Sulfinyl Radical Research Articles

Computational Investigation of Vicinal Disulfoxides and Other Sulfinyl Radical Dimers

The structure and energetics of the simplest vic-disulfoxide CH3S(O)S(O)CH3 and its key isomers are reported. Computational methods include a CASSCF[2,2] treatment with a large basis set on sulfur and oxygen, followed by multireference MP2 refinement, and also the hybrid density functional B3LYP. Calculations at the G3 level were also carried out. In contrast to previous reports, a weak bond does exist for both vic-disulfoxide diastereomers. A bond enthalpy near 20 kcal mol-1 is reported. The isomeric sulfenylsulfinate is slightly lower in energy than the vic-disulfoxide. The B3LYP structure obtained for 5,5-di-t-butyltetrathiolane-2,3-dioxide reproduces the essential structural parameters of the one experimentally known vic-disulfinyl compound, 5-(1-adamantyl)-5-tert-butyltetrathiolane-2,3-dioxide.

Radical β‐Elimination of a Sulfinyl Group to Afford Allenes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Radical β-Elimination of a Sulfinyl Group to Afford Allenes

An unprecedented radical β-elimination of vinyl sulfoxides has opened a new route to functionalized allenes. The radical precursors were obtained in two steps from a carbonyl derivative and a vinyl sulfoxide. The radical translocation stratagem could also be used to trigger the β-elimination of the sulfinyl radical. Variation of the nature of the sulfur group on a common precursor gave no allene with the thio, sulfonyl, and iminosulfinyl moieties, in contrast to the analogous vinyl sulfoxide and vinylsulfimide precursors. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

EPR studies on a stable sulfinyl radical observed in the iron-oxygen-reconstituted Y177F/I263C protein R2 double mutant of ribonucleotide reductase from mouse.

Ribonucleotide reductase (RNR) catalyzes the biosynthesis of deoxyribonucleotides. The active enzyme contains a diiron center and a tyrosyl free radical required for enzyme activity. The radical is located at Y177 in the R2 protein of mouse RNR. The radical is formed concomitantly with the mu-oxo-bridged diferric center in a reconstitution reaction between ferrous iron and molecular oxygen in the protein. EPR at 9.6 and 285 GHz was used to investigate the reconstitution reaction in the double-mutant Y177F/I263C of mouse protein R2. The aim was to produce a protein-linked radical derived from the Cys residue in the mutant protein to investigate its formation and characteristics. The mutation Y177F hinders normal radical formation at Y177, and the I263C mutation places a Cys residue at the same distance from the iron center as Y177 in the native protein. In the reconstitution reaction, we observed small amounts of a transient radical with a probable assignment to a peroxy radical, followed by a stable sulfinyl radical, most likely located on C263. The unusual radical stability may be explained by the hydrophobic surroundings of C263, which resemble the hydrophobic pocket surrounding Y177 in native protein R2. The observation of a sulfinyl radical in RNR strengthens the relationship between RNR and another free radical enzyme, pyruvate formate-lyase, where a similar relatively stable sulfinyl radical has been observed in a mutant. Sulfinyl radicals may possibly be considered as stabilized forms of very short-lived thiyl radicals, proposed to be important intermediates in the radical chemistry of RNR.

Electron paramagnetic resonance study of nitroxides generated from nitric oxide by reaction with transient radicals

AbstractPhotochemical or thermal decomposition of azo‐compounds (such as 2,2′‐azobisisobutyronitrile, 2,2′‐azobis(2‐methylpropionamidine) dihydrochloride, dialkyl peroxides (such as tert‐butyl peroxide and diacyl peroxides (such as benzoyl peroxide) in anaerobic nitric oxide (NO)‐saturated dimethylsulfoxide (DMSO) or aqueous solutions yielded nitroxides. Well‐characterized electron paramagnetic resonance spectra of nitroxides revealed that NO was favorable for reacting with carbon‐centered and less stereo‐inhibited transient alkyl radicals, giving kinds of nitrosoalkane, typically nitrosomethane, which act sequentially as C‐nitroso compounds to trap transient radicals present in solution, yielding spin‐trapping adducts, i.e. nitroxides. Radicals, including sulfinyl radicals from UV‐irradiated DMSO, were trapped by the in situ formed CH3NO. O‐centered radicals could not add to the freshly formed C‐nitroso compounds. Possible mechanisms are suggested. Copyright © 2002 John Wiley & Sons, Ltd.

