Perthiyl Radicals Research Articles

UV photochemistry of the L-cystine disulfide bridge in aqueous solution investigated by femtosecond X-ray absorption spectroscopy

The photolysis of disulfide bonds is implicated in denaturation of proteins exposed to ultraviolet light. Despite this biological relevance in stabilizing the structure of many proteins, the mechanisms of disulfide photolysis are still contested after decades of research. Herein, we report new insight into the photochemistry of L-cystine in aqueous solution by femtosecond X-ray absorption spectroscopy at the sulfur K-edge. We observe homolytic bond cleavage upon ultraviolet irradiation and the formation of thiyl radicals as the single primary photoproduct. Ultrafast thiyl decay due to geminate recombination proceeds at a quantum yield of >80 % within 20 ps. These dynamics coincide with the emergence of a secondary product, attributed to the generation of perthiyl radicals. From these findings, we suggest a mechanism of perthiyl radical generation from a vibrationally excited parent molecule that asymmetrically fragments along a carbon-sulfur bond. Our results point toward a dynamic photostability of the disulfide bridge in condensed-phase.

Hydropersulfides inhibit lipid peroxidation and ferroptosis by scavenging radicals

Ferroptosis is a type of cell death caused by radical-driven lipid peroxidation, leading to membrane damage and rupture. Here we show that enzymatically produced sulfane sulfur (S0) species, specifically hydropersulfides, scavenge endogenously generated free radicals and, thereby, suppress lipid peroxidation and ferroptosis. By providing sulfur for S0 biosynthesis, cysteine can support ferroptosis resistance independently of the canonical GPX4 pathway. Our results further suggest that hydropersulfides terminate radical chain reactions through the formation and self-recombination of perthiyl radicals. The autocatalytic regeneration of hydropersulfides may explain why low micromolar concentrations of persulfides suffice to produce potent cytoprotective effects on a background of millimolar concentrations of glutathione. We propose that increased S0 biosynthesis is an adaptive cellular response to radical-driven lipid peroxidation, potentially representing a primordial radical protection system.

Radical Substitution Provides a Unique Route to Disulfides.

Radical substitution on tetrasulfides is demonstrated to be a highly effective means to prepare unsymmetric disulfides. Alkyl and aryl radicals generated thermally or photochemically underwent substitution on readily prepared dialkyl, diaryl, and diacyl tetrasulfides to yield the corresponding disulfides in good to excellent yields. Classic and contemporary thermal and photochemical radical sources could be employed; while photoredox catalysis approaches led to either oxidation or reduction of the tetrasulfide, energy transfer photocatalysis was particularly useful. The success of the approach is driven by the thermodynamic stability of the perthiyl radicals formed upon substitution on the tetrasulfide; they simply combine under the reaction conditions to provide the starting tetrasulfide. Competition kinetic experiments reveal that alkyl radical substitution on tetrasulfides is a rapid reaction (6 × 105 M-1 s-1) that is enhanced at least 6-fold upon moving from dialkyl tetrasulfide to diacyl tetrasulfide due to favorable polar effects. This unique and versatile reaction enables introduction of disulfide moieties from a variety of radical precursors and straightforward access to hydropersulfides.

Interaction of Quinone-Related Electron Acceptors with Hydropersulfide Na2S2: Evidence for One-Electron Reduction Reaction.

