Bonded Radical Cation Research Articles

N-Terminal Decarboxylation as a Probe for Intramolecular Contact Formation in γ-Glu-(Pro)n-Met Peptides.

The kinetics of intramolecular-contact formation between remote functional groups in peptides with restricted conformational flexibility were examined using designed peptides with variable-length proline bridges. As probes for this motion, free radicals were produced using the •OH-induced oxidation at the C-terminal methionine residue of γ-Glu-(Pro)n-Met peptides (n = 0–3). The progress of the radicals’ motion along the proline bridges was monitored as the radicals underwent reactions along the peptides’ backbones. Of particular interest was the reaction between the sulfur atom located in the side chain of the oxidized Met residue and the unprotonated amino group of the glutamic acid moiety. Interactions between them were probed by the radiation-chemical yields (expressed as G values) of the formation of C-centered, α-aminoalkyl radicals (αN) on the Glu residue. These radicals were monitored directly or via their reaction with p-nitroacetophenone (PNAP) to generate the optically detected PNAP•– radical anions. The yields of these αN radicals were found to be linearly dependent on the number of Pro residues. A constant decrease by 0.09 μM J–1 per spacing Pro residue of the radiation-chemical yields of G(αN) was observed. Previous reports support the conclusion that the αN radicals in these cases would have to result from (S∴N)+-bonded cyclic radical cations that arose as a result from direct contact between the ends of the peptides. Furthermore, by analogy with the rate constants for the formation of intramolecularly (S∴S)+-bonded radical cations in Met-(Pro)n-Met peptides (J. Phys. Chem. B2016, 120, 973227513096), the rate constants for the formation of intramolecularly (S∴N)+-bonded radical cations are activated to the same extent for all of the γ-Glu-(Pro)n-Met peptides. Thus, the continuous decrease of G(αN) with the number of Pro residues (from 0 to 3) suggests that the formation of a contact between the S-atom in the C-terminal Met residue and the N-atom of a deprotonated N-terminal amino group of Glu is controlled in peptides with 0 to 3 Pro residues by the relative diffusion of the S•+ and unoxidized N-atom. The overall rate constants of cyclization to form the (S∴N)-bonded radical cations were estimated to be 3.8 × 106, 1.8 × 106, and 8.1 × 105 s–1 for peptides with n = 0, 1, and 2 Pro residues, respectively. If activation is the same for all of the peptides, then these rate constants are a direct indication for the end-to-end dynamics along the chain.

Comparison of the One-Electron Oxidations of CO-Bridged vs Unbridged Bimetallic Complexes: Electron-Transfer Chemistry of Os2Cp2(CO)4 and Os2Cp*2(μ-CO)2(CO)2 (Cp = η5-C5H5, Cp* = η5-C5Me5)

The one-electron oxidations of two dimers of half-sandwich osmium carbonyl complexes have been examined by electrochemistry, spectro-electrochemistry, and computational methods. The all-terminal carbonyl complex Os2Cp2(CO)4 (1, Cp = η5-C5H5) undergoes a reversible one-electron anodic reaction at E1/2 = 0.41 V vs ferrocene in CH2Cl2/0.05 M [NBu4][B(C6F5)4], giving a rare example of a metal–metal bonded radical cation unsupported by bridging ligands. The IR spectrum of 1+ is consistent with an approximately 1:1 mixture of anti and gauche structures for the 33 e– radical cation in which it has retained all-terminal bonding of the CO ligands. Density functional theory (DFT) calculations, including orbital-occupancy-perturbed Mayer bond-order analyses, show that the highest-occupied molecular orbitals (HOMOs) of anti-1 and gauche-1 are metal–ligand delocalized. Removal of an electron from 1 has very little effect on the Os–Os bond order, accounting for the resistance of 1+ to heterolytic cleavage. The Os–Os bo...

