Alcoxyl radicals

The increase in the yield of alcohol among the products of peroxyl radicals disproportionation with increasing temperature is the result of acceleration of hydrotrioxide decomposition to the alcoxyl radical and H02 . The proposed scheme is valid for the disproportionation of tertiary peroxyl radicals as well (see earlier). The rate constants of disproportionation of primary and secondary peroxyl radicals are presented in Table 2.17. 

Rate Constants of Decomposition of Alcoxyl Radicals R1 R2RCO R1 R2C(0) + R ... 

Enthalpies, Activation Energies and Rate Constants of Hydrogen Atom Intramolecular Abstraction in Alcoxyl Radicals (Experimental and Calculated)... 

Alcoxyl radical RO 10- Possessing efficiency of interaction with lipids being intermediate between ROO and OH... 

Low molecular weight antioxidants react with ROS in cell compartments which for some reasons are lack of antioxidant enzymes. Thus, suppression of bifurcate chain reactions of lipid peroxidation in hydrophobic core of cell membrane is mostly effectively performed by vitamin E (a-tocopherol). Interaction of lipid molecules with hydroxyl radical in the absence of vitamin E results in bifurcation of oxidative processes and formation of peroxyl and alcoxyl radicals. They are quickly accumulated in the restricted volume of the membrane and reaction began to be uncontrolled. a-Tocopherol interacts with peroxyl radicals with high affinity, reduces them and is then oxidized itself into relatively nonactive phenoxyl radical [8]. The latter can be accumulated within the bilayer until it will be returned to initial state by reduction by ascorbate [9]. Pair Vitamin E - Vitamin C is a good example of a mutual interaction between hydrophobic and hydrophilic low molecular weight antioxidants. Recently, tight relations were demonstrated for several natural antioxidants which interaction balances the red/ox state of the cell [3.5.10-12]. Figure 4 demonstrates such interaction between some of them. 

Oxidation is initiated by radicals present in living organisms (e.g., hydroperoxide H0 2, hydroxide OH, peroxide ROO, alcoxyl RO, alkyl L ) or by thermal or photochemical homolytic cleavage of an R-H bond. 

Termination of free radical oxidative reactions occurs when two radical species (peroxyl, alcoxyl, or alkyl) react with each other to form a non-radical adduct as in Equation (8.4). 

Hydroperoxides formed at the propagation stage of the free radical oxidation, as well as those produced by photooxidation and enzyme-catalyzed oxidation, can disintegrate and yield alcoxyl, alkyl, and peroxyl radicals, which reinitiate the oxidation of nnsatnrated FA. Hydroperoxide decomposition may be triggered by tem-peratnre and/or light, bnt most important in this respect is the activity of transition metals, mainly iron and copper [see Eqnations (8.5) and (8.6)]. 

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