We previously reported that 9,10-phenanthraquinone (9,10-PQ), an atmospheric electron acceptor, undergoes redox cycling with dithiols as electron donors, resulting in the formation of semiquinone radicals and monothiyl radicals; however, monothiols have little reactivity. Because persulfide and polysulfide species are highly reducing, we speculate that 9,10-PQ might undergo one-electron reduction with these reactive sulfides. In the present study, we explored the redox cycling capability of a variety of quinone-related electron acceptors, including 9,10-PQ, during interactions with the hydropersulfide Na2S2 and its related polysulfides. No reaction occurred when 9,10-PQ was incubated with Na2S; however, when 5 μM 9,10-PQ was incubated with either 250 μM Na2S2 or Na2S4, we detected extensive consumption of dissolved oxygen (84 μM). Under these conditions, both the semiquinone radicals of 9,10-PQ and their thiyl radical species were also detected using ESR, suggesting that a redox cycle reaction occurred utilizing one-electron reduction processes. Notably, the perthiyl radicals remained stable even under aerobic conditions. Similar phenomenon has also been observed with other electron acceptors, such as pyrroloquinoline quinone, vitamin K3, and coenzyme Q10. Our experiments with N-methoxycarbonyl penicillamine persulfide (MCPSSH), a precursor for endogenous cysteine persulfide, suggested the possibility of a redox coupling reaction with 9,10-PQ inside cells. Our study indicates that hydropersulfide and its related polysulfides are efficient electron donors that interact with quinones. Redox coupling reactions between quinoid electron acceptors and such highly reactive thiols might occur in biological systems.

Hydropersulfides: H-Atom Transfer Agents Par Excellence.

Hydropersulfides (RSSH) are formed endogenously via the reaction of the gaseous biotransmitter hydrogen sulfide (H2S) and disulfides (RSSR) and/or sulfenic acids (RSOH). RSSH have been investigated for their ability to store H2S in vivo and as a line of defense against oxidative stress, from which it is clear that RSSH are much more reactive to two-electron oxidants than thiols. Herein we describe the results of our investigations into the H-atom transfer chemistry of RSSH, contrasting it with the well-known H-atom transfer chemistry of thiols. In fact, RSSH are excellent H-atom donors to alkyl (k ∼ 5 × 108 M-1 s-1), alkoxyl (k ∼ 1 × 109 M-1 s-1), peroxyl (k ∼ 2 × 106 M-1 s-1), and thiyl (k > 1 × 1010 M-1 s-1) radicals, besting thiols by as little as 1 order and as much as 4 orders of magnitude. The inherently high reactivity of RSSH to H-atom transfer is based largely on thermodynamic factors; the weak RSS-H bond dissociation enthalpy (∼70 kcal/mol) and the associated high stability of the perthiyl radical make the foregoing reactions exothermic by 15-34 kcal/mol. Of particular relevance in the context of oxidative stress is the reactivity of RSSH to peroxyl radicals, where favorable thermodynamics are bolstered by a secondary orbital interaction in the transition state of the formal H-atom transfer that drives the inherent reactivity of RSSH to match that of α-tocopherol (α-TOH), nature's premier radical-trapping antioxidant. Significantly, the reactivity of RSSH eclipses that of α-TOH in H-bond-accepting media because of their low H-bond acidity (α2H ∼ 0.1). This affords RSSH a unique versatility compared to other highly reactive radical-trapping antioxidants (e.g., phenols, diarylamines, hydroxylamines, sulfenic acids), which tend to have high H-bond acidities. Moreover, the perthiyl radicals that result are highly persistent under autoxidation conditions and undergo very rapid dimerization (k = 5 × 109 M-1 s-1) in lieu of reacting with O2 or autoxidizable substrates.

193 - Investigating the Redox Chemistry of Perthiyl Radicals: An Implication of Their Biological Functions

Hydrogen sulfide (H2S) is reported to be an important endogenously generated signaling molecule with a number of beneficial biological effects. Though H2S is proposed to have numerous physiological functions, the specific roles, mechanisms and biological targets of H2S are not well defined. More recently, it has been considered that much of the proposed functions attributed to H2S are in fact largely due to its oxidized congeners, hydropolysulfides (RSnR) and hydropersulfides (RSSH/RSS–), making H2S merely a marker for their reactivity. To relieve this controversy, it seems reasonable to study the chemistry of hydropersulfides independent from that of H2S. Until now, such a task has been difficult. To address this issue, others and we have explored the use of hydropersulfide donors, which undergo spontaneous rearrangement for in situ generation of hydropersulfides. This has allowed for a more direct examination of the chemistry associated with hydropersulfides and their related redox species in the absence of other potentially interfering sulfur adducts. Of specific interest is the ability of hydropersulfides, in comparison to thiols, to act as potent one-electron reductants. Single electron oxidation of a hydropersulfide generates a perthiyl radical (RSS●), which in comparison to thiyl radicals (RS●), are considered to be relatively unreactive due to their enhanced stability through resonance. For these reasons, we have examined the redox chemistry of hydropersulfides and perthiyl radicals with biological molecules (i.e. O2 and nitric oxide) and we report the potential role of the RSSH/RSS● redox system as it pertains to biological processes.