Formation of Dimer Radical Cations of Selenourea on Oxidation: Pulse Radiolysis Studies

Reactions of oxidizing radicals like hydroxyl (·OH) radical, specific electron transfer agents like N 3 ·, and I 2 −. radicals were studied with selenourea (SeU) and compared with thiourea (ThU) using pulse radiolysis technique in microsecond time scales. Both the compounds efficiently react with ·OH radicals, however, SeU undergoes easier oxidation by secondary oxidants as compared to ThU. The results were supported by cyclic voltammetry studies. The radical cations of both SeU and ThU formed on oxidation undergo dimerization with the parent molecule to form two-centered three-electron-hemi bonded radical cations absorbing at 410 and 400 nm respectively with the stabilization energies of 21.1 and 20.5 kcal/mol for SeU and ThU, respectively. Preliminary studies indicated that at low concentration of SeU, the dimerization is prevented and the oxidation reaction produced metallic Se nanoparticles.

Sigma Bonded Radical Cation Complexes: A Theoretical Study

A theoretical study on the nature of bonding in several weakly bound radical cation complexes from second and third row hydrides is presented. It is shown that characterization of a two-center three-electron or 2c−3e bond based on its bond distance and binding energy may be misleading in many cases. It is also observed that the ab inito quantum chemical bond order index cannot be taken as a definite signature of a 2c−3e bond. Instead, it is suggested that appropriate localized molecular orbital need be used to test the presence of a 2c−3e bond. Localization of relevant molecular orbital in the 2c−3e bonded systems also suggests that the newly formed bond is of sigma character. Normal-mode analysis is performed to identify the stretching mode in the 2c−3e bonded radical cation complexes. Geometry optimizations are carried out at MP2 and restricted open shell Becke's half-and-half (BHH) nonlocal exchange and Lee−Yang−Parr (LYP) nonlocal correlation functionals (BHHLYP) with 6-31++G(d,p) basis set. Hessian c...

Redox Processes of Methionine Relevant to β-Amyloid Oxidation and Alzheimer's Disease

This minireview gives an overview over the oxidation mechanisms of methionine (Met) relevant for analogous processes which may lead to the oxidation of β-amyloid (βA) peptides. The CuII-catalyzed oxidation of a C-terminal Met35 residue in βA peptides may be a key to the known propensities of these peptides to form H2O2 and free radicals. Though the reduction potentials of CuII and Met would seem unfavorable, there are several structural features of βA, which may promote a one-electron oxidation of Met. The potentially close association of the Met sulfur with the C=O group C-terminal of Ile31 in the C-terminus of βA may support the formation of an S–O bonded radical cation intermediate. Evidence for such S–O bond formation has recently been obtained for a model, N-acetylmethionine amide. Additional support for a potential catalytic role of an oxygen-containing functional group comes from numerous studies with organic model sulfides.

The metal-catalyzed oxidation of methionine in peptides by Fenton systems involves two consecutive one-electron oxidation processes

The one-electron oxidation of methionine (Met) plays an important role in the redox reactions of Met in peptides and proteins under conditions of oxidative stress, e.g., during the metal-catalyzed oxidation of β-amyloid peptide (βA). However, little information is available with regard to mechanisms and product formation during the metal-catalyzed oxidation of Met. Here, we demonstrate that two-electron oxidation of Met in Fenton reactions, carried out aerobically by [Fe II(EDTA)] 2− and H 2O 2 (EDTA = ethylenediaminetetra acetate) is the consequence of two consecutive one-electron transfer reactions carried out by either free or complexed hydroxyl radicals, followed by the reaction of an intermediary sulfur-nitrogen bonded radical cation (sulfuranyl radical) with O 2. The model peptide Met-Met represents an ideal substrate for these investigations as its one-electron oxidation, followed by reaction with molecular oxygen, produces unique intermediates, azasulfonium diastereomers, which can be chemically isolated before hydrolysis to sulfoxide occurs.

