Thiyl radicals abstract hydrogen atoms from the (alpha)C-H bonds in model peptides: absolute rate constants and effect of amino acid structure

Thionitrite (SNO − ) and Perthionitrite (SSNO − ) are Simple Synthons for Nitrosylated Iron Sulfur Clusters

S/N crosstalk species derived from the interconnected reactivity of H₂ S and NO facilitate the transport of reactive sulfur and nitrogen species in signaling, transport, and regulatory processes. We report here that simple Fe²⁺ ions, such as those that are bioavailable in the labile iron pool (LIP), react with thionitrite (SNO^- ) and perthionitrite (SSNO^- ) to yield the dinitrosyl iron complex [Fe(NO)₂ (S₅ )]^- . In the reaction of FeCl₂ with SNO^- we were able to isolate the unstable intermediate hydrosulfido mononitrosyl iron complex [FeCl₂ (NO)(SH)]^- , which was characterized by X-ray crystallography. We also show that [Fe(NO)₂ (S₅ )]^- is a simple synthon for nitrosylated Fe-S clusters via its reduction with PPh₃ to yield Roussin's Red Salt ([Fe₂ S₂ (NO)₄ ]^2- ), which highlights the role of S/N crosstalk species in the assembly of fundamental Fe-S motifs.

Chemical Stability of Proteins in Foods: Oxidation and the Maillard Reaction

Protein is a major nutrient present in foods along with carbohydrates and lipids. Food proteins undergo a wide range of modifications during food production, processing, and storage. In this review, we discuss two major reactions, oxidation and the Maillard reaction, involved in chemical modifications of food proteins. Protein oxidation in foods is initiated by metal-, enzyme-, or light-induced processes. Food protein oxidation results in the loss of thiol groups and the formation of protein carbonyls and specific oxidation products of cysteine, tyrosine, tryptophan, phenylalanine, and methionine residues, such as disulfides, dityrosine, kynurenine, m-tyrosine, and methionine sulfoxide. The Maillard reaction involves the reaction of nucleophilic amino acid residues with reducing sugars, which yields numerous heterogeneous compounds such as α-dicarbonyls, furans, Strecker aldehydes, advanced glycation end-products, and melanoidins. Both protein oxidation and the Maillard reaction result in the loss of essential amino acids but may positively or negatively impact food structure and flavor. Expected final online publication date for the Annual Review of Food Science and Technology, Volume 13 is March 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Matrix-isolated trifluoromethylthiyl radical: sulfur atom transfer, isomerization and oxidation reactions

By high-vacuum flash pyrolysis of bis(trifluoromethyl)disulfane oxide (CF₃S(O)SCF₃) at 400 °C, the elusive trifluoromethylthiyl radical (CF₃S˙) has been efficiently generated in the gas phase. Subsequent isolation of CF₃S˙ in cryogenic matrixes (Ne, Ar, and N₂) allows a first time characterization with IR and UV-vis spectroscopy by combining with computations at the CCSD(T)/aug-cc-pV(T + d)Z level. In addition to the photo-induced sulfur atom transfer (SAT) from CF₃S˙ to N₂ and CO and the isomerization to ˙CF₂SF, the O₂-oxidation via the intermediacy of the novel thiylperoxy radical CF₃SOO˙ has been observed.

Antioxidants and radical damage in a hydrophilic environment: chemical reactions and concepts

Known endogenous antioxidants are unlikely to prevent radical damage due to oxidative stress or achieve complete repair by established reaction mechanisms. While near complete prevention seems very unrealistic, some of the initial damage can be repaired. Depending on tissue, this may be even a large fraction. Antioxidants, however, will efficiently break radical reaction chains and, therefore, certainly limit the damage caused by radicals. It is not clear if chemical antioxidant action is strictly limited to electron-transfer processes or if additional reaction mechanisms may contribute.

