Thiol antioxidant

Another example where antioxidant performance can be improved dramatically lies in the mechanochemlcally initiated addition of reactive antioxidants on rubbers (5.10) or unsaturated thermoplastics such as ABS (12). For example, using thiol antioxidants 2 and 3 as the reactive antioxidants, Kharasch-type addition of the thiol function to the polymer double bond takes place during melt processing to give bound antioxidant adduct (see Equation 1) the polymer becomes much more substantive under aggressive environments. 

Iijima, N. et al., Selective regulation of CD40 expression in murine dendritic cells by thiol antioxidants, Immunology, 110, 197, 2003. 

Sen CK. 1998. Redox signaling and the emerging therapeutic potential of thiol antioxidants. Biochem Pharmacol 55 1747-1758. 

Han D., Sen C. K., Roy S., Kobayashi M. S., Tritschler H. J., and Packer L. (1997). Protection against glutamate-induced cytotoxicity in C6 glial cells by thiol antioxidants. Am. J. Physiol 273 R1771-R1778. 

The same process can be used to graft vinyl antioxidants to polymer chains during processing of elastomer or to initiate the formation of polymer adducts with thiol antioxidants (21, 22, 24). 

Rubber-modified plastics also react readily with thiol antioxidants mechanochemically to give high yields of bound antioxidants (18). An interesting feature of this method of preparation in ABS is the increase in effectiveness that occurs with increase in concentration (see Figure 7). Moreover, the effectiveness of the mechanochemically formed adducts is higher than the corresponding latex adducts. 

Thiol antioxidants can also be reacted with polyethylene and polypropylene (40). Figure 10 illustrates the superiority of MADA-B in PP compared with conventional thermal antioxidants when subjected to continuous hot water leaching. It is also an effective processing stabilizer and photoantioxidant. Table 15 shows that the latter effect is resistant to solvent extraction. The loss of activity after extraction corresponds to the amount of unbound MADA present. 

Recently there has been a widespread interest in thiol antioxidants, glutathione, and related enzymes, hydrogen sulfide, sesame oil components, L-arginine (L-Arg) and other nitric oxide modulators [129]. While the former compounds may have direct antioxidant actions, L-Arg is both a direct precursor of nitric oxide as well as an antioxidant [110]. These compounds may assist in lowering high blood pressure. 

Modification of rubbers with thiol antioxidants by the Kharasch reaction... 

Droge, W., 2002. Aging-related changes in the thiol/disulfide redox state implications for the use of thiol antioxidants. Exp. Gerontol. 37, 1333-1345. 

Although aminyl radicals are stable towards oxygen, they can oxidi2e other aromatic amines, phenols and thiols (10), and regenerate the diarylamine. Thus, mixtures of phenols and diarylamines frequendy show better antioxidant activity than either one alone. This is called synergism. 

Ethyleneamines are used in certain petroleum refining operations as well. Eor example, an EDA solution of sodium 2-aminoethoxide is used to extract thiols from straight-mn petroleum distillates (314) a combination of substituted phenol and AEP are used as an antioxidant to control fouling during processing of a hydrocarbon (315) AEP is used to separate alkenes from thermally cracked petroleum products (316) and TEPA is used to separate carbon disulfide from a pyrolysis fraction from ethylene production (317). EDA and DETA are used in the preparation and reprocessing of certain... 

Antioxidant free radicals (A H) produced in this way are relatively stable, but they may react with another lipid free radical to form a quinone [15.3] and [15.4]. Quinones can react with amine or thiol groups of proteins, forming polymerisable yellow or red coloured compounds (Pokorny, 1987). Antioxidant free radicals may also react with either another antioxidant free radical or a lipid free radical, forming dimers [15.5] or copolymers [15.6] and [15.7], respectively. Oligomeric or condensated products of antioxidant free radicals usually possess moderate antioxidant activities too (Pokorny et al, 1974) ... 

Table 4.1 Effect of selected thiols, disulphides, amino acids and antioxidants on the time to the onset and the time to reach maximal ischaemic contracture in isolated perfused rat hearts. Hearts were perfused for a control period of 10 min at the end of which global low-flow (10% of control) ischaemia was initiated. The interventions described above were included in the perfusion fluid 5 min prior to the onset and throughout the ischaemic period. The data are shown as means standard errors of the means (n = 6)...

Reduced scavenger capacity is deduced from studies demonstrating low plasma and cellular levels of antioxidants such as glutathione, vitamin E, thiols, magnesium and ascorbic acid, as well as reduced levels of scavenger enzymes such as neutrophil glutathione peroxidase and red cell superoxide dismutase (Lyons, 1991 Sinclair /., 1992). 

Phenols are important antioxidants, with vitamin E being the most important endogenous phenolic membrane-bound antioxidant. Membrane levels of vitamin E are maintained through recycling of the vitamin E radical with ascorbate and thiol reductants. Vitamin E is a mixture of four lipid-soluble tocopherols, a-tocopherol being the most efiective radical quencher. The reaction of a-tocopherol with alkyl and alkylperoxyl radicals of methyl linoleate was recently reported. These are facile reactions that result in mixed dimer adducts (Yamauchi etal., 1993). 

Trott, A. West, J. D. Klaic, L. Westerheide, S. D. Silverman, R. B. Morimoto, R. I. Morano, K. A. Activation of heat shock and antioxidant responses by the natural product celastrol transcriptional signatures of a thiol-targeted molecule. Mol. Biol. Cell2008,19, 1104-1112. 

Table 2.3 as a completely worked out example using quantitative solvent extraction, ash content determination, TGA, FTIR, XRF, GC-MS, HS-GC-MS, PyFTIR, ICP, and s-NMR. Information on the cure and antidegradant systems was obtained (assigned species/possible origin), as follows cyclohexane thiol/CBS accelerator benzothiazole/MBT, MBTS or CBS accelerators N, A-dimethylformamide/TMTD accelerator phthalim-ide/Santoguard PVI and IV-phenylbenzene amine/possi-bly a diphenyl/acetone amine antioxidant. 

Figure 2. Effect of hot (100 0 water extraction on impact strength of PP in the absence and presence of a combination of conventional antioxidants (Nickel dibutyl dithiocarbamate and Irganox 1076, NiDBC+irg.1076) and a thiol bound antioxidant (3), Bound AO. (Reproduced with permission from Ref. 13. Copyright 1983 App. Sci. Pub.)...

Garlic s proven mechanisms of action include (a) inhibition of platelet function, (b) increased levels of two antioxidant enzymes, catalase and glutathione peroxidase, and (c) inhibition of thiol enzymes such as coenzyme A and HMG coenzyme A reductase. Garlic s anti-hyperlipidemic effects are believed to be in part due to the HMG coenzyme A reductase inhibition since prescription medications for hyperlipidemia have that mechanism of action (statins). It is unknown whether garlic would have the same drug interactions, side effects, and need for precautions as the statins. 

Everett, S.A., Dennis, M.F., Patel, K.B., Maddix, S., Kundu, S.C., and Willson, R.L. 1996. Scavenging of nitrogen dioxide, thiol, and sulphonyl free radicals by the nutritional antioxidant (3-carotene. J. Biol. Chem. 271 3988-3994. 

Mascio, P.D., Murphy, M.E., Sies, H., 1991. Antioxidant defense systems The role of carotenoids, tocopherols, and thiols. Am J Clin Nutr. 53, 194S-200S. 

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