Kinetics of the Reactions between Sulfide Radical Cation Complexes, [S∴S]+ and [S∴N]+, and Superoxide or Carbon Dioxide Radical Anions

The reaction of superoxide radical anions with sulfide radical cation complexes represents an important sulfoxide-forming process. Here, absolute rate constants for the reaction of sulfur-sulfur and sulfur-nitrogen three-electron bonded sulfide radical cation complexes with superoxide and, for comparison, carbon dioxide radical anion have been measured by pulse radiolysis. For two different sulfur-sulfur bonded species, the intermolecular complex from dimethyl sulfide and the intramolecular complex from 1,5-dithia-3-hydroxycyclooctane, the rate constants for the reaction with superoxide are on the order of 1.6 {times} 10{sup 10}M{sup {minus}1} s{sup {minus}1} and with the carbon dioxide radical anion on the order of (6.5 {+-} 0.2) {times} 10{sup 9} M{sup {minus}1} s{sup {minus}1}. The fact that the stronger reducing carbon dioxide radical anion shows the lower rate constant can be rationalized by the higher internal reorganization energy of {sup {sm{underscore}bullet}}CO{sub 2}{sup {minus}} as compared to O{sub 2}{sup {sm{underscore}bullet}{minus}}. The rate constant for the reaction of superoxide with the sulfur-nitrogen bonded radical cation of Met-Gly, k = 5.3 x 10{sup 9} M{sup {minus}1} s{sup {minus}1}, is 3-fold lower as compared to that of the reaction with the sulfur-sulfur bonded radical cation complexes.

Oxidation of (Carboxyalkyl)thiopropionic Acid Derivatives by Hydroxyl Radicals. Mechanisms and Kinetics of Competitive Inter- and Intramolecular Formation of σ- and σ*-type Sulfuranyl Radicals

The substituent effects on kinetics and yields of specific intermediates and products for the one-electron oxidation by hydroxyl radicals of various (carboxylalkyl)thiopropionic acid derivatives, 3-(methylthio)propionic acid (3-MTPA), 3,3‘-thiodipropionic acid (3,3‘-TDPA), 3-(carboxymethylthio)propionic acid (3-CMTPA), and 2-(carboxymethylthio)succinic acid (2-CMTPA) have been investigated employing pulse radiolysis on the nanosecond to microsecond time scale, and γ-radiolysis. For each derivative, the initial step was a formation of a hydroxysulfuranyl radical proceeding with absolute rate constants of kOH+3-MTPA = 9.1 × 109 M-1 s-1 and kOH+3,3‘-TDPA = 5.8 × 109 M-1 s-1. The subsequent formation of one-electron-oxidized intermediates such as dimeric sulfur−sulfur (S∴S)-three-electron-bonded and monomeric sulfur−carboxylate oxygen (S−O)-bonded sulfide radical cations strongly depended on pH, thioether concentration, and the availability of α- or β-positioned carboxylate functions. A spectral resolution procedure permitted the quantification of all transients present in solution at any time after the pulse. Whereas both (S∴S)- and (S−O)-bonded intermediates were formed for 3-MTPA at neutral solution, electrostatic repulsion nearly prohibited the formation of dimeric (S∴S)-bonded intermediates between an overall negatively charged sulfide radical cation of 3,3‘-TDPA and a second nonoxidized, overall twice negatively charged, molecule of 3,3‘-TDPA. Neither sulfide radical cation complex (S∴S)+ was observed for 3-CMTPA and 2-CMPTA rationalized by a fast decarboxylation of the α-positioned carboxylate group, yielding α-(alkylthio)alkyl radicals which were the only products optically observed on the pulse radiolysis time scale. For 3,3‘-TDPA, the conversion of the initially formed hydroxysulfuranyl radicals into the (S−O)-bonded intermediates occurred unimolecularly with k ≅ 108 s-1 whereas the formation of the (S∴S)-bonded intermediates proceeded bimolecularly with k = (1.9−2.0) × 108 M-1 s-1. These processes did not occur competitively, as the intercepts of plots of pseudo-first-order rate constants for the formation of the S∴S bonded intermediates as a function of thioether concentration were too small (2.7 × 107 s-1) to contain the unimolecular rate constant for the formation of the (S−O)-bonded intermediate (k = 108 s-1). Therefore, a mechanism was proposed according to which initially formed hydroxysulfuranyl radicals rapidly converted into the σ*-type (S−O)-bonded intermediate. Subsequently, these either converted into (S∴S)-bonded radical cation complexes via a displacement of the carboxylate oxygen by a second nonoxidized sulfide function, or reversibly ring-opened to yield the monomeric sulfur-centered radical cation. The latter either associated with a nonoxidized sulfide or irreversibly cyclized to a σ-type (S−O)-bonded sulfuranyl radical.