Thiyl Radical Reactions in the Chemical Degradation of Pharmaceutical Proteins

Free radical pathways play a major role in the degradation of protein pharmaceuticals. Inspired by biochemical reactions carried out by thiyl radicals in various enzymatic processes, this review focuses on the role of thiyl radicals in pharmaceutical protein degradation through hydrogen atom transfer, electron transfer, and addition reactions. These processes can lead to the epimerization of amino acids, as well as the formation of various cleavage products and cross-links. Examples are presented for human insulin, human and mouse growth hormone, and monoclonal antibodies.

A Photochemically Generated Selenyl Free Radical Observed by High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy

Selenium-based selenyl free radicals are chemical entities that may be involved in a range of biochemical processes. We report the first X-ray spectroscopic observation of a selenyl radical species generated photochemically by X-ray irradiation of low-temperature solutions of l-selenocysteine. We have employed high energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD-XAS) and electron paramagnetic resonance (EPR) spectroscopy, coupled with density functional theory calculations, to characterize and understand the species. The HERFD-XAS spectrum of the selenyl radical is distinguished by a uniquely low-energy transition with a peak energy at 12 659.0 eV, which corresponds to a 1s → 4p transition to the singly occupied molecular orbital of the free radical. The EPR spectrum shows the broad features and highly anisotropic g-values that are expected for a selenium free radical species. The availability of spectroscopic probes for selenyl radicals may assist in understanding why life chooses selenium over sulfur in selected biochemical processes.

Reaction rates of glutathione and ascorbate with alkyl radicals are too slow for protection against protein peroxidation in vivo

Reaction kinetics of amino acid and peptide alkyl radicals with GSH and ascorbate, the two most abundant endogenous antioxidants, were investigated by pulse radiolysis. Rate constants in the order of 10⁶ M^-1s^-1 were found. Alkyl radicals react at almost diffusion controlled rates and irreversibly with oxygen to form peroxyl radicals, and competition with this reaction is the benchmark for efficient repair in vivo. We consider repair of protein radicals and assume comparable rate constants for the reactions of GSH/ascorbate with peptide alkyl radicals and with alkyl radicals on a protein surface. Given physiological concentrations of oxygen, GSH and ascorbate, protein peroxyl radicals will always be a major product of protein alkyl radicals in vivo. Therefore, if they are formed by oxidative stress, protein alkyl radicals are a probable cause for biological damage.

Sulfur Radical-Induced Redox Modifications in Proteins: Analysis and Mechanistic Aspects

SIGNIFICANCE

The sulfur-containing amino acids cysteine (Cys) and methionine (Met) are prominent protein targets of redox modification during conditions of oxidative stress. Here, two-electron pathways have received widespread attention, in part due to their role in signaling processes. However, Cys and Met are equally prone to one-electron pathways, generating intermediary radicals and/or radial ions. These radicals/radical ions can generate various reaction products that are not commonly monitored in redox proteomic studies, but they may be relevant for the fate of proteins during oxidative stress. Recent Advances: Time-resolved kinetic studies and product analysis have expanded our mechanistic understanding of radical reaction pathways of sulfur-containing amino acids. These reactions are now studied in some detail for Met and Cys in proteins, and homocysteine (Hcy) chemically linked to proteins, and the role of protein radical reactions in physiological processes is evolving.

CRITICAL ISSUES

Radical-derived products from Cys, Hcy, and Met can react with additional amino acids in proteins, leading to secondary protein modifications, which are potentially remote from initial points of radical attack. These products may contain intra- and intermolecular cross-links, which may lead to protein aggregation. Protein sequence and conformation will have a significant impact on the formation of such products, and a thorough understanding of reaction mechanisms and specifically how protein structure influences reaction pathways will be critical for identification and characterization of novel reaction products.

FUTURE DIRECTIONS

Future studies must evaluate the biological significance of novel reaction products that are derived from radical reactions of sulfur-containing amino acids. Antioxid. Redox Signal. 26, 388-405.

