Formation Of Initiating Radicals Research Articles

Evaluating UV254 absorbance reductions in landfill leachate for municipal sewage co-treatment through timed UV/electrooxidation

Landfill leachate contains dissolved organic matter (DOM) exhibiting high ultraviolet absorbance at 254 nm (UVA254). The UVA254 limits leachate co-treatment with municipal sewage by hindering the downstream UV disinfection efficiency at wastewater treatment plants. Here, we alleviated the UVA254 by timing the radiation in a UV/electrooxidation (UV/EO) process to accelerate reactive species formation. At 200 A·m−2, the UV radiation was delayed by 10 min to accumulate 21 mg·L−1 as Cl2, which enhanced the initial radical formation rate by 5.25 times compared with a simultaneous UV/EO. The timed operation increased the steady-state concentrations of ClO• by 700 times to 4.11 × 10−14 M and reduced the leachate UVA254 by 78.2% after 60 min. We identified that aromatic formulas with low oxygen content were susceptible to UV/EO from Fourier-transform ion cyclotron resonance mass spectrometry analysis. The toxicity of the treated leachate and generated byproducts was assessed through specific oxygen uptake rates (SOUR) and developmental assays with Platynereis dumerilii. After quenching the residual chlorine, leachate co-treatment at 3.5% v/v presented minimal toxicological risk. Our findings provide operational insights for applying UV/EO in high UVA254 matrices such as landfill leachate.

Ne-22 Ion-Beam Radiation Damage to DNA: From Initial Free Radical Formation to Resulting DNA-Base Damage.

We report on the physicochemical processes and the products of DNA damage involved in Ne-22 ion-beam radiation of hydrated (12 ± 3 H2O/nucleotide) salmon testes DNA at 77 K. Free radicals trapped at 77 K were identified using electron spin resonance (ESR) spectroscopy. The measurement of DNA damage using two different techniques of mass spectrometry revealed the formation of numerous DNA products. Results obtained by ESR spectroscopy showed that as the linear energy transfer (LET) of the ion-beam radiation increases along the beam track, the production of DNA radicals correspondingly increases until just before the Bragg peak is reached. Yields of DNA products along the ion-beam track were in excellent agreement with the radical production. This work is the first to use the combination of ESR spectroscopy and mass spectrometric techniques enabling a better understanding of mechanisms of radiation damage to DNA by heavy ion beams detailing the formation of DNA free radicals and their subsequent products.

Facile access to nitroalkanes: Nitration of alkanes by selective C[sbnd]H nitration using metal nitrate, catalyzed by in-situ generated metal oxide

Direct C − H functionalization of inactive alkanes is an important strategy to streamline the preparation of functional molecules. Herein, we describe an operationally simple and effective alkane C − H nitration reaction to access versatile nitroalkanes without cleavage of the C − C skeleton. Nontoxic and inexpensive metal nitrate (Fe(NO3)3·9H2O) plays a dual role as catalyst precursors as well as nitro sources for the transformation. Experimental evidence and theoretical modeling have shown the formation of iron oxide as a key catalytic species for the alkane C − H and NO2 activation, which favors a stepwise radical mechanism with initial alkyl radical formation.

Simulation of Laser-Induced Plasma Ignition in a Hypersonic Crossflow

A numerical study of a laser-induced plasma ignition system in a scramjet-like geometry is conducted. Simulations are based on experiments in which hydrogen is injected into a hypersonic air-crossflow, compressed by a ramp from a freestream Mach number of 8 to below hydrogen autoignition conditions. Ignition of the jet is attempted using a laser pulse, modeled in the simulations as a small region of high temperature and pressure corresponding to the energy input measured in the experiments. To capture the highly unsteady nature of the fuel-air interface and its coupling with the dynamics of the expanding laser spark, wall-modeled large-eddy Simulations are used to resolve the majority of turbulent motion. Thermal nonequilibrium and detailed finite-rate chemistry models are included in the calculations. The simulations indicate that the cloud of hydroxyl radicals detected in the experiments is created on the jet’s forward surface, as the expanding plasma kernel drives a blast wave into the bowshock surrounding the jet. This event raises the temperature and pressure in the upstream mixing layer, causing spontaneous ignition of the mixture. Within tens of microseconds the laser-induced combustion detaches from the jet and combustion ceases due to unfavorable conditions for flame anchoring in this area. Downstream of the jet, the simulations suggest that the plasma kernel remains can initiate radical formation in the wake region, but no stable flame forms due to nominally low pressures in the experimental condition studied.

The Formation Time of Ti-O• and Ti-O•-Ti Radicals at the n-SrTiO3/Aqueous Interface during Photocatalytic Water Oxidation.

The initial step of photocatalytic water oxidation reaction at the metal oxide/aqueous interface involves intermediates formed by trapping photogenerated, valence band holes on different reactive sites of the oxide surface. In SrTiO3, these one-electron intermediates are radicals located in Ti-O• (oxyl) and Ti-O•-Ti (bridge) groups arranged perpendicular and parallel to the surface respectively, and form electronic states in the band gap of SrTiO3. Using an ultrafast sub band gap probe of 400 nm and white light, we excited transitions between these radical states and the conduction band. By measuring the time evolution of surface reflectivity following the pump pulse of 266 nm light, we determined an initial radical formation time of 1.3 ± 0.2 ps, which is identical to the time to populate the surface with titanium oxyl (Ti-O•) radicals. The oxyl was separately observed by a subsurface vibration near 800 cm-1 from Ti-O located in the plane right below Ti-O•. Second, a polarized transition optical dipole allows us to assign the 1.3 ps time constant to the production of both O-site radicals. After a 4.5 ps delay, another distinct surface species forms with a time constant of 36 ± 10 ps with a yet undetermined structure. As would be expected, the radicals' decay, specifically probed by the oxyl's subsurface vibration, parallels that of the photocurrent. Our results led us to propose a nonadiabatic kinetic mechanism for generating radicals of the type Ti-O• and Ti-O•-Ti from valence band holes based on their solvation at aqueous interfaces.

