldehyde oxidase, monoamine oxidase, cyclooxygenase, and lipoxygenase [ 5 ].

Generally, antioxidant enzymes efficiently control these radicals. Another generating source of ROS is the structural alteration of essential macromolecules of the cell DNA, protein, and lipids by irreversible chemical reactions. These reactions generate derivatives, such as malonaldehyde and hydroperoxides that propagate oxidative damage. These species are generated in small amounts during normal cellular processes such as cell signaling, neurotransmission, muscle relaxation, peristalsis, platelet aggregation, blood pressure modulation, immune system control, phagocytosis, production of cellular energy, and regulation of cell growth [ 6 ].

Table 1 shows the most representative FR present during the process of energy production in aerobic biological systems. These transformations are summarized in Figure 1. Free radicals produce diverse actions on the metabolism of immediate principles, which can be the origin of cell damage [ 7 ]: In the polyunsaturated lipids of membranes, producing loss of fluidity and cell lysis because of lipid peroxidation Figure 2.

In the glycosides, altering cellular functions such as those associated with the activity of interleukins and the formation of prostaglandins, hormones, and neurotransmitters Figure 3 [ 8 ].

In proteins, producing inactivation and denaturation Figure 4 [ 9 ]. In nucleic acids, by modifying bases Figure 5 [ 8 ], producing mutagenesis and carcinogenesis. The human body responds to oxidative stress with antioxidant defense, but in certain cases, it may be insufficient, triggering different physiological and physiopathological processes.

Currently, many processes are identified related to the production of free radicals. Among them are mutagenesis, cell transformation, cancer, arteriosclerosis, myocardial infarction, diabetes, inflammatory diseases, central nervous system disorders, and cell aging [ 10 , 11 ].

Biological systems in oxygenated environments have developed defense mechanisms, both physiological and biochemical. Among them, at the physiological level, is a microvascular system with the function of maintaining the levels of O 2 in the tissues, and at a biochemical level, the antioxidant defense can be enzymatic or nonenzymatic, as well as being a system for repairing molecules.

SOD is the most important and most powerful detoxification enzyme in the cell. SOD is a metalloenzyme and, therefore, requires a metal as a cofactor for its activity. Depending on the type of metal ion required as a cofactor by SOD, there are several forms of the enzyme [ 12 , 13 ]. CAT uses iron or manganese as a cofactor and catalyzes the degradation or reduction of hydrogen peroxide H 2 O 2 to produce water and molecular oxygen, thus completing the detoxification process initiated by SOD [ 14 , 15 ].

CAT is highly efficient at breaking down millions of H 2 O 2 molecules in a second. CAT is mainly found in peroxisomes, and its main function is to eliminate the H 2 O 2 generated during the oxidation of fatty acids. GPx is an important intracellular enzyme that breaks down H 2 O 2 in water and lipid peroxides in their corresponding alcohols; this happens mainly in the mitochondria and sometimes in the cytosol [ 16 ].

The activity of GPx depends on selenium. Among glutathione peroxidases, GPx1 is the most abundant selenoperoxidase and is present in virtually all cells. The enzyme plays an important role in inhibiting the process of lipid peroxidation and, therefore, protects cells from oxidative stress [ 18 ]. Low GPx activity leads to oxidative damage of the functional proteins and the fatty acids of the cell membrane. GPx, particularly GPx1, has been implicated in the development and prevention of many diseases, such as cancer and cardiovascular diseases [ 19 ].

DT-diaphorase catalyzes the reduction of quinone to quinol and participates in the reduction of drugs of quinone structure [ 20 ]. DNA regulates the production of these enzymes in cells. This system of antioxidants consists of antioxidants that trap FR. They capture FR to avoid the radical initiation reaction. Neutralize the radicals or capture them by donating electrons, and during this process, the antioxidants become free radicals, but they are less reactive than the initial FR.

FR from antioxidants are easily neutralized by other antioxidants in this group. The flavonoids that are extracted from certain foods interact directly with the reactive species to produce stable complexes or complexes with less reactivity, while in other foods, the flavonoids perform the function of co-substrate in the catalytic action of some enzymes. Enzymes that repair or eliminate the biomolecules that have been damaged by ROS, such as lipids, proteins, and DNA, constitute the repair systems.