Oxidative stress, free radicals and protein peroxides

Primary free radicals generated under oxidative stress in cells and tissues produce a cascade of reactive secondary radicals, which attack biomolecules with efficiency determined by the reaction rate constants and target concentration. Proteins are prominent targets because they constitute the bulk of the organic content of cells and tissues and react readily with many of the secondary radicals. The reactions commonly lead to the formation of carbon-centered radicals, which generally convert in vivo to peroxyl radicals and finally to semistable hydroperoxides. All of these intermediates can initiate biological damage. This article outlines the advantages of the application of ionizing radiations to studies of radicals, with particular reference to the generation of desired radicals, studies of the kinetics of their reactions and correlating the results with events in biological systems. In one such application, formation of protein hydroperoxides in irradiated cells was inhibited by the intracellular ascorbate and glutathione.

Chemical degradation of proteins in the solid state with a focus on photochemical reactions

Protein pharmaceuticals comprise an increasing fraction of marketed products but the limited solution stability of proteins requires considerable research effort to prepare stable formulations. An alternative is solid formulation, as proteins in the solid state are thermodynamically less susceptible to degradation. Nevertheless, within the time of storage a large panel of kinetically controlled degradation reactions can occur such as, e.g., hydrolysis reactions, the formation of diketopiperazine, condensation and aggregation reactions. These mechanisms of degradation in protein solids are relatively well covered by the literature. Considerably less is known about oxidative and photochemical reactions of solid proteins. This review will provide an overview over photolytic and non-photolytic degradation reactions, and specially emphasize mechanistic details on how solid structure may affect the interaction of protein solids with light.

Thiyl radicals and induction of protein degradation

Thiyl radicals are important intermediates in the redox biology and chemistry of thiols. These radicals can react via hydrogen transfer with various C-H bonds in peptides and proteins, leading to the generation of carbon-centered radicals, and, potentially, to irreversible protein damage. This review summarizes quantitative information on reaction kinetics and product formation, and discusses the significance of these reactions for protein degradation induced by thiyl radical formation.

Reactivity and selectivity patterns in hydrogen atom transfer from amino acid C-H bonds to the cumyloxyl radical: polar effects as a rationale for the preferential reaction at proline residues

Absolute rate constants for hydrogen atom transfer (HAT) from the C-H bonds of N-Boc-protected amino acids to the cumyloxyl radical (CumO(•)) were measured by laser flash photolysis. With glycine, alanine, valine, norvaline, and tert-leucine, HAT occurs from the α-C-H bonds, and the stability of the α-carbon radical product plays a negligible role. With leucine, HAT from the α- and γ-C-H bonds was observed. The higher kH value measured for proline was explained in terms of polar effects, with HAT that predominantly occurs from the δ-C-H bonds, providing a rationale for the previous observation that proline residues represent favored HAT sites in the reactions of peptides and proteins with (•)OH. Preferential HAT from proline was also observed in the reactions of CumO(•) with the dipeptides N-BocProGlyOH and N-BocGlyGlyOH. The rate constants measured for CumO(•) were compared with the relative rates obtained previously for the corresponding reactions of different hydrogen-abstracting species. The behavior of CumO(•) falls between those observed for the highly reactive radicals Cl(•) and (•)OH and the significantly more stable Br(•). Taken together, these results provide a general framework for the description of the factors that govern reactivity and selectivity patterns in HAT reactions from amino acid C-H bonds.

Protein thiyl radical reactions and product formation: a kinetic simulation

Protein thiyl radicals are important intermediates generated in redox processes of thiols and disulfides. Thiyl radicals efficiently react with glutathione and ascorbate, and the common notion is that these reactions serve to eliminate thiyl radicals before they can enter potentially hazardous processes. However, over the past years increasing evidence has been provided for rather efficient intramolecular hydrogen transfer processes of thiyl radicals in proteins and peptides. Based on rate constants published for these processes, we have performed kinetic simulations of protein thiyl radical reactivity. Our simulations suggest that protein thiyl radicals enter intramolecular hydrogen transfer reactions to a significant extent even under physiologic conditions, i.e., in the presence of 30 µM oxygen, 1 mM ascorbate, and 10 mM glutathione. At lower concentrations of ascorbate and glutathione, frequently observed when tissue is exposed to oxidative stress, the extent of irreversible protein thiyl radical-dependent protein modification increases.

