Computational Characterization of Sulfur−Oxygen-Bonded Sulfuranyl Radicals Derived from Alkyl- and (Carboxyalkyl)thiopropionic Acids: Evidence for σ*-Type Radicals (original) (raw)
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Sulfonyl radicals, sulfinic acid, and related species: an ab initio molecular orbital study
Canadian Journal of Chemistry, 1980
. Extensive a b initio molecular orbital calculations on six sulfonyl radicals (XSO, where X = H, CH,, NH,, OH, F, and Cl), the simplest sulfinic acid HS02H3 and its isomeric sulfone H2S02, the HS0,-anion of sulfinic acid, the isomeric anion S02H-. and for completeness. the S0,H radical are presented. By use of the STO-3G* basis set, which includes d-type polarization functions on second-row atoms, all geometrical parameters are varied until the total energy is minimized. subject only to certain symmetry restrictions specified for each system. The inclusion of d orbitals on S is observed to affect the S-0 bond lengths by as much as 0.45 A. The calculations suggest that the radical site in sulfonyl radicals is significantly delocalized over the entire functional group and that the geometrical parameters of the SO, functional group in sulfonyl radicals are nearly independent of the substituent (r(S-0) = 1.47 + 0.01 A, < OSO = 123 + 2"). Estimates of the X-S bond energy in CH,SO,, NH2S02, and OHSO, are consistent with the chemistry of alkylsulfonyl and aminosulfonyl radicals and lead to an interesting prediction for alkoxylsulfonyl radicals. Furthermore the calculations yield lower total energies for HSO,H, HS02-, and S 0 2 H than for the respective isomeric forms H2S02, S02H-, and H S 0 2 .
The Journal of Physical Chemistry A, 2004
The mechanism of the • OH-induced oxidation of S-ethylthioacetate (SETAc) and S-ethylthioacetone (SETA) was investigated in aqueous solution using pulse radiolysis and steady-state γ radiolysis combined with chromatographic and ESR techniques. For each compound, • OH radicals were added, as an initial step, to the sulfur moiety, forming hydroxysulfuranyl radicals. Their subsequent decomposition strongly depended on the availability of Ror -positioned acetyl groups, pH, and the thioether concentration. For SETAc, which contains the R-positioned acetyl group, hydroxysulfuranyl radicals SETAc(> • S-OH) subsequently decay into secondary products, which do not include intermolecularly three-electron-bonded dimeric radical cations, even at high concentrations of SETAc. At low pH, these observations are rationalized in terms of the highly unstable nature of sulfur monomeric radical cations SETAc(>S +• ) because of their rapid conversion via deprotonation to the R-(alkylthio)alkyl radicals H 3 C-• CH-S-C(dO)-CH 3 (λ max ) 420 nm). However, at low proton concentrations, the R-positioned acetyl group destabilizes SETAc(> • S-OH) radicals within a five-membered structure that leads to the formation of alkyl-substituted radicals, H 3 C-CH 2 -S-C(dO)-• -CH 2 . A somewhat different picture is observed for SETA, which contains a -positioned acetyl group. The main pathway involves the formation of hydroxysulfuranyl radicals SETA(> • S-OH) and R-(alkylthio)alkyl radicals H 3 C-CH 2 -S-• CH-C(dO)-CH 3 (λ max ) 380 nm). The latter radicals are highly stabilized through the combined effect of both substituents in terms of the captodative effect. At low pH, SETA(> • S-OH) radicals undergo efficient conversion to intermolecularly three-electron-bonded dimeric radical cations SETA-(>S∴S<) + (λ max ) 500 nm), especially for high SETA concentrations. In contrast, at low proton concentrations, SETA(> • S-OH) radicals decompose via the elimination of water, formed through intramolecular hydrogen transfer within a six-membered structure that leads to the formation of alkyl-substituted radicals, H 3 C-CH 2 -S-CH 2 -C(dO)-• CH 2 . The latter radicals undergo a 1,3-hydrogen shift and intramolecular hydrogen abstraction within the six-membered structure, leading to the R-(alkylthio)alkyl radicals H 3 C-CH 2 -S-• CH-C(dO)-CH 3 and H 3 C-• CH-S-CH 2 -C(dO)-CH 3 , respectively. To support our conclusions, quantum mechanical calculations were performed using density functional theory (DFT-B3LYP) and second-order Møller-Plesset perturbation theory (MP2) to calculate the bond-formation energies of some key transients and the location and strength of their associated optical absorptions. * Corresponding we have extended our studies to the reactions of the • OH radicals with two model thioether compounds, S-ethylthioacetate and S-ethylthioacetone, containing acetyl groups in the R and positions with respect to the sulfur atom, respectively. Furthermore, to support our conclusions, we performed quantum mechanical ab initio calculations by using density functional theory (DFT-B3LYP) and second-order Møller-Plesset perturbation theory (MP2) to calculate the bond formation energies of some of the key transients and the location and strength of their associated optical absorptions.
