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[en] We introduce a new strategy for reduction in volumetric shrinkage of free radical photopolymerization. Our strategy is based on the reversible reaction of disulfide bonds under UV irradiation. Here, we synthesized 2,2′-dithiodiethanol diacrylate (DSDA), an acrylate monomer with disulfide bonds. The homolytic photocleavage of DSDA under UV irradiation generates thiyl radicals that can initiate polymerization. Volumetric shrinkage can decrease to 0.1% through a repeated “contraction–expansion–contraction” volume-adjustable process. We identified the mechanism that underlies volumetric shrinkage reduction. The photocleavage rate of DSDA under UV irradiation is slower than that of the added photoinitiator. Moreover, in the presence of the photoinitiator, most of the generated thiyl radicals undergo restoration and exchange reactions instead of polymerization initiation or chain termination. The free volume and structure of the polymer network are effectively tuned by the dynamic and reversible processes of gradual disulfide-bond homolysis and recombination during fast photopolymerization.
[en] The mechanisms of quinone reduction by thiols containing α-hydrogen atoms were established using chemically induced dynamic nuclear polarization effects. It was found that substituents in the quinone nucleus change the nature of the primary radical pair. In the photolysis of 2,6-dimethyl-1,4-benzoquinone (1), the radical pair consists of semiquinone and thioalkyl radicals, whereas in the case of 2,6-diphenyl-1,4-benzoquinone (2), the radical pair is composed of semiquinone and thiyl radicals. Quinone 2 is readily photolyzed with any thiol to give dibenzofuran derivative as the final products.
[en] The thiyl radical derived from glutathione (GSH) is shown to decay rapidly in aqueous solution by intramolecular rearrangement reactions into the non-sulphur-centred radical 1. The reaction is induced by OH- with a rate constant of 5 x 109 dm3 mol-1 and is also observable at near-neutral conditions (at physiological pH values around 7.5 the rate of formation of 1 amounts to ∼ 1 x 103 s-1). The activation enthalpy and entropy at pH 8.4 and 20oC were found to be 26.7 kJ mol-1 and -77 J mol-1 K-1, respectively. Radical 1 was unequivocally identified by EPR as the α-amino radical at the glutamyl residue of GSH. It is relatively long-lived with typical biomolecular decay rate constants of the order of (2-20) x 106dm3 mol-1 s-1. At higher GSH concentrations the formation of 1 is retarded but not inhibited. All radicals, sulphur- as well as non-sulphur-centred ones are connected via equilibria, partly under the action of 'repair' processes of GSH. These repair processes, however, are slow (k << 1.4 x 105 dm3 mol-1 S -1). (Author)
[en] Mild oxidation of -SH-containing proteins (serum albumin, hemoglobin) by Ce(IV)-ions in the presence of the spin trap phenyl-tert-butylnitrone (PBN) resulted in the appearance of strongly immobilized nitroxide free radicals which evidences the formation of thiyl radicals on the thiol site of the proteins. In hydroxyl free radical generating system a fraction of strongly immobilized nitroxide radicals was also detected in these proteins, which implies that the oxidation of a fraction of the thiol groups was also involved in the free radical reaction. According to the differential scanning calorimetry (DSC) experiments the melting processes of the proteins were calorimetrically irreversible, therefore the two-state kinetic model was used to evaluate the experiments. The results support the view that site-specific interaction of SH-containing proteins with hydroxyl and thiyl free radicals is able to modify the internal dynamics of proteins and affect the conformation of large molecules