Reactions of the cumyloxyl radical with secondary amides. The influence of steric and stereoelectronic effects on the hydrogen atom transfer reactivity and selectivity

A time-resolved kinetic study of the hydrogen atom transfer (HAT) reactions from secondary alkanamides to the cumyloxyl radical was carried out in acetonitrile. HAT predominantly occurs from the N-alkyl α-C-H bonds, and a >60-fold decrease in kH was observed by increasing the steric hindrance of the acyl and N-alkyl groups. The role of steric and stereoelectronic effects on the reactivity and selectivity is discussed in the framework of HAT reactions from peptides.

Kinetics of hydrogen atom abstraction from substrate by an active site thiyl radical in ribonucleotide reductase

Ribonucleotide reductases (RNRs) catalyze the conversion of nucleotides to deoxynucleotides in all organisms. Active E. coli class Ia RNR is an α₂β₂ complex that undergoes reversible, long-range proton-coupled electron transfer (PCET) over a pathway of redox active amino acids (β-Y₁₂₂ → [β-W₄₈] → β-Y₃₅₆ → α-Y₇₃₁ → α-Y₇₃₀ → α-C₄₃₉) that spans ∼35 Å. To unmask PCET kinetics from rate-limiting conformational changes, we prepared a photochemical RNR containing a [Re^I] photooxidant site-specifically incorporated at position 355 ([Re]-β₂), adjacent to PCET pathway residue Y₃₅₆ in β. [Re]-β₂ was further modified by replacing Y₃₅₆ with 2,3,5-trifluorotyrosine to enable photochemical generation and spectroscopic observation of chemically competent tyrosyl radical(s). Using transient absorption spectroscopy, we compare the kinetics of Y· decay in the presence of substrate and wt-α₂, Y₇₃₁F-α₂ ,or C₄₃₉S-α₂, as well as with 3′-[²H]-substrate and wt-α₂. We find that only in the presence of wt-α₂ and the unlabeled substrate do we observe an enhanced rate of radical decay indicative of forward radical propagation. This observation reveals that cleavage of the 3′-C–H bond of substrate by the transiently formed C₄₃₉· thiyl radical is rate-limiting in forward PCET through α and has allowed calculation of a lower bound for the rate constant associated with this step of (1.4 ± 0.4) × 10⁴ s^–1. Prompting radical propagation with light has enabled observation of PCET events heretofore inaccessible, revealing active site chemistry at the heart of RNR catalysis.

Antioxidant activities of cysteine derivatives against lipid oxidation in anhydrous media

This study investigated antioxidant activities of cysteine derivatives of amino and carboxylic acid moieties against lipid oxidation in anhydrous acetonitrile. Only cysteine derivatives bearing free amino or carboxylate ion were found to exert potent antioxidant activities. Sequential proton loss and electron transfer-like proton shift and subsequent electron transfer (PS-ET) mechanism may facilitate the antioxidant activities of cysteine derivatives against lipid oxidation in anhydrous media.

Cardiovascular redox and ox stress proteomics

SIGNIFICANCE

Oxidative post-translational modifications (OPTMs) have been demonstrated as contributing to cardiovascular physiology and pathophysiology. These modifications have been identified using antibodies as well as advanced proteomic methods, and the functional importance of each is beginning to be understood using transgenic and gene deletion animal models. Given that OPTMs are involved in cardiovascular pathology, the use of these modifications as biomarkers and predictors of disease has significant therapeutic potential. Adequate understanding of the chemistry of the OPTMs is necessary to determine what may occur in vivo and which modifications would best serve as biomarkers.

RECENT ADVANCES

By using mass spectrometry, advanced labeling techniques, and antibody identification, OPTMs have become accessible to a larger proportion of the scientific community. Advancements in instrumentation, database search algorithms, and processing speed have allowed MS to fully expand on the proteome of OPTMs. In addition, the role of enzymatically reversible OPTMs has been further clarified in preclinical models.