New substrates for the preparation of electroactive materials: the syntheses of chiral tetrathiafulvalene derivatives with hydroxy-functionalised butane-1,4-dithio bridges

The syntheses of two derivatives of TTF containing (2R,3R)-2,3-dihydroxybutane-1,4-dithio bridges is described, for use in the preparation of chiral hydrogen bonded radical cation salts as potentially electroactive materials. The bridges are introduced by reaction of 2-thioxo-1,3-dithiole-4,5-dithiolate with 1,4-disubstituted derivatives of bis-O-protected butane-2,3-diol to give a bicyclic thione. The X-ray crystal structure of the isopropylidene ketal derivative shows that all four sp3 carbon atoms in the eight-membered ring are displaced to the same side of the dithiole plane. Ease of deprotection led to the use of methoxyethoxymethyl (MEM) protecting groups rather than the ketal. Treatment of the bis-MEM-protected bicyclic thione with mercuric acetate and acetic acid led not only to exchange of the exocyclic sulfur for oxygen but also to removal of just one of the MEM groups, which may have wider synthetic applications.

Mechanism of One-Electron Oxidation of β-, γ-, and δ-Hydroxyalkyl Sulfides. Catalysis through Intramolecular Proton Transfer and Sulfur−Oxygen Bond Formation

The mechanism of photoinduced electron transfer between sulfur-containing alcohols and the 4-carboxybenzophenone (CB) triplet state in aqueous solution was investigated using laser flash photolysis and steady-state photolysis techniques. Bimolecular rate constants for quenching of the CB triplet state by five hydroxyalkyl sulfides, 2-(methylthio)ethanol (2-MTE), 2,2‘-dihydroxydiethyl sulfide (2,2‘-DHE), 3-(methylthio)propanol (3-MTP), 3,3‘-dihydroxydipropyl sulfide (3,3‘-DHP), and 4-(methylthio)butanol (4-MTB), with varying numbers of OH groups and varying locations with respect to the sulfur atom, were determined to be in the range (3.3−4.8) × 109 M-1 s-1 for neutral solutions. The intermediates identified were the CB ketyl radical anion (CB•-), the CB ketyl radical (CBH•), and intermolecularly (S∴S)-bonded radical cations. An additional absorption band at approximately 400 nm in the transient spectra for some of the hydroxyalkyl sulfides was assigned to the intramolecularly (•S−O)-bonded species (only f...

Theoretical Studies on Some S.'.N Three-Electron Bonded Radical Cations.

Theoretical Studies on Some S.'.N Three-Electron Bonded Radical Cations.

Reaction of hydroxyl radicals with 1-bromo- N-iodoalkanes in aqueous solution: 2C–3E bonded radical cations

The transient optical absorption bands ( λ max = 310, 350 nm) formed on reaction of OH radicals with 1-bromo-4-iodobutane (pH = 6) are assigned to OH-adduct. In acidic solutions (pH = 3.0), the OH adduct is converted to a transient band with λ max = 440 nm. This band is assigned to intramolecular radical cation formed on p-orbital overlap of oxidized iodine with bromine. In case of 1-bromo-5-iodopentane, the transient optical absorption bands formed on the reaction of OH radicals at pH = 6.0 are at λ max = 310 and 350 nm. In acidic aqueous solution (pH = 3.0), the OH adducts are converted to intramolecularly coordinated solute radical cation ( λ max = 410 nm) through acid catalysed dehydration reaction. In case of 1-bromo-3-iodopropane, the hydroxyl radicals react to form absorption band with λ max = 400 nm and a small shoulder in 310–330 nm region at neutral pH. In acidic solutions (pH = 3.0), the absorption of 310–330 nm band decreases and that of 400 nm band increases. The transient band at 400 nm is assigned to intramolecular radical cation. A fraction of OH radical is able to react by electron transfer mechanism at neutral pH. The fraction of OH radical forming OH-adduct at neutral pH decreases and is converted to radical cation in acidic pH.

