DL-Piperidinium-2-carboxylate bis(hydrogen peroxide): unusual hydrogen-bonded peroxide chains

The title compound, C6H11NO2·2H2O2, is the richest (by molar ratio) in hydrogen peroxide among the peroxosolvates of aliphatic α-amino acids. The asymmetric unit contains a zwitterionic pipecolinic acid molecule and two hydrogen peroxide molecules. The two crystallographically independent hydrogen peroxide molecules form a different number of hydrogen bonds: one forms two as donor and two as acceptor ([2,2] mode) and the other forms two as donor and one as acceptor ([2,1] mode). The latter hydrogen peroxide molecule forms infinite hydrogen-bonded hydroperoxo chains running along the c-axis direction, which is unusual for aliphatic α-amino acid peroxosolvates.

Stabilization of hydrogen peroxide by hydrogen bonding in the crystal structure of 2-aminobenzimidazole perhydrate

2-Aminobenzimidazole peroxosolvate – the third H2O2 crystalline adduct stabilized with the maximum possible number of hydrogen bonds formed by one hydrogen peroxide molecule.

Crystallographic study of dioxygen chemistry in a copper-containing nitrite reductase fromGeobacillus thermodenitrificans

Copper-containing nitrite reductases (CuNIRs) are multifunctional enzymes that catalyse the one-electron reduction of nitrite (NO2−) to nitric oxide (NO) and the two-electron reduction of dioxygen (O2) to hydrogen peroxide (H2O2). In contrast to the mechanism of nitrite reduction, that of dioxygen reduction is poorly understood. Here, results from anaerobic synchrotron-radiation crystallography (SRX) and aerobic in-house radiation crystallography (iHRX) with a CuNIR from the thermophileGeobacillus thermodenitrificans(GtNIR) support the hypothesis that the dioxygen present in an aerobically manipulated crystal can bind to the catalytic type 2 copper (T2Cu) site ofGtNIR during SRX experiments. The anaerobic SRX structure showed a dual conformation of one water molecule as an axial ligand in the T2Cu site, while previous aerobic SRXGtNIR structures were refined as diatomic molecule-bound states. Moreover, an SRX structure of the C135A mutant ofGtNIR with peroxide bound to the T2Cu atom was determined. The peroxide molecule was mainly observed in a side-on binding manner, with a possible minor end-on conformation. The structures provide insights into dioxygen chemistry in CuNIRs and hence help to unmask the other face of CuNIRs.

OXIDATION OF FERRIC XYLENOL ORANGE CHELATES BY HYDROGEN PEROXIDE IN AQUEOUS SOLUTION: CONCEPTION OF OXYGEN SINGLET ATOMS GENERATION FROM HYDROGEN PEROXIDE

We observed for the first time the reaction of oxidation of ferric xylenol orange chelates by hydrogen peroxide in aqueous solution. The reaction is accompanied with decoloration of the violet aqueous solution. Based on generally accepted conception, there is a process of free radical chain oxidation of indicator molecule in the solution. However, after investigating the final colorless solution by 1H NMR-spectroscopy we found the modified but not broken structure in which the initial hydrocarbon core remained mainly unchanged. We concluded that kind of reaction was an oxyfunctionalization by hydrogen peroxide versus free radical chain destruction. We argued steps of the reaction such as N-oxidation, Cope’s elimination, and certain rearrangements with possible products oligomerization. There was a need to explain the mechanism of interaction between the ferric iron ion and the hydrogen peroxide molecule and to argue the nature of intermediate reactive oxygen species. There is similarity between the ferric-catalyzed hydroperoxide xylenol orange oxidation and the peroxygenase-catalyzed biochemical oxyfunctionalization reactions. However, based on literature data and molecular orbital modeling, we proposed another mechanism of interaction between the ferric iron ion and the hydrogen peroxide molecule instead the tetravalent iron generation. Concretely, we proposed the hydrogen peroxide zwitter-ionization (isomerization to oxywater molecule) and subsequent intramolecular disproportionation with generation of a water molecule and a singlet oxygen atom as a reactive oxygen species. In this view, the iron ion oxidation state is unchanged during the reaction and remains ferric. An oxyfunctionalization of any organic substrate by hydrogen peroxide in the presence of ferric iron ions is promising approach in organic synthesis. However, the usage of organic ligands for ferric iron ions as components of catalysts is limited and requires only non-oxidizable compounds. On the other hand, one can choose an oxidation substrate as a ligand for ferric iron ions that is the formation of chelate complex of ferric catalyst with an organic substrate.Forcitation:Chumakov A.A., Kotelnikov O.A., Slizhov Yu.G. Oxidation of ferric xylenol orange chelates by hydrogen peroxide in aqueous solution: conception of oxygen singlet atoms generation from hydrogen peroxide. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2018. V. 61. N 2. P. 15-22

VERIFICATION OF NON-CATALYTIC HYDROGEN PEROXIDE DISPROPORTIONATION MECHANISM BY THERMODYNAMIC ANALYSIS OF ONE-ELECTRON REDOX REACTIONS