Common examples include systems of DNA repair enzymes polymerases, glycosylases, and nucleases and proteolytic enzymes proteinases, proteases, and peptidases found in both the cytosol and the mitochondria of mammalian cells.

These enzymes act as intermediaries in the repair process of the oxidative damage caused by the attack of excess ROS. Any environmental factor that inhibits or modifies a regular biological activity becomes a condition that favors the appearance or reinforcement of oxidative stress. The main characteristic of a compound or antioxidant system is the prevention or detection of a chain of oxidative propagation, by stabilizing the generated radical, thus helping to reduce oxidative damage in the human body [ 21 ].

Gordon [ 22 ] provided a classification of antioxidants, mentioning that characteristic. There are two main types of antioxidants, the primary breaking the chain reaction, free radical scavengers and the secondary or preventive.

The secondary antioxidant mechanisms may include the deactivation of metals, inhibition of lipid hydroperoxides by interrupting the production of undesirable volatiles, the regeneration of primary antioxidants, and the elimination of singlet oxygen. A compound that reduces in vitro radicals does not necessarily behave as an antioxidant in an in vivo system. This is because FR diffuse and spread easily.

Some have extremely short life spans, on the order of nanoseconds, so it is difficult for the antioxidant to be present at the time and place where oxidative damage is being generated. Additionally, the reactions between antioxidants and FR are second order reactions. Therefore, they not only depend on the concentration of antioxidants and free radicals but are also dependent on factors related to the chemical structure of both reagents, the medium and the reaction conditions.

The phenolic compounds constitute a wide group of chemical substances, with diverse chemical structures and different biological activities, encompassing more than different compounds which are a significant part of the human and animal diet [ 24 ].

The phenolic compounds are important components in the mechanism of signaling and defense of plants. These compounds combat the stress brought about by pathogenic organisms and predators.

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.


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.


Free Radicals

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 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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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.

Shashank Kumar, Abhay K. There has been increasing interest in the research on flavonoids from plant sources because of their versatile health benefits reported in various epidemiological studies. Since flavonoids are directly associated with human dietary ingredients and health, there is need to evaluate structure and function relationship. The bioavailability, metabolism, and biological activity of flavonoids depend upon the configuration, total number of hydroxyl groups, and substitution of functional groups about their nuclear structure. Fruits and vegetables are the main dietary sources of flavonoids for humans, along with tea and wine. Most recent researches have focused on the health aspects of flavonoids for humans. Many flavonoids are shown to have antioxidative activity, free radical scavenging capacity, coronary heart disease prevention, hepatoprotective, anti-inflammatory, and anticancer activities, while some flavonoids exhibit potential antiviral activities.

Что он делает здесь, в Испании, зачем спорит с этим психованным подростком. Беккер резким движением взял парня под мышки, приподнял и с силой посадил на столик. - Слушай, сопливый мозгляк. Убирайся отсюда немедленно, или я вырву эту булавку из твоих ноздрей и застегну ею твой поганый рот. Парень побелел. Беккер попридержал его еще минутку, потом отпустил.

The Scientific World Journal

Сирена выла не преставая. Сьюзан подбежала к. - Коммандер. Стратмор даже не пошевелился. - Коммандер.

Но Цифровая крепость никогда не устареет: благодаря функции меняющегося открытого текста она выдержит людскую атаку и не выдаст ключа. Новый стандарт шифрования. Отныне и навсегда. Шифры, которые невозможно взломать. Банкиры, брокеры, террористы, шпионы - один мир, один алгоритм.

 - Еще чуточку. Сьюзан удалось протиснуть в щель плечо. Теперь ей стало удобнее толкать. Створки давили на плечо с неимоверной силой. Не успел Стратмор ее остановить, как она скользнула в образовавшийся проем.

 Лиланд Фонтейн, - представился он, протягивая руку.  - Я рад, что вы живы-здоровы. Сьюзан не отрывала глаз от директора.

 Нет, коммандер! - вскрикнула Сьюзан.  - Нет. Хейл сжал ее горло.

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