Photodissociation dynamics of the methyl perthiyl radical at 248 and 193 nm using fast-beam photofragment translational spectroscopy.

The photodissociation dynamics of the methyl perthiyl radical (CH3SS) have been investigated using fast-beam coincidence translational spectroscopy. Methyl perthiyl radicals were produced by photodetachment of the CH3SS(-) anion followed by photodissociation at 248 nm (5.0 eV) and 193 nm (6.4 eV). Photofragment mass distributions and translational energy distributions were measured at each dissociation wavelength. Experimental results show S atom loss as the dominant (96%) dissociation channel at 248 nm with a near parallel, anisotropic angular distribution and translational energy peaking near the maximal energy available to ground state CH3S and S fragments, indicating that the dissociation occurs along a repulsive excited state. At 193 nm, S atom loss remains the major fragmentation channel, although S2 loss becomes more competitive and constitutes 32% of the fragmentation. The translational energy distributions for both channels are very broad at this wavelength, suggesting the formation of the S2 and S atom products in several excited electronic states.

ChemInform Abstract: Perthiyl Radicals, Trisulfide Radical Ions, and Sulfate Formation. A Combined Photolysis and Radiolysis Study on Redox Processes with Organic Di- and Trisulfides.

The formation of perthiyl and thiyl radicals has been studied in a combined photochemical and radiation chemical investigation of the di- and trisulfides of penicillamine and cysteine, together with glutathione disulfide, cystamine dihydrochloride, and dithiodiglycolic acid in aqueous solution.

Reactions of Aliphatic Thiyl Radicals in the Solid State: Photoisomerization of trans-4,5-Dihydroxy-1,2-dithiacyclohexane and Oxidation of Dithiothreitol

A description of free-radical reactions in the solid state is important for some processes causing long-term stability problems of natural and synthetic products. Recent studies revealed that, in the solid state, mercaptooctadecanethiyl radicals, C(18)H(37)S., do not abstract a hydrogen atom from mercaptooctadecane, C(18)H(37)SH, but yield perthiyl radicals, C(18)H(37)SS., via a net sulfur transfer (Faucitano et al. ChemPhysChem 2005, 6, 1100-1107). Here, we demonstrate that such a sulfur transfer is not a general phenomenon of thiyl-radical reactions in the solid state, providing experimental evidence for a solid-state hydrogen-transfer reaction between a dithiyl radical, generated through the photolysis of trans-4,5-dihydroxy-1,2-dithiacyclohexane (DTT(ox)), and dithiothreitol. The photolysis of crystalline solid deposits of DTT(ox) yields two isomers of 2,3-dihydroxy-1-mercaptotetrahydrothiophene with a combined quantum yield of Phi(F) = 0.39 +/- 0.02. This quantum yield was increased to Phi(F) = 0.87 +/- 0.13 when the solid deposits contained an additional dithiol, dl-1,4-dimercapto-2,3-butanediol (DTT), at a ratio of DTT/DTT(ox) = 10:1. This increase in quantum yield depended, in part, on the presence of oxygen but was independent of residual moisture in the solid samples. Mechanistically, the formation of 2,3-dihydroxy-1-mercaptotetrahydrothiophene can be rationalized by the H transfer from DTT to a photochemically formed dithiyl radical from DTT(ox), yielding 2 equiv of monothiyl radicals from DTT, followed by a series of radical transformations.