Sensitized photo-oxidation of sulfur-containing amino acids and peptides in aqueous solution

Qualitative and quantitative studies of the photo-oxidation of sulfur-containing amino acids and methionine-containing dipeptides and tripeptides in aqueous solution sensitized by 4-carboxybenzophenone (CB) are reviewed. The mechanism of the photo-oxidation reaction was investigated using the techniques of flash photolysis, steady state photolysis and pulse radiolysis. The rate constants for quenching of the CB triplet by twelve sulfur-containing amino acids and six methionine-containing peptides were determined to be in the range 10 8–10 9 M −1 s −1 for both neutral and alkaline solutions. The amino acids varied in structure, having different numbers of COCH and NH 2 terminal groups and their sulfur atom at different locations relative to the terminal groups. The methionine-containing peptides were MetGly, GlyMet, MetMet MetGlyGly, GlyMetGly and GlyGlyMet. Time-resolved transient spectra accompanying the quenching events were assigned to the triplet states of CB, ketyl radicals of CB, radical anions of CB and radical anions of CB and radical cations derived from the amino acids and peptides. The radical cations identified were intermolecularly (S.·.S) +-bonded cations, intramolecularly (S.·.N) +-bonded cations and an intramolecularly (S.·.S) +-bonded radical cation that was observed in experiments with MetMet. The quantum yields of the transients and their kinetics of formation and decay were measured by flash photolysis. The quantum yields of CO 2 formation were determined by steady state photolysis. Electron transfer from the sulfur atom to the triplet state of the ketone was found to be a primary photochemical step. A detailed mechanism of the CB-sensitized photo-oxidation of sulfur-containing amino acids and methionine-containing peptides, including primary and secondary photoreactions, is proposed and discussed. Within the mechanism, contrasting behavior between the peptides and amino acids as quenchers is emphasized.

Mechanism of 4-carboxybenzophenone-sensitized photooxidation of methionine-containing dipeptides and tripeptides in aqueous solution

The mechanism of 4-carboxybenzophenone (CB)-sensitized photooxidation of methionine-containing dipeptides (Met-Gly and Gly-Met) and tripeptides (Met-Gly-Gly, Gly-Met-Gly, and Gly-Gly-Met) was investigated using nanosecond flash photolysis and steady-state photolysis. The rate constants for quenching of the CB triplet by sulfur-containing peptides were determined to be in the range (1.8-2.3) x 10{sup 9} M{sup -1} S{sup -1} for neutral and alkaline solutions. The presence of the various electron-transfer intermediates accompanying the CB triplet quenching events was identified through the use of a multiple-regression procedure that was used to resolve the experimental transient spectra into components. The intermediates identified were the CB ketyl radical anion, the CB ketyl radical, intermolecularly (S.{sup {center_dot}}.S)-bonded radical cations, and intramolecularly (S.{sup {center_dot}}.N)-bonded radical cations derived from peptides. The types of intermediates were found to depend on the pH of the solution and on the location of the methionine unit with respect to the terminal functions. The quantum yields of all the transients and the kinetics of their formation and decay were measured by flash photolysis, and quantum yields of CO{sub 2} formation were measured by steady-state photolysis. 50 refs., 6 figs., 3 tabs.

Photoinduced Electron Transfer between Sulfur-Containing Carboxylic Acids and the 4-Carboxybenzophenone Triplet State in Aqueous Solution

The mechanism of photoinduced electron transfer was investigated using laser flash photolysis and steady-state photolysis techniques. Bimolecular rate constants for quenching of the CB triplet state by six sulfur-containing acids, with varying numbers of COO[sup [minus]] groups and varying locations with respect to the sulfur atom, were found to be in the range (0.3-2.1) x 10[sup 9]M[sup [minus]1] s[sup [minus]1] depending on the charge of the acid molecule. The observation of ketyl radical anions and intermolecular (S S)-bonded radical cations of some of the acids was direct evidence for the participation of electron transfer in the mechanism of quenching. An additional absorption band at approximately 410 nm in the transient absorption spectra for some of the acids was assigned to intramolecularly (S O)-bonded species (for acids for which a five-member ring structure was sterically favorable). Quantum yields of formation of intermediates from flash photolysis experiments and quantum yields of CO[sub 2] formation and CB disappearance from the steady-state measurements were determined. The values of these quantum yields clearly indicated that the diffusion apart (escape of the radical ions) of the charge-transfer complex, formed as a primary photochemical step, is the main photochemical pathway. 29 refs., 4 figs., 3 tabs.

