Carbon-centered free radicals properties

Aminocyclopropane derivatives are known to possess enzyme inhibitory properties.In particular, aminocyclopropanes are potent inhibitors of cytochrome P-450 mono-oxygenases. A carbon-centered free radical, generated by one-electron oxidation of aminocyclopropane by P-450 to form a nitrogen-centered radical cation and subsequent ring opening, may play an important role in the destruction of the enzyme. 

The chemical properties of these free radicals were studied in small molecules such as dithiothreitol, lipoamide and glutathione etc... (For reviews see 7, 132). The thiyl radical is oxidizing (table 5). However, it is in tautomeric equilibrium with a carbon-centered free radical (49, 133) formed through base-catalyzed intramolecular rearrangement. This latter form is a reductant (49) and its fate is unknown. Thiyl radicals are also formed in proteins. They are somewhat uneasy to visualize because of their low extinction coefficient (table 3). In addition when they are formed by one-electron oxidation, tyrosinyl radicals are also created and thus thiyl radicals are hidden by their strong absorption (134, 135). However they may be observed when formed by reduction (37). 

V. Barone, M. Biczysko, P. Cimino, Interplay of stereo electronic vibrational and enviromnental effects in mning physico-chemical properties of carbon centered radicals, in Carbon-Centered Free Radicals and Radical Cations, M. D. E. Forbes, Ed., Willey, Hoboken, NJ, 2010, pp. 105-139. 

For the evaluation of AOPs in an aqueous phase, it is essential to know the absorption properties of the auxiliary oxidants and of the most important intermediate reactive species. These data are collected in Tab. 6-2. Carbon-centered peroxyl radicals play an important role in AOPs, since free carbon radicals RCH, which are formed for example via hydrogen abstraction by hydroxyl radicals react rapidly with dissolved molecular oxygen with formation of the corresponding peroxyl radicals RCH2O. Two examples of transient absorption characteristics are included in... 

Carotenoid radicals — Many of the important oxidations are free-radical reactions, so a consideration of the generation and properties of carotenoid radicals and of carbon-centered radicals derived from carotenoids by addition of other species is relevant. The carotenoid radicals are very short-lived species. Some information has been obtained about them by the application of radiation techniques, particularly pulse radiolysis. Carotenoid radicals can be generated in different ways. "... 

The mechanism of melatonin s interaction with reactive species probably involves donation of an electron to form the melatoninyl cation radical or through a radical addition at the site C3. Other possibilities include hydrogen donation from the nitrogen atom or substitution at position C2, C4, and C7 and nitrosation [169]. The mechanisms by which melatonin protects against LP most likely involve direct or indirect antioxidant and free-radical scavenging activities of this indoleamine [169,171]. 2-Phenyl indole derivatives have redox properties because of the presence of an electron-rich aromatic ring system that allows the indoleamine to easily function as an electron donor. For these derivatives, the possible antioxidant mechanism might be most probably toward carbon-centered radicals described by Antosiewicz et al. [172]. 

Hydrogen-atom donation from thiols to carbon-centered radicals was too often assumed to be the only property of thiols that is important in free radical processes in biology. Moreover, it was a common presumption that reaction (1) was the end of the biological pathway in which thiols repair radicals. Equilibria turned out to be much more important in sulfur radical chemistry than was first thought. For instance, the hydrogen-donation reaction was found to be a reversible equilibrium over 30 years after it was first observed. ... 

In polar solvents the first process is more efficient and leads to the decarboxylation, and this yields a radical centered on the ot-carbon with respect to the carboxylic acid group. In less polar and viscous solvents there is no separation of the radical ions and an addition-type secondary reaction is more likely for example, proton transfer to produce a free radical centered on a-carbon with respect to the carboxylate group takes place. In this case, no decarboxylation of the amino acid is observed. Based on this and the properties of A-phenylglycine [166-169], Scheme 20 describes the photochemical properties of dye NPG photoinitiator systems. 

Generalized to any type of carbon-centered radical, this approach has led to use of C—H bond dissociation energies for estimating the unpaired electron delocalization energies of these species. As shown in Tables XXXIII, XXXIV, and XXXV, bond dissociation energies which, except for a constant, are equal to heats of reaction do not provide satisfactory resonance (or stabilization) energies of free radicals. Indeed, as stated before, a heat of reaction can never be used for determining any property of one of the species involved in that reaction. 

OH free radicals react with almost all amino-acids. For aliphatic residues, rate constants are correlated with the strength of the X-H bond(X = S, C or N) (1). Thus the reaction is relatively slow with glycine (k = 1.7 x 10 mol 1 s ) and fast with the -SH function of cysteine (k = 1.9 x 10 mol M s i). The thiyl radical formed upon oxidation of cysteine, whose spectral properties are in table 3, is formed but a carbon-centered radical is also present (50, 51). In the presence of oxygen, thiyl radical fixes O2 giving peroxy radicals (52). These radicals may photoisomerize into sulfonyl radicals RS02 (53). In small molecules, disulfide groups can also be oxidized. This reaction was not demonstrated in proteins, but cannot be neglected. A disulfide radical cation is formed (54). Final compounds are not known. 

Photopolymerization systems, like thermally initiated systems, contain initiator, monomer, and other additives that impart desired properties (color, strength, flexibility, etc) (6). The reaction is initiated by active centers that are produced when light is absorbed by the photoinitiator. One important class of active centers includes free-radical species, which possess an impaired electron (5,7). The highly reactive free-radical active centers attack carbon-carbon double bonds in imsaturated monomers to form pol5nner chains. Although the kinetic treatment of photopoljnner systems is similar to that in thermal systems, significant differences arise in the description of the initiation step, which in turn affect the... 

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