Formation of perthiyl free radicals in irradiated glutathione

The irradiation of reduced (GSH) or oxidized (GSSG) glutathione in the solid state was performed using gamma-rays from Co-60 or high-energy electrons from an accelerator. In GSSG, perthiyl radicals (GSS') were created in addition to carbon-centred radicals as seen by electron paramagnetic resonance spectroscopy. The quantity was independent of the dose rate (electrons or gamma-rays) and of the presence of oxygen. In GSH, perthiyl radicals were also created, although in a smaller quantity. Among end-products in irradiated GSSG, we find GSH. Thus, sulphur functions are reduced by irradiation in the solid state.

The Influence of Solid‐State Molecular Organization on the Reaction Paths of Thiyl Radicals

Electron paramagnetic resonance (EPR) spectroscopy has been employed to investigate the effect of solid-state molecular organization on the reaction of thiyl radicals with thiols. In an irradiated C18H37SH/thiourea clathrate, the conversion of thiyl to perthiyl radicals is substantial, due to the head-to-head arrangement of the reactants within the channels and the suppression of other possible competing reactions due to hindrance by the clathrate walls. The perthiyl radical was identified using EPR analysis of its molecular dynamics within the clathrate channels. Irradiated polyethylene film containing 30% C18H37SH afforded a negligible conversion of thiyl to perthiyl radicals because of the random distribution of reactants. These results suggest that in the absence of favorable structure-control effects, the reaction between RS* and RSH is unimportant with respect to other competing reactions. Perthiyl radicals are also the major product in the vacuum solid-state radiolysis of lysozyme. A proposal of the mechanism involved in all cases is based on the equilibrium RS* + RSH <==> RSS*(H)R, followed by the irreversible conversion of the sulfuranyl radical to the perthiyl radical. As the equilibrium is strongly shifted to the left, the intermediate sulfuranyl radicals were not detected, but the lack of other competing reactions for the thiyl radicals caused the formation of perthiyl radicals to become the major path in the clathrate and in solid lysozyme radiolysis.

Radiolysis of proteins in the solid state: an approach by EPR and product analysis

Radio-induced modifications in proteins have been studied using several techniques. Electron paramagnetic resonance (EPR) was used to characterize free radicals, and analysis methods (high-performance liquid chromatography, capillary electrophoresis) were employed to visualize final degraded forms. Whereas EPR indicates that perthiyl radicals are formed, analysis does not detect any compound in which such bonds would be broken. Since EPR signals decay with time, it is concluded that rearrangements occur at subsequent steps, in which the solvent used during the analysis might play a role.

Protein thiyl radicals in disordered systems: A comparative EPR study at low temperature