CRITICAL ISSUES

The identification of OPTMs suffers from limitations in analytic detection based on the methodology, instrumentation, sample complexity, and bioinformatics. Currently, each type of OPTM requires a specific strategy for identification, and generalized approaches result in an incomplete assessment.

FUTURE DIRECTIONS

Novel types of highly sensitive MS instrumentation that allow for improved separation and detection of modified proteins and peptides have been crucial in the discovery of OPTMs and biomarkers. To further advance the identification of relevant OPTMs in advanced search algorithms, standardized methods for sample processing and depository of MS data will be required.

Intramolecular hydrogen transfer reactions of thiyl radicals from glutathione: formation of carbon-centered radical at Glu, Cys, and Gly

Glutathione thiyl radicals (GS(•)) were generated in H(2)O and D(2)O by either exposure of GSH to AAPH, photoirradiation of GSH in the presence of acetone, or photoirradiation of GSSG. Detailed interpretation of the fragmentation pathways of deuterated GSH and GSH derivatives during mass spectrometry analysis allowed us to demonstrate that reversible intramolecular H-atom transfer reactions between GS(•) and C-H bonds at Cys[(α)C], Cys[(β)C], and Gly[(α)C] are possible.

Reversible hydrogen transfer reactions in thiyl radicals from cysteine and related molecules: absolute kinetics and equilibrium constants determined by pulse radiolysis

The mercapto group of cysteine (Cys) is a predominant target for oxidative modification, where one-electron oxidation leads to the formation of Cys thiyl radicals, CysS(•). These Cys thiyl radicals enter 1,2- and 1,3-hydrogen transfer reactions, for which rate constants are reported in this paper. The products of these 1,2- and 1,3-hydrogen transfer reactions are carbon-centered radicals at position C(3) (α-mercaptoalkyl radicals) and C(2) ((•)C(α) radicals) of Cys, respectively. Both processes can be monitored separately in Cys analogues such as cysteamine (CyaSH) and penicillamine (PenSH). At acidic pH, thiyl radicals from CyaSH permit only the 1,2-hydrogen transfer according to equilibrium 12, (+)H(3)NCH(2)CH(2)S(• )⇌ (+)H(3)NCH(2)(•)CH-SH, where rate constants for forward and reverse reaction are k(12) ≈ 10(5) s(-1) and k(-12) ≈ 1.5 × 10(5)s(-1), respectively. In contrast, only the 1,3-hydrogen transfer is possible for thiyl radicals from PenSH according to equilibrium 14, ((+)H(3)N/CO(2)H)C(α)-C(CH(3))(2)-S(•) ⇌ ((+)H(3)N/CO(2)H)(•)C(α)-C(CH(3))(2)-SH, where rate constants for the forward and the reverse reaction are k(14) = 8 × 10(4) s(-1) and k(-14) = 1.4 × 10(6) s(-1). The (•)C(α) radicals from PenSH and Cys have the additional opportunity for β-elimination of HS(•)/S(•-), which proceeds with k(39) ≈ (3 ± 1) × 10(4) s(-1) from (•)C(α) radicals from PenSH and k(-34) ≈ 5 × 10(3) s(-1) from (•)C(α) radicals from Cys. The rate constants quantified for the 1,2- and 1,3-hydrogen transfer reactions can be used as a basis to calculate similar processes for Cys thiyl radicals in proteins, where hydrogen transfer reactions, followed by the addition of oxygen, may lead to the irreversible modification of target proteins.

Investigation of reactions postulated to occur during inhibition of ribonucleotide reductases by 2'-azido-2'-deoxynucleotides