[en] A large number of compounds that have toxic effects can be metabolised to free radicals and secondary reactive oxygen species. These may be directly damaging or affect cell function by altering regulatory mechanisms through changing redox status. Protection is provided by an integrated system of antioxidant defenses. This includes reduced glutathione (GSH), one of the functions of which is as a free radical scavenger. For GSH to be an effective radical scavenging antioxidant, therefore, it must act in concert with superoxide dismutase to remove the superoxide so generated. Superoxide is produced in a variety of metabolic processes. It is also a secondary product of radicals reacting with oxygen either directly or through GSH. The biological reactivity of superoxide has been the subject of much debate ever since the discovery of superoxide dismutase in 1968. It has more recently become apparent that its rapid reaction with nitric oxide to give peroxynitrite, and its ability to reversibly oxidise and inactivate iron sulphur enzymes, contribute to the toxicity of superoxide. Another mechanism that could be important involves addition reactions of superoxide with other radicals to give organic peroxides. This reaction, to form a tyrosine peroxide, has come to authors attention through the study of the scavenging of tyrosyl radicals by GSH. It is also shown that a tyrosine peroxide is a major product of the oxidation of tyrosine by neutrophils
[en] In order to investigate the radical chemistry of 3-(2-mercaptoethyl)quinazoline-2,4(1H,3H)-dione (MECH) in homogeneous and liposomal solutions experiments were performed with pulse radiolysis, γ radiolysis and the chemical radical initiator 2,2'-azobis(2-amidinopropane)dihydrochloride (AAPH). It is shown that the thiol group represents the most sensitive group to radical attack. The thiyl radical originating from MECH is detected indirectly by product analysis and by pulse radiolysis. The thiyl radical can abstract bisallylic hydrogens from polyunsaturated fatty acids. This is shown by pulse radiolysis in homogeneous and liposomal solutions via the formation of the pentadienyl radical which has a strong and characteristic absorption band at 280 nm. (author)
[en] Thiyl radicals (RS radical) formed by the reaction of radiolytically generated OH radicals with thiols, e.g. 1,4-dithiothreitol (DTT), react with cis- and trans-2,5-dimethyltetrahydrofuran by abstracting an H atom in the α-position to the ether function (k approx.5 x 103dm3mol-1s-1). The so-formed planar ether radical is 'repaired' by the thiol (k = 6 x 108dm3mol-1s-1) thereby regenerating a cis-or trans-2,5-dimethyltetrahydrofuran molecule. In this reaction a thiyl radical is reproduced. Thus trans-2,5-Me2 THF from cis-2,5-Me2THF and vice versa are formed in a chain reaction: at a dose rate of 2.8 x 10-3Gy s-1 and a trans-2,5-Me2THF concentration of 1 x 10-2mol dm-3 using DTT as the thiol, G(cis-2,5-Me2THF) = 160 has been found. The chain reaction is very sensitive to impurities and also to disulphides such as those radiolytically formed. 2,5-Me2THF can be regarded as a model for the sugar moiety of DNA where the C(4')-radical is known to lead to DNA strand breakage. The possible role of cellular thiols in the repair of the C(4')DNA radical, and also the conceivable role of thiyl radicals inducing DNA strand breakage, are discussed. (author)
[en] Complete text of publication follows. Sulfur-centered radicals represent an important class of radicals since they exhibit very interesting redox chemistry, including biological redox processes, and different spectral and kinetic properties as compared to carbon-centered radicals. This is due to the fact that the lone electron pairs present in sulfur atom influenced on the overall electronic structure of radicals. In the past few years unprecedented progress has been made in the recognition and understanding the role of structures and reaction mechanisms of sulfur-centered radicals. Research on these transients flourished particularly in systems that are relevant in biology, biochemistry and medicine. Relevant examples will highlight only the very recent achievements emerging from radiation chemical studies. These include radical processes connected with the intramolecular addition of cysteine thiyl radicals (CysS·) to phenylalanine, reversible H-atom transfer between CysS· and amino acids within a peptide chain, addition reaction of thiyl radicals (RS·) involving double bonds in pyrimidines, RS·-catalyzed cis/trans isomerization of lipid double bonds in unsaturated fatty acids, ·OH and ·H-induced degradation of molecules containing sulfur atoms, stabilization of monomeric sulfur radical cations (>S+·) with peptide bonds, and stabilization of disulfide radical anions (>S.·.S<) in uracil-derived disulfides. Important outputs from these studies are new directions for improving our knowledge how sulfur-centered radicals interact with major cellular targets during oxidative stress, i.e. proteins, DNA and lipids.