Site-specific mechanism

Fukuzawa, K. and Fujii, T. 1992. Peroxide dependent and independent lipid peroxidation Site specific mechanisms of initiation by chelated iron and inhibition by a-tocopherol. Lipids 27 227-233. 

Chevion. M. A site-specific mechanism for free radical induced biological damage the essential role of redox-active transition metals. Free Radic. Biol Med 5 2" -37 1988. 

The kinetics of the reduction of hemoglobin by Fe(III) and Cu(II) complexes indicate the presence of both simple outer-sphere and site-specific electron transfer mechanisms. With the Fe(III) chelate oxidants the pathway is dependent on the reduction potential and the stability of the Hb complex, while for Cu(II) oxidations the outer-sphere process occurs at the a subunits, with the site-specific mechanism involving metal binding at the Cys p-93 residue. 

There are also indications that the adsorption of small molecules on coal, such as methanol, occurs by a site-specific mechanism (Ramesh et al., 1992). In such cases, it appears that the adsorption occurs first at high-energy sites but with increasing adsorption the (methanol) adsorbate continues to bind to the surface rather than to other (polar) methanol molecules and there is evidence for both physical and chemical adsorption. In addition, at coverages below a monolayer, there appears to be an activation barrier to the adsorption process. Whether or not such findings have consequences for surface area and pore distribution studies remains to be seen. But there is the very interesting phenomenon of the activation barrier which may also have consequences for the interpretation of surface effects during coal combustion (Chapters 14 and 15). As an aside, adsorption studies of small molecules on coal has been claimed to confirm the copolymeric structure of coal (Milewska-Duda, 1991). 

Hydroxyl radical is endowed with unique properties due to a combination of high electrophilicity, high thermochemical reactivity, and a mode of prodnction that can occur in the vicinity of DNA (site specific mechanism), it can both abstract H atoms from the sugar in the DNA helix and add to DNAbases, leading to single strand breaks and nucleobase (8- hydroxydesoxyguanosine) oxidation, respectively. 

The WOF is inhibited by additives which bind Fe ions, e. g., polyphosphates, phytin, and EDTA. In comparison, antioxidants are almost ineffective. Therefore, it is assumed that a site specific mechanism is involved in the formation of WOF. The Fe ions liberated in the cooking process are bound by the phospholipids via the negatively charged phosphate residues and, consequently, adjoin the unsaturated acyl residues of these lipids. Radicals from the Fenton reaction of Fe ions with hydroperoxides (cf. 3.7.2.1.8) attack only the unsaturated acyl residues, starting their peroxidation. This hypothesis can also explain the observation that multivalent ions (Ca , Al ) inhibit WOF as they probably displace the Fe ions from the phospholipids. 

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