Model 3'-azido-3'-deoxynucleosides with thiol or vicinal dithiol substituents at C2' or C5' were synthesized to study reactions postulated to occur during inhibition of ribonucleotide reductases by 2'-azido-2'-deoxynucleotides. Esterification of 5'-(tert-butyldiphenylsilyl)-3'-azido-3'-deoxyadenosine and 3'-azido-3'-deoxythymidine (AZT) with 2,3-S-isopropylidene-2,3-dimercaptopropanoic acid or N-Boc-S-trityl-L-cysteine and deprotection gave 3'-azido-3'-deoxy-2'-O-(2,3-dimercaptopropanoyl or cysteinyl)adenosine and the 3'-azido-3'-deoxy-5'-O-(2,3-dimercaptopropanoyl or cysteinyl)thymidine analogs. Density functional calculations predicted that intramolecular reactions between generated thiyl radicals and an azido group on such model compounds would be exothermic by 33.6-41.2 kcal/mol and have low energy barriers of 10.4-13.5 kcal/mol. Reduction of the azido group occurred to give 3'-amino-3'-deoxythymidine, which was postulated to occur with thiyl radicals generated by treatment of 3'-azido-3'-deoxy-5'-O-(2,3-dimercaptopropanoyl)thymidine with 2,2'-azobis-(2-methyl-2-propionamidine) dihydrochloride. Gamma radiolysis of N(2)O-saturated aqueous solutions of AZT and cysteine produced 3'-amino-3'-deoxythymidine and thymine most likely by both radical and ionic processes.

Forging isopeptide bonds using thiol-ene chemistry: site-specific coupling of ubiquitin molecules for studying the activity of isopeptidases

Chemical methods for modifying proteins can enable studies aimed at uncovering biochemical function. Herein, we describe the use of thiol-ene coupling (TEC) chemistry to report on the function of branched (also referred to as forked) ubiquitin trimers. We show how site-specific isopeptide (Nε-Gly-L-homothiaLys) bonds are forged between two molecules of Ub, demonstrating the power of TEC in protein conjugation. Moreover, we demonstrate that the Nε-Gly-L-homothiaLys isopeptide bond is processed to a similar extent by deubiquitinases (DUBs) as that of a native Nε-Gly-L-Lys isopeptide bond, thereby establishing the utility of TEC in the generation of Ub-Ub linkages. TEC is then applied to the synthesis of branched Ub trimers. Interrogation of these branched derivatives with DUBs reveals that the relative orientation of the two Ub units has a dramatic impact on how they are hydrolyzed. In particular, cleavage of K48C-linkages is suppressed when the central Ub unit is also conjugated through K6C, whereas cleavage proceeds normally when the central unit is conjugated through either K11C or K63C. The results of this work presage a role for branched polymeric Ub chains in regulating linkage-selective interactions.

Cysteine residues as catalysts for covalent peptide and protein modification: a role for thiyl radicals?

Cysteine thiyl radicals engage in reversible intramolecular hydrogen-transfer reactions with amino acid residues in peptides and proteins. These reactions can be experimentally demonstrated through covalent hydrogen-deuterium exchange when experiments are carried out in (2)H2O. To this end, hydrogen-transfer reactions have been observed between cysteine thiyl radicals and glycine, alanine, serine, valine and leucine in both model peptides and a protein, insulin. The relevance of such reactions for protein oxidation under conditions of oxidative stress is discussed.

Modeling of the ribonucleotide reductases substrate reaction. Hydrogen atom abstraction by a thiyl free radical and detection of the ribosyl-based carbon radical by pulse radiolysis

The 1,4-anhydro-5-deoxy-6-thio-D-ribo-hexofuranitol (1) was prepared from 1,2-O-isopropylidene-α-D-glucose in 10 steps. In a key step treatment of the 1,2-O-isopropylidenehexofuranose derivative with BF3/Et3SiH effected deacetonization and reductive deoxygenation at carbon 1. Pulse radiolysis experiments with 6-thiohexofuranitol 1 and its disulfide derivative demonstrated formation of the ribosyl-based carbon-centered radical upon generation of 6-thiyl radical in basic medium. The proposed [1,5]-hydrogen shift abstraction with generation of the C3 radical mimics the initial substrate reaction of RNRs. The reversible H-atom transfer has been quantified and was correlated with the computed rate constants for the internal H atom abstraction from C1, C2, C3 and C4 by the thiyl radical. The energy barrier for the H3 and H4 abstractions were calculated to be most favorable with the corresponding barriers of 11.1 and 11.2 kcal/mol, respectively.

