OH-Induced Oxidative Cleavage of Dimethyl Disulfide in the Presence of NO

A radical approach to radicals

Nitroxide radicals are characterized by a long-lived spin-unpaired electronic ground state and are strongly sensitive to their chemical surroundings. Combined with electron paramagnetic resonance spectroscopy, these electronic features have led to the widespread application of nitroxide derivatives as spin labels for use in studying protein structure and dynamics. Site-directed spin labelling requires the incorporation of nitroxides into the protein structure, leading to a new protein-ligand molecular model. However, in protein crystallographic refinement nitroxides are highly unusual molecules with an atypical chemical composition. Because macromolecular crystallography is almost entirely agnostic to chemical radicals, their structural information is generally less accurate or even erroneous. In this work, proteins that contain an example of a radical compound (Chemical Component Dictionary ID MTN) from the nitroxide family were re-refined by defining its ideal structural parameters based on quantum-chemical calculations. The refinement results show that this procedure improves the MTN ligand geometries, while at the same time retaining higher agreement with experimental data.

Two-step reaction mechanism reveals new antioxidant capability of cysteine disulfides against hydroxyl radical attack

Cysteine disulfides, which constitute an important component in biological redox buffer systems, are highly reactive toward the hydroxyl radical (^•OH). The mechanistic details of this reaction, however, remain unclear, largely due to the difficulty in characterizing unstable reaction products. Herein, we have developed a combined approach involving mass spectrometry (MS) and theoretical calculations to investigate reactions of ^•OH with cysteine disulfides (Cys-S-S-R) in the gas phase. Four types of first-generation products were identified: protonated ions of the cysteine thiyl radical (⁺Cys-S^•), cysteine (⁺Cys-SH), cysteine sulfinyl radical (⁺Cys-SO^•), and cysteine sulfenic acid (⁺Cys-SOH). The relative reaction rates and product branching ratios responded sensitively to the electronic property of the R group, providing key evidence to deriving a two-step reaction mechanism. The first step involved ^•OH conducting a back-side attack on one of the sulfur atoms, forming sulfenic acid (-SOH) and thiyl radical (-S^•) product pairs. A subsequent H transfer step within the product complex was favored for protonated systems, generating sulfinyl radical (-SO^•) and thiol (-SH) products. Because sulfenic acid is a potent scavenger of peroxyl radicals, our results implied that cysteine disulfide can form two lines of defense against reactive oxygen species, one using the cysteine disulfide itself and the other using the sulfenic acid product of the conversion of cysteine disulfide. This aspect suggested that, in a nonpolar environment, cysteine disulfides might play a more active role in the antioxidant network than previously appreciated.

The mechanism of the ozonolysis on the surface of C70 fullerene: the electron localizability indicator study

The formation of C₇₀O from C₇₀O₃ monomolozonide is a three-step process with the isomer dependent last step leading either to c,c-C₇₀O epoxide or d,d-C₇₀O oxidoannulene. The process involves the open intermediate (first O-O then C_c-C_c/C_d-C_d bonds broken), oxidoannulene-like structure intermediate (new C_c-O/C_d-O bond formed) and finally the oxide product. On the formation of c,c-C₇₀O isomer, the final release of O₂ is followed by the restoration of C_c-C_c bond, which stabilizes the product. Neither C_d-C_d bond is restored nor the total energy essentially lowered upon d,d-C₇₀O formation. At all steps of the studied process, the four CC bonds adjacent to C_c-C_c or C_d-C_d bond, respectively, play a crucial role donating or withdrawing the necessary electron density. C₇₀(O)O₂ products, with O₂ bridging one of the bonds adjacent to the parent C_c-C_c/C_d-C_d one, may compete with the oxide products. The OO bond in such structures is weak as suggested by its low electron population. For both c,c-C₇₀O₃ and d,d-C₇₀O₃, the shape of the potential energy surfaces (0 K) and the related, reported earlier, room temperature-free energy surfaces differ. Graphical abstract.

Hydrogen detachment driven by a repulsive 1πσ* state – an electron localization function study of 3-amino-1,2,4-triazole

Electron localization function analysis reveals the details of a charge induced hydrogen detachment mechanism of 3-amino-1,2,4-triazole, identified recently as responsible for phototautomerization of the molecule.

The OH-Initiated Oxidation of CS2 in the Presence of NO: FTIR Matrix-Isolation and Theoretical Studies

We studied the photochemistry of the carbon disulfide-nitrous acid system with the help of Fourier transform infrared (FTIR) matrix isolation spectroscopy and theoretical methods. The irradiation of the CS2···HONO complexes, isolated in solid argon, with the filtered output of the mercury lamp (λ > 345 nm) was found to produce OCS, SO2, and HNCS; HSCN was also tentatively identified. The (13)C, (15)N, and (2)H isotopic shifts as well as literature data were used for product identifications. The evolution of the measured FTIR spectra with irradiation time and the changes in the spectra after matrix annealing indicated that the identified molecules are the products of different reaction channels: OCS being a product of another reaction path than SO2 and HNCS or HSCN. The possible reaction channels between SC(OH)S/SCS(OH) radicals and NO were studied using DFT/B3LYP/aug-cc-pVTZ method. The SC(OH)S and/or SCS(OH) intermediates are formed when HONO attached to CS2 photodissociates into OH and NO. The calculations indicated that SC(OH)S radical can form with NO two stable adducts. The more stable SC(OH)S···NO structure is a reactant for a simple one-step process leading to OCS and HONS molecules. An alternative, less-stable complex formed between SC(OH)S and NO leads to formation of OCS and HSNO. The calculations predict only one stable complex between SCS(OH) radical and NO, which can dissociate along two channels leading to HNCS and SO2 or HSCN and SO2 as the end products. The identified photoproducts indicate that both SC(OH)S and SCS(OH) adducts are intermediates in the CS2 + OH + NO reaction leading to different reaction products.