A systematic study concerning the EPR spectroscopic properties of alkyl thiyl radicals in disordered systems under various conditions in proteins and in low molecular weight thiols is reported. Thiyl radicals were generated by UV photolysis at 77 K and studied at this temperature in frozen aqueous solution of the R1 protein of Escherichia coli ribonucleotide reductase (RNR), and of bovine serum albumin (BSA), in lyophilized BSA, and in a BSA film, and for comparison, in frozen aqueous solution of cysteine and in polycrystalline cysteine. A correlation of the g-tensor with the polarity of the environment (hydrogen bonding to the sulfur) of the thiyl radicals, as well as unusual anisotropic relaxation of the radicals have been observed. The stability of the thiyl radicals in various matrices was studied, including their reactions with oxygen and thiols in the matrix, which give rise to secondary radicals. The EPR line shape of alkyl thiyl radicals in disordered systems is characterized by an axial symmetric large g-anisotropy with an intense g⊥ component and a broad and weak g∥ component. A considerable g-strain, and in proteins, a number of different thiyl radical sites strongly broaden the g∥ component in spectra of frozen glasses. Nevertheless, g∥ is completely resolved in X-band spectra also in disordered systems. Protein thiyl radicals in BSA and R1 protein exhibit similar EPR spectroscopic properties in frozen glassy solution as thiyl radicals derived from low molecular weight thiols. The g∥ component of about 2.10–2.30 is a unique feature and thus characteristic for thiyl radicals in disordered systems. However, this fingerprint is particularly broad and weak in protein thiyl radicals and can most clearly be detected in absorption display. Protein-based thiyl radicals, as well as thiyl radicals in low molecular weight systems, exhibit a very anisotropic relaxation behavior. The microwave power for half-saturation of the g∥ component at 80 K is about 100 times larger than that of the g⊥ component indicating much faster relaxation for the g∥ component. This relaxation behavior was observed for the first time in disordered systems and represents a general property of thiyl radicals. Upon UV irradiation of proteins and cysteine in frozen solution at 77 K, thiyl radicals (major species) as well as perthiyl and C-centered radicals (accompanying minor species) have been identified as primary photolysis products. Thiyl radicals in the different systems are generally stable up to 120 K; at temperatures above 270 K, however, they are unstable. The stability of thiyl radicals is larger in crystalline samples than in frozen aqueous solution. Protein-associated thiyl radicals of R1 protein and BSA in frozen aqueous glasses are considerably more stable than thiyl radicals from cysteine in the same frozen solution, which is traced back to the effect of the protein matrix. Thiyl radicals from proteins and from cysteine in a frozen solid glass exhibit a temperature-dependent irreversible reaction with oxygen and with sulfhydryl groups of cysteine. Thereby, sulfinyl, sulfonyl, thiyl peroxyl, and perthiyl radicals have been observed as secondary reaction products.

5] Perthiols as antioxidants: Radical-scavenging andprooxidative mechanisms

Publisher Summary Thiols (RSH) are recognized for their radical-scavenging role in protection against cellular oxidative stress and in the repair of radical-induced DNA damage. The administration of exogenous thiol drugs offers defense against many free radical–associated diseases and protects normal tissues in cancer therapy using radiation or drugs. The antioxidant efficiency of perthiols reflects the free radical-scavenging ability of perthiols and prooxidative effects associated with perthiyl radical (RSS) formation. The methods used for investigating thiol free radical reactivity can be equally applied to assess the radical-scavenging and prooxidative properties of perthiols. Pulse radiolysis has proved to be a useful tool to quantify the free radical–scavenging ability of perthiols. Chemical models have been designed to probe the interaction of perthiols with free radical species commonly associated with cellular oxidative stress and to elucidate subsequent prooxidative reactions. The free radical–scavenging reactions of perthiols are qualitatively similar to, but quantitatively different from, those of the corresponding thiol antioxidants. Perthiols are not only more efficient hydrogen donors than thiols, but as perthiolate anions they are highly efficient electron donors. The antioxidant-derived radical, in this case the perthiyl radicals, are significantly less reactive than their thiyl radical counterparts and are less likely to pose a threat if generated within the cellular environment. In view of the essential role thiols play in controlling cellular oxidative stress, the encouraging performance of perthiols as free radical scavengers suggests they will be of use as exogenous antioxidants.

Sulfane-activated reduction of cytochrome c by glutathione.

The inorganic sulfane tetrathionate (-O3SSSSO3-) resembles glutathione trisulfide (GSSSG) in that it remarkably activates the reduction of cytochrome c by GSH, both under aerobic and anaerobic conditions. These observations can be explained by the formation of the persulfide GSS-, due to nucleophilic displacements of sulfane sulfur. The GSS- species has previously been proposed to act as a chain carrier in the catalytic reduction of cytochrome c, and perthiyl radicals GSS., formed in the reduction step, were thought to recycle to sulfane via dimerization to GSSSSG.2 The present study provides some arguments in favour of a chain mechanism involving the GSS. + GS-<-->(GSSSG).- equilibrium and sulfane regeneration by a second electron transfer from (GSSSG).- to cytochrome c. Thiosulfate sulfurtransferase (rhodanese) is shown to act as a cytochrome c reductase in the presence of thiosulfate and GSH, and again the generation of GSS- can be envisaged to explain this result.