MAPping the chiral inversion and structural transformation of a metal-tripeptide complex having ni-superoxide dismutase activity

The metal abstraction peptide (MAP) tag is a tripeptide sequence capable of abstracting a metal ion from a chelator and binding it with extremely high affinity at neutral pH. Initial studies on the nickel-bound form of the complex demonstrate that the tripeptide asparagine-cysteine-cysteine (NCC) binds metal with 2N:2S, square planar geometry and behaves as both a structural and functional mimic of Ni superoxide dismutase (Ni-SOD). Electronic absorption, circular dichroism (CD), and magnetic CD (MCD) data collected for Ni-NCC are consistent with a diamagnetic Ni(II) center. It is apparent from the CD signal of Ni-NCC that the optical activity of the complex changes over time. Mass spectrometry data show that the mass of the complex is unchanged. Combined with the CD data, this suggests that chiral rearrangement of the complex occurs. Following incubation of the nickel-containing peptide in D(2)O and back-exchange into H(2)O, incorporation of deuterium into non-exchangeable positions is observed, indicating chiral inversion occurs at two of the α carbon atoms in the peptide. Control peptides were used to further characterize the chirality of the final nickel-peptide complex, and density functional theory (DFT) calculations were performed to validate the hypothesized position of the chiral inversions. In total, these data indicate Ni-SOD activity is increased proportionally to the degree of structural change in the complex over time. Specifically, the relationship between the change in CD signal and change in SOD activity is linear.

Hydrogen exchange equilibria in glutathione radicals: rate constants

The reduction of oxidized glutathione GSSG by hydrated electrons and hydrogen atoms to form GSSG^•− is quantitative. The radical anion dissociates into GS^• and GS⁻, and the S-centered radical subsequently abstracts a hydrogen intramolecularly. We observe sequential development of UV absorbance signatures that indicate the formation of both α- and β-carbon-centered radicals. From experiments performed at pH 2 and pH 11.8, we determined forward and reverse rate constants for the overall equilibrium between sulfur-centered and carbon-centered radicals: k_forward = 3·10⁵ s⁻¹, k_reverse = 7·10⁵ s⁻¹, and K = 0.4. Furthermore, on the basis of the differences between the kinetics traces at 240 and 280 nm, we estimate that α- and β-carbon-centered radicals are formed at a surprising ratio of 1:3. The ratios found at pH 2 also apply to pH 7, with the conclusion that the equilibrium ratio of S-centered:β-centered:α-centered radicals is, very approximately, 8:3:1. The formation of carbon-centered radicals could lead to irreversible damage in proteins via the formation of carbon−carbon bonds or backbone fragmentation.

Mobile protons versus mobile radicals: gas-phase unimolecular chemistry of radical cations of cysteine-containing peptides

A combination of electrospray ionization (ESI), multistage, and high-resolution mass spectrometry experiments are used to examine the gas-phase fragmentation reactions of radical cations of cysteine containing di- and tripeptides. Two different chemical methods were used to form initial populations of radical cations in which the radical sites were located at different positions: (1) sulfur-centered cysteinyl radicals via bond homolysis of protonated S-nitrosocysteine containing peptides; and (2) alpha-carbon backbone-centered radicals via Siu's sequence of reactions (J. Am. Chem. Soc.2008, 130, 7862). Comparison of the fragmentation reactions of these regiospecifically generated radicals suggests that hydrogen atom transfer (HAT) between the alpha C-H of adjacent residues and the cysteinyl radical can occur. In addition, using accurate mass measurements, deuterium labeling, and comparison with an authentic sample, a novel loss of part of the N-terminal cysteine residue was shown to give rise to the protonated, truncated N-formyl peptide (an even-electron x(n) ion). DFT calculations were performed on the radical cation [GCG]*(+) to examine: the relative stabilities of isomers with different radical and protonation sites; the barriers associated with radical migration between four possible radical sites, [G*CG](+), [GC*G](+), [GCG*](+), and [GC(S*)G](+); and for dissociation from these sites to yield b(2)-type ions.