There is two-electron transfer during the process of hydrogen peroxide decomposition into water and oxygen. The detailed mechanism of non-catalytic hydrogen peroxide disproportionation is not verified until now. We assumed that any poly-electron redox process is a complex and consists of one-electron redox reactions. We have formulated equations of possible one-electron transfers during hydrogen peroxide disproportionation. Based on known laws and equations of thermochemistry we calculated standard thermodynamic functions for a total reaction and each one-elect-ron redox reaction using reference values of standard thermodynamic functions of reagents and products of reactions. Results show that the total reaction leads to significant decrease in Gibbs free energy -246.0 kJ/mol in gas phase but there is increase +39.9 kJ/mol in Gibbs free energy during the first proposed step. It is substantiation for known dependence of hydrogen peroxide dismutation kinetics at thermal, photochemical or catalytic activation. The first proposed step of non-catalytic process is one-electron plus one-proton transfer in thermally or photochemically activated dimeric hydrogen peroxide associate (H2O2)2 with simultaneous generation of hydroperoxyl HO2• and hydroxyl HO• free radicals and water molecule. There is thermodynamic argumentation for radical chain mechanism of hydrogen peroxide disproportionation after the activation. We made the graphic illustration of thermodynamically supported scheme of non-catalytic hydrogen peroxide decomposition. There is a cyclic alternation of two radical-molecular interactions during the hydrogen peroxide chain decomposition. The hydroxyl radical generates the hydroperoxyl radi-cal from a hydrogen peroxide molecule and then the hydroperoxyl radical interacts with a next hydrogen peroxide molecule followed by the hydroxyl radical generation. Interactions between the homonymic or heteronymic free radicals are the reactions of chain breaking.Forcitation:Chumakov A.A., Batalova V.N., Slizhov Yu.G., Minakova T.S. Verification of non-catalytic hydrogen peroxide disproportionation mechanism by thermodynamic analysis of one-electron redox reactions. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2017. V. 60. N 6. P. 40-44.

PEROXIDE CROSS-LINKING OF EPDMs HAVING HIGH FRACTIONS OF ETHYLENIC UNITS

ABSTRACT The considerable amount of research in the literature has practically allowed the elucidation of the mechanism of peroxide cross-linking of ethylene–propylene–diene–monomer rubber (EPDM), which occurs through a radical chain reaction initiated by the thermal decomposition of the peroxide molecule. According to this radical chain reaction, all types of labile hydrocarbon bonds (i.e., allylic, methynic, and methylenic CH bonds) would be exposed to alkoxy radicals and involved in the formation of the elastomeric network. However, for high fractions of ethylenic units (typically ≥60 mol.%), simple chemical kinetics and thermochemical analyses have shown that the radical attack would essentially occur on the methylenic CH bonds. Starting from this assertion, a simplified mechanistic scheme has been proposed for the three commercial EPDMs under study. The corresponding kinetic model, derived from this new scheme by using the basic concepts of the chemical kinetics, provides access to the changes in concentration of the main reactive chemical functions (against exposure time), among which are double bonds and changes in cross-linking density. The validity of these predictions has been eventually successfully verified by five distinct analytical techniques frequently used for studying the cross-linking of rubbers.

Generation of hydrogen peroxide on a pyridine-like nitrogen-nickel doped graphene surface

ABSTRACTDensity functional theory and molecular dynamics were used to study the generation of hydrogen peroxide around a nickel atom anchored on a pyridine-like nitrogen-doped graphene (PNG) layer. First, we found that two hydrogen molecules are adsorbed around the nickel atom, with adsorption energy 0.95 eV/molecule. Then we studied the interaction of oxygen molecules with this system at atmospheric pressure and 300 K. It is found that two hydrogen peroxide molecules are formed. However, at 700 K, one hydrogen peroxide molecule, and one water molecule are desorbed. One oxygen atom stays bound to the nickel atom.

Vibronic Origin of the “SKEWED” Anticline Configuration of the Hydrogen Peroxide Molecule

The vibronic origin of instability of the symmetrical forms (D¥ h, C2h and C2v) of the hydrogen peroxide molecule H2O2 was revealed using ab initio calculations of the electronic structure and the adiabatic potential energy curves. The vibronic constants in this approach were estimated by fitting of the ab initio calculated adiabatic potential in the vicinity of the high-symmetry nuclear configurations to its analytical expression. It was shown that the equilibrium “skewed” anticline shape of the C2 symmetry can be realized in two ways: D¥h ® C2v® C2 or D¥h ® C2h® C2 with the decreasing of the adiabatic potential energy at every step.

Detection of Structural Change of Superoxide Dismutase in Solution

We have confirmed that dissociation of the dimeric SOD molecule into a monomeric one can be readily detected in solution by the use of capillary electrophoresis (CE), which is based on the fact that the peak height in the CE profile is highly dependent on the aggregation conditions of the protein molecule. Based on this fact, it has become apparent that the hydrogen peroxide molecule induces the dissociation of the dimeric structure of SOD, and this should give reasonable explanation for the inactivation of SOD by hydrogen peroxide. Our results may give a convenient way for the early detection of the amyotrophic lateral sclerosis in patients, because we can estimate whether the SOD molecule is of a rigid or loosed dimeric structure by the use of this technique. The loosed one has been assumed to exhibit inherent toxicity of the copper center, so-called “gain-of-function” of the mutant SOD.