SSOH and HSSO Radicals: An Experimental and Theoretical Study of [S 2 OH] 0/+/- Species
The Journal of Physical Chemistry A, 2007
New radicals containing sulfur-sulfur bonds are detected in the gas phase: disulfur hydroxide SSOH and thiosulfenoxide HSSO, stable toward dissociation by ca. 50 and 40 kcal mol -1 , respectively. Energetic, structural features and fragmentation pathways of these [S 2 OH] radicals and their charged species [S 2 OH] + and [S 2 OH]are experimentally and computationally investigated by mass spectrometric techniques and ab initio calculations.
Chemical Physics Letters, 2011
Two-center three-electron anions have been reported as a pitfall case for density functional theory, owing to their important dynamical correlation and the inherent difficulty of a balanced description of odd-vs. even-number electron bonds. The critical roles of the correlation functional, and of the amount of exact Hartree-Fock exchange included are investigated in order to pinpoint the most efficient approach for simulating disulfide radical anions (ease of formation and inter-sulfur distances). Several recently-developed functionals, such as B1B95, BMK, B2PLYP(D) and numerous range-separated hybrid yield a quantitatively correct description, and offer a pragmatic and superior solution to the Becke's half-and-half usually recommended for this delicate situation.
International Journal of Molecular Sciences
Hydroxyl radicals (HO•) have long been regarded as a major source of cellular damage. The reaction of HO• with methionine residues (Met) in peptides and proteins is a complex multistep process. Although the reaction mechanism has been intensively studied, some essential parts remain unsolved. In the present study we examined the reaction of HO• generated by ionizing radiation in aqueous solutions under anoxic conditions with two compounds representing the simplest model peptide backbone CH3C(O)NHCHXC(O)NHCH3, where X = CH2CH2SCH3 or CH2SCH3, i.e., the Met derivative in comparison with the cysteine-methylated derivative. We performed the identification and quantification of transient species by pulse radiolysis and final products by LC–MS and high-resolution MS/MS after γ-radiolysis. The results allowed us to draw for each compound a mechanistic scheme. The fate of the initial one-electron oxidation at the sulfur atom depends on its distance from the peptide backbone and involves tra...
Conformational Influence on the Type of Stabilization of Sulfur Radical Cations in Cyclic Peptides
ChemPhysChem, 2007
The free‐radical chemistry of two oxidized cyclic dipeptides is investigated using time‐resolved optical and conductivity detection. Two cyclic dipeptides, cyclo‐Gly‐L‐Met and cyclo‐D‐Met‐L‐Met, are synthesized and irradiated with nanosecond pulses of electrons, which initiate the oxidation of the methionine side chains with hydroxyl radicals from the radiolysis of water. The cyclic peptides are taken to be models for the interior of proteins where there are no terminal groups. This opens up the possibility that neighboring‐group effects can be studied directly between the initially formed sulfur radical cations and the heteroatoms associated with the peptide bonds. Such complexation of the sulfur radical cations is observed with the amide nitrogen atoms. In addition, intermolecular stabilization with the unoxidized sulfur atoms on separate cyclic dipeptide molecules is observed. Little or no intramolecular stabilization by the unoxidized sulfur in the neighboring methionine occurs ...
The journal of physical chemistry. B, 2017
The gas-phase structural modifications induced by the oxidation of methionine of two peptides of reverse sequence, methionine-valine (Met-Val) and valine-methionine (Val-Met), have been studied by mass-selected infrared multiple photon dissociation (IRMPD) spectroscopy in the 800-2000 cm-1 fingerprint range at the Centre Laser Infrarouge d'Orsay (CLIO) free electron laser facility. Oxidation has been achieved by •OH radicals generated by γ-radiolysis. IRMPD spectra were interpreted by static and harmonic DFT calculations and Born-Oppenheimer molecular dynamics simulations, employed to take into account all anharmonic and finite temperature effects. The diagnostic signature of the sulfoxide group in the final products of Met-Val and Val-Met oxidation, which is missing in the spectra of native peptides, has been recorded. Evidence has also been gathered that a mixture of R and S isomers of close energies is formed. Interconversion between different isomers has been unveiled in the...