Perthiyl radicals, trisulfide radical ions, and sulfate formation: a combined photolysis and radiolysis study on redox processes with organic di- and trisulfides

The formation of perthiyl and thiyl radicals has been studied in a combined photochemical and radiation chemical investigation of the di- and trisulfides of penicillamine and cysteine, together with glutathione disulfide, cystamine dihydrochloride, and dithiodiglycolic acid in aqueous solution.

An ESR Study of Radical Kinetics in L-α-Amino-n-Butyric Acid Hydrochloride Containing L-Cysteine Hydrochloride

On annealing at temperatures near 100 degrees C, carbon-centered radicals migrate to sulfur-centered radicals in X-irradiated crystals of L-alpha-amino-n-butyric acid hydrochloride, CH3CH2CH(NH3-Cl)COOH, containing L-cysteine hydrochloride, SHCH2CH(NH3Cl)COOH. Samples containing 0, 0.5, 1.0, and 1.5% L-cysteine hydrochloride were studied. When no cysteine is present, the carbon-centered radical formed by X irradiation, CH3CH2CHOOH, decays according to a second-order diffusion-controlled rate equation. In samples containing cysteine, the same carbon-centered radicals are formed, but on annealing, they migrate to cysteine, where a perithiyl radical, RSS, is formed. The transfer of carbon-centered radicals to perthiyl radicals follows a pseudo first-order rate equation with an activation energy of 1.15 eV. A decrease in the initial concentration of the carbon-centered radicals or an increase in the initial concentration of cysteine results in an increase in the transfer efficiency. The rate of growth of the perthiyl radical depends on both the initial concentration of cysteine and the initial concentration of carbon-centered radicals. The pseudo first-order rate constant increases when either the initial carbon-centered radical concentration increases or the initial cysteine concentration increases. The mechanism by which radicals move from one lattice site to another in the crystalline material is most likely hydrogen abstraction from a neighboring molecule.

An ESR investigation of the reactions of glutathione, cysteine and penicillamine thiyl radicals: competitive formation of RSO., R., RSSR-., and RSS(.).

The reactions of the cysteine, glutathione and penicillamine thiyl radicals with oxygen and their parent thiols in frozen aqueous solutions have been elucidated through electron spin resonance spectroscopy. The major sulfur radicals observed are: (1) thiyl radicals, RS.; (2) disulfide radical anions. RSSR-.; (3) perthiyl radicals, RSS. and upon introduction of oxygen; (4) sulfinyl radicals, RSO., where R represents the remainder of the cysteine, glutathione or penicillamine moiety. The radical product observed depends on the pH, concentration of thiol, and presence or absence of molecular oxygen. We find that the sulfinyl radical is a ubiquitous intermediate in the free radical chemistry of these important biological compounds, and also show that peroxyl radical attack on thiols may lead to sulfinyl radicals. We elaborate the observed reaction sequences that lead to sulfinyl radicals, and, using 17O isotopic substitution studies, demonstrate that the oxygen atom in sulfinyl radicals originates from dissolved molecular oxygen. In addition, the glutathione thiyl radical is found to abstract hydrogen from the alpha-carbon position on the cysteine residue of glutathione to form a carbon-centered radical.

ChemInform Abstract: Thermochemistry of Perthiyl Radicals.

AbstractMS is used to measure the appearance energies for the reaction (I)‐(II)+(III).

Thermochemistry of perthiyl radicals

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTThermochemistry of perthiyl radicalsJ. A. Hawari, D. Griller, and F. P. LossingCite this: J. Am. Chem. Soc. 1986, 108, 12, 3273–3275Publication Date (Print):June 1, 1986Publication History Published online1 May 2002Published inissue 1 June 1986https://doi.org/10.1021/ja00272a021RIGHTS & PERMISSIONSArticle Views228Altmetric-Citations27LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (415 KB) Get e-Alerts Get e-Alerts