Free radicals oxygen species

Under physiological conditions, ascorbic acid is mainly used as a reductant in a variety of functions. It is essential in chloroplasts for the removal of photoproduced active oxygen species (Miyake and Asada, 1992). Ascorbate can also regenerate some membrane-bound radical quenchers such as a-tocopherol and zeaxanthin (Foyer et al, 1991). Ascorbate function is in addition extended to the removal of free radical oxygen species produced by air pollutants, certain herbicides, and other cytotoxic compounds derived from lipid peroxidation (Polle et al., 1990 Penel and Castillo, 1991 Luwe et al., 1993). 

Free radicals are species that contain unpaired electrons The octet rule notwithstand mg not all compounds have all of their electrons paired Oxygen (O2) is the most famil lar example of a compound with unpaired electrons it has two of them Compounds that have an odd number of electrons such as nitrogen dioxide (NO2) must have at least one unpaired electron... 

Among the oxygen-derived free radicals, the species of primary concern include superoxide anion (O -), hydrogen peroxide (H202), and hydroxyl radical (OH-). The superoxide is further converted in peroxynitrite (ONOO-) by reacting with nitric oxide. 

Termination can also occur by the reaction of polymer free radicals with primary initiator radicals (called primary termination) or free-radical scavenging species, especially oxygen. Activation energies for propagation and termination for some typical monomers are listed in Table 2. 

Chemists call this type of reaction a photochemical reaction because light causes the reaction to occur. Oxygen atoms contain an unpaired electron (sometimes shown by a dot in the symbol, or 0-), making them very, very reactive. Chemists call species that contain one or more unpaired electrons, free radicals. Although the concentration of free radical oxygen atoms is only on the order of parts of million, the concentration of molecules is... 

Tocotrienols differ from tocopherols by the presence of three isolated double bonds in the branched alkyl side chain. Oxidation of tocopherol leads to ring opening and the formation of tocoquinones that show an intense red color. This species is a significant contributor to color quaUty problems in oils that have been abused. Tocopherols function as natural antioxidants (qv). An important factor in their activity is their slow reaction rate with oxygen relative to combination with other free radicals (11). 

A few free radicals are indefinitely stable. Entries 1, 4, and 6 in Scheme 12.1 are examples. These molecules are just as stable under ordinary conditions of temperature and atmosphere as typical closed-shell molecules. Entry 2 is somewhat less stable to oxygen, although it can exist indefinitely in the absence of oxygen. The structures shown in entries 1, 2, and 4 all permit extensive delocalization of the unpaired electron into aromatic rings. These highly delocalized radicals show no tendency toward dimerization or disproportionation. Radicals that have long lifetimes and are resistant to dimerization or other routes for bimolecular self-annihilation are called stable free radicals. The term inert free radical has been suggested for species such as entry 4, which is unreactive under ordinary conditions and is thermally stable even at 300°C. ... 

One of the important consequences of neuronal stimulation is increased neuronal aerobic metabolism which produces reactive oxygen species (ROS). ROS can oxidize several biomoiecules (carbohydrates, DNA, lipids, and proteins). Thus, even oxygen, which is essential for aerobic life, may be potentially toxic to cells. Addition of one electron to molecular oxygen (O,) generates a free radical [O2)) the superoxide anion. This is converted through activation of an enzyme, superoxide dismurase, to hydrogen peroxide (H-iO,), which is, in turn, the source of the hydroxyl radical (OH). Usually catalase... 

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