Disulfide free radicals

In hen egg white lysozyme, out ofthe four disulfide bridges, one is much more easily reduced than the others (Fig. 3 A and B) [9]. A study by quantum chemistry could justify this selectivity by the stabilization ofthe resulting disulfide free radical by the positively charged end of arginine. The distances between sulphur atoms and the central carbon of arginine confirm the strong interaction between these groups (Fig. 3). 

As for reduction processes, C02 free radicals were shown to react specifically with disulfide bonds (122). They were extensively used to study the redox properties of disulfide bonds, thiyl and disulfide free radicals in proteins. This is discussed in paragraph 5. However, they do react with thiol functions also (37). For proteins containing a prosthetic group, the reduction concerns also oxidized valencies of metals and flavins. Flavin adenine dinucleotide (FAD) or Flavin Mononucleotide (FMN). The proportion of reduced disulfide/reduced prosthetic group varies considerably with the protein. For instance, lipoamide dehydrogenase contains one disulfide bond close to a flavin (FAD). Free radicals can reduce only the flavin, although both are in the active site (123). In chicken egg white riboflavin binding protein, competitive formation of both disulfide and semireduced flavin is observed (124). 

THIYL AND DISULFIDE FREE RADICALS IN PEPTIDES AND PROTEINS... 

The reactive species that iaitiate free-radical oxidatioa are preseat ia trace amouats. Exteasive studies (11) of the autoxidatioa mechanism have clearly estabUshed that the most reactive materials are thiols and disulfides, heterocycHc nitrogen compounds, diolefins, furans, and certain aromatic-olefin compounds. Because free-radical formation is accelerated by metal ions of copper, cobalt, and even iron (12), the presence of metals further compHcates the control of oxidation. It is difficult to avoid some metals, particularly iron, ia fuel systems. 

Treatment of 2-methylthiirane with t-butyl hydroperoxide at 150 °C in a sealed vessel gave very low yields of allyl disulfide, 2-propenethiol and thioacetone. The allyl derivatives may be derived from abstraction of a hydrogen atom from the methyl group followed by ring opening to the allylthio radical. Percarbonate derivatives of 2-hydroxymethylthiirane decompose via a free radical pathway to tar. Acrylate esters of 2-hydroxymethylthiirane undergo free radical polymerization through the double bond. 

The free-radical chemistry of fluoroalkanesulfenyl chlorides with hydrocarbons was also investigated [S, 9], Depending upon the structures of the sulfenyl chloride and the hydrocarbon, these reactions yield as major products up to three of the following four types of organic compounds thiols, disulfides, sulfides, and chlorohydrocarbons (equation 6), Perfluoroisobutanesulfenyl chloride is unique m that the only major products detected are the thiol and chlorohydrocarbon [ ] (equation 6) (Table 3). 

The stabilizing activity of polyamine disulfides is specified by the imine group and the disulfide bridge, which participate simultaneously in the breaking of the chain by suppressing the radicals and in the free radical destruction of peroxides and hydroxides. 

During the chain oxidation of hydrocarbons, sulfides and disulfides terminate chains by reacting with peroxyl radicals [40,42,44], which, as opposed to phenols, are weak inhibitors (see Table 17.6). The mechanism and stoichiometry of the termination reaction by sulfides remain yet unclear. Since sulfenic acid is an efficient scavenger of free radicals, the oxidation of tetralin in the presence of dialkylsulfoxide occurs only if the initiation rate v > vimin is proportional to the concentration of sulfoxide [5], so that the rate of oxidation is... 

Copper is part of several essential enzymes including tyrosinase (melanin production), dopamine beta-hydroxylase (catecholamine production), copper-zinc superoxide dismutase (free radical detoxification), and cytochrome oxidase and ceruloplasmin (iron conversion) (Aaseth and Norseth 1986). All terrestrial animals contain copper as a constituent of cytochrome c oxidase, monophenol oxidase, plasma monoamine oxidase, and copper protein complexes (Schroeder et al. 1966). Excess copper causes a variety of toxic effects, including altered permeability of cellular membranes. The primary target for free cupric ions in the cellular membranes are thiol groups that reduce cupric (Cu+2) to cuprous (Cu+1) upon simultaneous oxidation to disulfides in the membrane. Cuprous ions are reoxidized to Cu+2 in the presence of molecular oxygen molecular oxygen is thereby converted to the toxic superoxide radical O2, which induces lipoperoxidation (Aaseth and Norseth 1986). 

When free radicals or other reactive organic species are generated in the electron transfer step, the pattern of surface coverage may have a dramatic impact on the yields of various possible organic oxidation products. Consider the oxidation of mercaptans (RSH) by a metal oxide surface. One-equivalent oxidation forms mercapto radicals (RS1) which quickly couple with neighboring radicals to form a disulfide product (36). 

The SH group of B HnSH2- can be oxidized to disulfides linking two clusters, and further to the disulfide monoxide [57]. The disulfide has tumor-localizing properties superior to Na2Bi2HnSH [58], In acidic conditions, a free radical with intense blue color is obtained as an intermediate, which can be reduced by NaBI-U... 

N. S. Kosower and E. M. Kosower, The glutathione-glutathione disulfide system, Free Radicals Biol. 2, 55-84 (1976). 

Di-/erJ-butylthianthrene and 1,3,6,8-tetra-tert-butylthianthrene were obtained from the diaryl sulfides in moderate yields by reaction with c. H2SO4 in nitromethane (68CB2956). A modified diaryl disulfide approach was based on intramoleculeir free radical substitution an o-phenoxy- (95% yield) or o-phenylthio-substituent (40% yield) being displaced as a radical (75G841). 

Conjugated dienes such as 1,3-butadiene very readily polymerize free radically. The important thing to remember here is that there are double bonds still present in the polymer. This is especially important in the case of elastomers (synthetic rubbers) because some cross-linking with disulfide bridges (vulcanization) can occur in the finished polymer at the allylic sites still present to provide elastic properties to the overall polymers. Vulcanization will be discussed in detail in Chapter 18, Section 3. The mechanism shown in Fig. 14.3 demonstrates only the 1,4-addition of butadiene for simplicity. 1,2-Addition also occurs, and the double bonds may be cis or trans in their stereochemistry. Only with the metal complex... 

Cross-linking is a predominant process during irradiation of siloxane polymers. Chain scissions are negligible. ° ° The cross-link density increases linearly with a dose up to 160 Mrad (1,600 kGy). ° At 5.0 MGy (500 Mrad) the G(X) value is 0.5. Free radical scavengers, such as n-butyl and frrf-dode-cyl mercaptan and diethyl disulfide, are the most effective antirads. ° - ° At a concentration of 10%, two-thirds of the cross-links were prevented from forming however, the scission yield was also increased. 

Research Focus Method of preparing surface adhesives using phenolic resins either modified with disulfides or free radically copolymerized with ethane-dithol and then postreacted with epichlorohydrin. 

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