Generation and Electron Paramagnetic Resonance Spin Trapping Detection of Thiyl Radicals in Model Proteins and in the R1 Subunit of Escherichia coli Ribonucleotide Reductase

In the Escherichia coli class Ia ribonucleotide reductase (RNR), the best characterized RNR, there is no spectroscopic evidence for the existence of the postulated catalytically essential thiyl radical (R–S•) in the substrate binding subunit R1. We report first results on artificially generated thiyl radicals in R1 using two different methods: chemical oxidation by CeIV/nitrilotriacetate (NTA) and laser photolysis of nitric oxide from nitrosylated cysteines. In both cases, EPR spin trapping at room temperature using phenyl-N-t-butylnitrone, and controls with chemically blocked cysteines, has shown that the observed spin adduct originates from thiyl radicals. The EPR line shape of the protein-bound spin adduct is typical for slow motion of the nitroxide moiety, which indicates that the majority of trapped thiyl radicals are localized in a folded region of R1. In aerobic R1 samples without spin trap that were frozen after treatment with CeIV/NTA or laser photolysis, we observed sulfinyl radicals (R–S•=O) assigned via their g-tensor components 2.0213, 2.0094, and 2.0018 and the hyperfine tensor components 1.0, 1.1, and 0.9 mT of one β-proton. Sulfinyl radicals are the reaction products of thiyl radicals and oxygen and give additional evidence for generation of thiyl radicals in R1 by the procedures used.

Oxidation Processes ofN,S-Diacetyl-l-cysteine Ethyl Ester: Influence of S-Acetylation

The mechanism of the •OH-induced oxidation of N,S-diacetyl-l-cysteine ethyl ester (SNACET) was investigated in aqueous solutions using pulse radiolysis and steady-state γ-radiolysis combined with chromatographic and ESR techniques. The reaction of hydroxyl radicals with SNACET at slightly acidic to neutral pH results in the degradation into acetaldehyde. The underlying mechanism involves a very fast fragmentation of an initially formed hydroxysulfuranyl radical (kfragm ≥ 7.9 × 107 s-1) into acyl radicals (H3C−CO•) and the respective sulfenic acid (RSOH). Subsequently, these intermediates react via a hydrogen abstraction reaction that yields acetaldehyde and the respective sulfinyl radical (RSO•). In contrast, in very acidic solutions (pH < 2), the •OH-induced oxidation results in the formation of the monomeric sulfur radical cation (SNACET>S•+) which absorbs at λmax = 420 nm. This intermediate is formed with the absolute bimolecular rate constant k = 3.9 × 109 M-1 s-1. It decays in a SNACET concentration independent process (kd = 2.5 × 105 s-1) and in a SNACET concentration dependent process (kS-S = 2.2 × 107 M-1 s-1). The first process involves mainly fragmentation of the carbon−sulfur bond and yields acetylthiyl radical (CH2C(OH)−S•). The latter intermediate was identified via its reaction with oxygen as the thiylperoxyl radical (RSOO•), characterized by a transient absorption band with λmax = 540 nm. The second process represents the association of the monomeric sulfur radical cation (SNACET>S•+) with a second nonoxidized SNACET molecule to form intermolecularly three-electron-bonded (>S∴S<)+ dimeric radical cation. The low rate constant (kS-S) is in line with the high stability of the monomeric sulfur radical cation (SNACET>S•+) because of spin delocalization in the carbonyl group. The monomeric radical cation (SNACET>S•+) is alternatively produced in slightly acidic solutions using the sulfate radical anion, SO4•-, as an one-electron oxidant. This paper provides further evidence that the nature of the neighboring group affects the ultimate course of the sulfide oxidation.

Inactivation of pyruvate formate-lyase by dioxygen: defining the mechanistic interplay of glycine 734 and cysteine 419 by rapid freeze-quench EPR.

Pyruvate formate-lyase from Escherichia coli (EC 2.3.1.54; PFL) catalyzes the reversible anaerobic conversion of pyruvate and CoA into acetyl-CoA and formate. Active PFL contains a novel alpha-carbon centered glycyl radical at G734 that is required for its catalytic activity. Two adjacent cysteine residues, C418 and C419, are essential for PFL activity according to site-directed mutagenesis studies. Upon exposure to air, active PFL loses its activity with the concomitant loss of the glycyl radical. Previous EPR studies of dioxygen inactivation of PFL revealed protein-based peroxyl and sulfinyl radicals during the manual mixing and quenching process [Reddy et al. (1998) Biochemistry 37, 558-563]. To probe the mechanism of this process, we carried out experiments using rapid freeze-quench EPR spectroscopy. Upon mixing of active wild type or C418A PFL with oxygenated solution, a short-lived radical intermediate appears at the earliest time point (10 ms), followed by the appearance of a long-lived sulfinyl radical. The axial EPR spectrum of this short-lived radical (g = 2.034, 2.007) is characteristic of a peroxyl radical. When C419A PFL or the double mutant [C418A/C419A] PFL was mixed with oxygenated solution, the peroxyl radical was also observed at 10 ms but in this case persisted over 12 s. These observations provide compelling evidence to support a proposed mechanism in which dioxygen quenches the glycyl radical in the active enzyme and the resulting peroxyl radical may react further with the sulfhydryl group of the C419 residue to form the sulfinyl radical.