Cleavage of the radical cations of alkenes; 1-butene and 4,4-dimethyl-1-pentene. Ab initio calculations on the interaction between the allyl and alkyl radical and carbocation moieties

Carbon–carbon bond cleavage of the radical cations of 1-butene [Formula: see text] and 4,4-dimethyl-1-pentene [Formula: see text] will generate the allyl and alkyl radical and carbocation fragments. Alternative bonding arrangements between the allyl and methyl moieties in [Formula: see text] and between the allyl and tert-butyl moieties in [Formula: see text] possible metastable intermediates or transition states preceding complete separation of the fragments, have been investigated by ab initio molecular orbital calculations. Structures were fully optimized at the UHF/6-31G* or UHF/STO-3G levels, and some of the calculations on [Formula: see text] were expanded with single point MP2/6-31G*//UHF/6-31G* computations. The C4H8+ radical cation, having a structure similar to that of 1-butene, is more stable than the separated fragments: 183 kj mol−1 lower in energy than the sum of the energies of the allyl cation and the methyl radical, and 385 kJ mol−1 lower than the sum of the energies of an allyl radical and a methyl cation, at the MP2/6-31G* level. The corresponding values at the UHF/STO-3G level are 276 and 415 kj mol−1, respectively. There is less bonding interaction between the allyl and tert-butyl moieties in [Formula: see text] The summation of the energies of the allyl radical and tert-butyl cation is 123 kj mol−1 lower than the summation of the energies of the allyl cation and tert-butyl radical, and 115 kJ mol−1 higher in energy than the bonded radical cation [Formula: see text] at the UHF/STO-3G level. These calculated values are compared with thermochemical data and with experimental results on the cleavage of these, and related, radical cations. Key words: radical cation, cleavage, ab initio calculations, electron transfer, photochemistry.

OH-induced oxidation of cyclo-met-met. A search for a complexed OH-radical

In the OH radical-induced oxidation of cyclo-Met-Met an OH adduct,cyclo-Met-Met(S∴OH), absorbing at γ max = 390 nm with ε 390cyclo-Met-Met(S∴OH) = 4500 dm 3 mol −1 cm −1 is formed. This adduct reversibly eliminates hydroxide ion, p K b = 5.4 ± 0.1, forming corresponding (S ∴ S)-bonded radical cation, cyclo-Met-Met(S ∴ S) +, which absorbs at γ max = 510 nm with ε 510cyclo-Met-Met(S ∴ S) + = 6200 ± 400 dm 3 mol -1 cm -1. The OH - elimination is catalysed by H + and buffers. The rate constants are k=(1.4 ± 0.1)×10 9dm 3mol −1 s −1 and k=(1.0±0.1)×10 4 s −1 for the H + catalysed and uncatalysed reactions, respectively. The rate constant for the reverse reaction is k[cyclo-Met-Met(S ∴ S) + + OH -] = (2.6 ± 0.3) × 10 9 dm 3 mol -1 s -1. A certain amount of cyclo-Met-Met(S |.: S) +, G ≈ 1, is formed immediately after the pulse, independently of the actual equilibrium position. Therefore, the existence of at least two different precursors of the cyclo-Met-Met(S ∴ S) + is inferred.

Reaction of a stable N ∴ N bonded radical cation with free radicals generated by pulse radiolysis: exceedingly rapid hydrogen abstraction from C–H bonds

Download options Please wait... Article information DOI https://doi.org/10.1039/P29860000277 Article type Paper Download Citation J. Chem. Soc., Perkin Trans. 2, 1986, 277-284 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Reaction of a stable N ∴ N bonded radical cation with free radicals generated by pulse radiolysis: exceedingly rapid hydrogen abstraction from C–H bonds R. W. Alder, M. Bonifačić and K. Asmus, J. Chem. Soc., Perkin Trans. 2, 1986, 277 DOI: 10.1039/P29860000277 To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Search articles by author Roger W. Alder Marija Bonifačić Klaus-Dieter Asmus