Oxidative Degradation of Pyruvate Formate-Lyase

The reaction mechanisms of oxidative degradation of the anaerobic radical enzyme pyruvate formate-lyase (PFL) have been examined at the theoretical hybrid Hartree−Fock/density functional theory level B3-LYP. It is concluded that the most likely scenario involves O2 addition at the glycyl radical site, followed by H• abstraction/transfer from C419 and subsequent rearrangements to generate an α-hydroxyglycine and the sulfinyl radical R−SO•, observed by EPR spectroscopy. In addition, the present model accounts for alternative fragmentations observed in wild-type PFL, and the formation and degradation of Gly−O-O• in the absence of C419 (as observed in C419A and C419AC418A mutants).

A new radical synthesis of allenes

The unprecedented radical β-elimination of vinylsulfoxides opens a new access to functionalized di- and trisubstituted allenes. The radical precursors are obtained in two steps from a carbonyl derivative and a vinylsulfoxide. The radical translocation trick can also be used to trigger the β-elimination of the sulfinyl radical.

Oxysulfur Compounds Derived from Dimethyl Disulfide: An ab Initio Study

The optimized geometries of oxysulfur compounds derived from dimethyl disulfide were calculated at the MP2/6-31G* level, and the relative energies of isomers in the CH3S2OCH3, CH3S2O2CH3, CH3S2O3CH3, and CH3S2O4CH3 series were compared. All these derivatives are characterized by long S−S bonds or long sulfenyl or sulfinyl S−O bonds for sulfur−oxygen-bridged isomers. According to the geometrical parameters calculated for vic-disulfoxide 4RR in the CH3S2O2CH3 series, this compound is better described as two interacting sulfinyl radicals, and the obtained results substantiate the experimental scheme proposed by Clennan28 for the intramolecular singlet oxygen addition on disulfides. The UV spectra of all these isomers were evaluated at a standard ab initio level: despite underestimated wavelengths, their evolution (wavelengths and molar extinction coefficients) is consistent with experiment and indicates a strong influence of the σS-S,nS → σ*S-S transition for sulfur−sulfur-bridged isomers and of nS,nO → σ*S...

Dioxygen inactivation of pyruvate formate-lyase: EPR evidence for the formation of protein-based sulfinyl and peroxyl radicals.

We here report EPR studies that provide evidence for radical intermediates generated from the glycyl radical of activated pyruvate formate-lyase (PFL) during the process of oxygen-dependent enzyme inactivation, radical quenching, and protein fragmentation. Upon exposure of active PFL to air, a long-lived radical intermediate was generated, which exhibits an EPR spectrum assigned to a sulfinyl radical (RSO*). The EPR spectrum of a sulfinyl radical was also generated from the activated C418A mutant of PFL, indicating that Cys 418 is not the site of sulfinyl radical formation. Exposure of the activated C419A mutant or C418AC419A double mutant to air on the other hand, resulted in a new EPR spectrum that we assign to the alpha-carbon peroxyl radical (ROO*) of the active-site glycine, G734. These findings suggest that C419 is the site of sulfinyl radical formation and that replacement of this cysteine with alanine results in the accumulation of the carbon peroxyl radical. The results also support the proposal that the peroxyl radical and the sulfinyl radical are intermediates in the oxygen-dependent inactivation and cleavage of the protein. Moreover, these observations are consistent with the hypothesis that C419 and G734 are in close proximity in the activated enzyme and may participate in a glycyl/thiyl radical equilibrium. A mechanism that accounts for the formation of the radical intermediates is proposed.

Photochemistry of benzyl β-naphthyl sulfoxide and characterization of the β-naphthylsulfinyl radical

Photolysis of benzyl β-naphthyl sulfoxide results mainly in α-cleavage. The isomeric sulfenic ester is the major product. Because the triplet energies of both the sulfoxide and sulfenic ester are below that of acetone, in contrast to the previously studied phenyl and tolyl cases, interpretation of sensitization experiments is straightforward. The sulfinyl radical is characterized by transient absorption and a model is proposed to account for its reactivity with nitroxide radicals.

Generation and Decay of Aryl Sulfinyl and Sulfenyl Radicals: A Transient Absorption and Computational Study

Absorption spectra and extinction coefficients of phenylsulfinyl and phenylsulfenyl (thiyl) radicals are determined by nanosecond laser photolysis in various solvents. Direct observation and characterization of arylsulfinyl radicals from the photolysis of several aromatic sulfoxides provides the strongest evidence to date for α-cleavage as the predominant primary photochemical process for these compounds. The absorption spectrum of phenylsulfinyl, with λmax = 300 and 450 nm and ε = 1.1 × 104 and 1.3 × 103 M-1 cm-1, is practically independent of solvent. Quantum yields of free sulfinyl radicals range from 0.09 to 0.18 in various solvents. Recombination rate constants very near diffusion control indicate that there is a large spin−orbital coupling in the radical pair. Rate constants for the reactions of arylsulfinyl radicals with stable nitroxide radicals are among the fastest known, but reactivity with O2 is very modest. Computations indicate that the singly occupied molecular orbital is a π* orbital largely localized on the sulfur and oxygen atoms.

Keto-sulfoxide derivatives as photoinitiators of polymerization

The photolysis of three aryl—alkyl sulfoxide photoinitiators is studied by time-resolved laser spectroscopy. The general diagram of the excited-state process is rather unusual: it shows a dual path of photolysis. Thus, whereas the singlet state cleaves at the S-alkyl bond, the triplet generates aryl and alkyl sulfinyl radicals. The conclusions arrived at are corroborated by product analysis by the coupled gas chromatography—mass spectroscopy technique. The photoinitiated polymerization activity has been checked and compared with that of other well known photoinitiators.

ChemInform Abstract: Altering the Stereochemistry of Allylation Reactions of Cyclic α‐ Sulfinyl Radicals with Diarylureas.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Altering the Stereochemistry of Allylation Reactions of Cyclic α‐ Sulfinyl Radicals: Effects of Solvents and Lewis Acids.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The Reaction of Superoxide with Reduced Glutathione

Superoxide, generated by a xanthine oxidase/hypoxanthine system, reacts with reduced glutathione (GSH) to cause an increase in oxygen consumption and oxidized glutathione (GSSG) formation, both of which are fully inhibited by superoxide dismutase. In this study we have shown that little, if any, of the additional oxygen consumed is converted to hydrogen peroxide. We have confirmed that approximately 90% of the GSH is oxidized to GSSG, the remainder being converted to the sulfonic acid. Approximately 1.2 mol of GSSG was formed for each additional mole of oxygen consumed in the presence of GSH. The efficiency of the reaction increased with increasing GSH concentration (1-8 mM), pH, and pO2 and with decreasing superoxide generation rate. The results are consistent with a superoxide-dependent chain that does not produce hydrogen peroxide and that is terminated primarily by superoxide dismutation. We propose that this occurs via an initial reaction of superoxide with GSH to produce a sulfinyl radical rather than hydrogen transfer to give the thiyl radical. Our data suggest a rate constant for the superoxide/GSH reaction in the 102-103 M−1 s−1 range. GSH at the millimolar concentrations found intracellularly should react with superoxide, but because superoxide is regenerated, it will not be an effective scavenger. Physiologically, superoxide dismutase is required to prevent chain oxidation of GSH.

The Formation and Structure of the Sulfoxyl Radicals RSO·, RSOO·, RSO2·, and RSO2OO· from the Reaction of Cysteine, Glutathione and Penicillamine Thiyl Radicals with Molecular Oxygen

This work reports an electron spin resonance study of the reactions of cysteine, glutathione and penicillamine thiyl radicals with molecular oxygen in frozen aqueous solutions at low temperatures. For all three thiols, the thiyl radical, RS., is found to react with oxygen to form the thiol peroxyl radical, RSOO(.). On the absorption of visible light, RSOO(.) photoisomerizes to the sulfonyl radical, RSO2(.), which subsequently reacts with molecular oxygen to form RSO2OO(.), the sulfonyl peroxyl radical. The identities of the sulfonyl and sulfonyl peroxyl radicals were confirmed by their production by a different route, from sulfinic acid. Sulfinyl radicals, RSO(.), are found as the final radical species in the reactions of thiyl radicals and oxygen. Parallel 17O hyperfine couplings (A parallel) are reported for each sulfoxyl radical and a correlation between the spin density on oxygen and the reactivity of the radical is suggested. As a result of this correlation sulfonyl peroxyl radicals are predicted to be far more reactive than thiol peroxyl radicals. We also report molecular orbital calculations on the nature of the spin density distribution and the molecular geometry of the model radicals CH3SO2(.) and CH3SO2OO(.).

ChemInform Abstract: An ESR Investigation of the Structure and Formation of Sulfinyl Radicals: Reaction of Peroxyl Radicals with Thiols.

AbstractMatrix peroxyl radicals generated by irradiation of organic matrices in the presence of O2 react with added simple thiols (R‐SH; R: ‐Me, ‐Bu, ‐tBu) and the radio‐protective thiols cysteamine and dithiothreitol to form sulfinyl radicals R‐SO..