Understanding the Reaction Chemistry of 2,2′:5′,2″-Terthiophene Films with Vapor-Deposited Ag, Al, and Ca

The reaction chemistry of vapor-deposited 2,2′:5′,2″-terthiophene (α-3T) solid-state thin films with postdeposited Ag, Al, and Ca is investigated in ultrahigh vacuum using Raman spectroscopy. Vapor-deposited Ag forms nanoparticles on these films and induces considerable surface enhanced Raman scattering (SERS) along with a change in molecular symmetry of adjacent α-3T and formation of Ag–S bonds; no other reaction chemistry is observed. Vapor-deposited Al and Ca undergo chemical reaction with α-3T initiated by metal-to-α-3T electron transfer. For Al, the resulting product is predominantly amorphous carbon through initial radical formation and subsequent decomposition reactions. For Ca, the spectral evidence suggests two pathways: one leading to α-3T polymerization and the other resulting in thiophene ring opening, both initiated by radical formation through Ca-to-α-3T electron transfer. These interfacial reactions reflect the complex chemistry that can occur between low work function metals and thiophene-based oligomers. This reactivity is strongly correlated with metal work function.

Radical Mechanism of Isocyanide-Alkyne Cycloaddition by Multicatalysis of Ag2CO3, Solvent, and Substrate

A combined DFT and experimental study was performed to reveal the mechanism of isocyanide-alkyne cycloaddition. Our results indicate that the mechanism of this valuable reaction is an unexpected multicatalyzed radical process. Ag2CO3 is the pivotal catalyst, serving as base for the deprotonation of isocyanide and oxidant to initiate the initial isocyanide radical formation. After the cycloaddition between isocyanide radical and silver-acetylide, substrate (isocyanide) and solvent (dioxane) replace the role of Ag2CO3. They act as a radical shuttle to regenerate isocyanide radical for the next catalytic cycle, simultaneously completing the protonation. Furthermore, the bulk solvent effect significantly increases the reactivity by decreasing the activation barriers through the whole reaction, serving as solvent as well as catalyst.

Atropisomerism of the Asn α Radicals Revealed by Ramachandran Surface Topology

C radicals are typically trigonal planar and thus achiral, regardless of whether they originate from a chiral or an achiral C-atom (e.g., C-H + (•)OH → C• + H2O). Oxidative stress could initiate radical formation in proteins when, for example, the H-atom is abstracted from the Cα-carbon of an amino acid residue. Electronic structure calculations show that such a radical remains achiral when formed from the achiral Gly, or the chiral but small Ala residues. However, when longer side-chain containing proteogenic amino acid residues are studied (e.g., Asn), they provide radicals of axis chirality, which in turn leads to atropisomerism observed for the first time for peptides. The two enantiomeric extended backbone structures, •βL and •βD, interconvert via a pair of enantiotopic reaction paths, monitored on a 4D Ramachandran surface, with two distinct transition states of very different Gibbs-free energies: 37.4 and 67.7 kJ/mol, respectively. This discovery requires the reassessment of our understanding on radical formation and their conformational and stereochemical behavior. Furthermore, the atropisomerism of proteogenic amino acid residues should affect our understanding on radicals in biological systems and, thus, reframes the role of the D-residues as markers of molecular aging.

Mechanism of antioxidant interaction on polymer oxidation by thermal and radiation ageing

The mechanism of polymer oxidation by radiation and thermal ageing was investigated for the life evaluation of cables installed in radiation environments. The antioxidant as a stabilizer was very effective for thermal oxidation with a small content in polymers, but was not effective for radiation oxidation. The ionizing radiation induced the oxidation to result in chain scission even at low temperature, because the free radicals were produced and the antioxidant could not stop the oxidation of radicals with the chain scission. A new mechanism of antioxidant effect for polymer oxidation was proposed. The effect of antioxidant was not the termination of free radicals in polymer chains such as peroxy radicals, but was the depression of initial radical formation in polymer chains by thermal activation. The antioxidant molecule was assumed to delocalize the activated energy in polymer chains by the Boltzmann statics (distribution) to result in decrease in the probability of radical formation at a given temperature. The interaction distance (delocalization volume) by one antioxidant molecule was estimated to be 5–10nm by the radius of sphere in polymer matrix, though the value would depend on the chemical structure of antioxidant.

Detection and imaging of the free radical DNA in cells--Site-specific radical formation induced by Fenton chemistry and its repair in cellular DNA as seen by electron spin resonance, immuno-spin trapping and confocal microscopy

Oxidative stress-related damage to the DNA macromolecule produces lesions that are implicated in various diseases. To understand damage to DNA, it is important to study the free radical reactions causing the damage. Measurement of DNA damage has been a matter of debate as most of the available methods measure the end product of a sequence of events and provide limited information on the initial free radical formation. We report a measurement of free radical damage in DNA induced by a Cu(II)-H2O2 oxidizing system using immuno-spin trapping supplemented with electron paramagnetic resonance. In this investigation, the short-lived radical generated is trapped by the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) immediately upon formation. The DMPO adduct formed is initially electron paramagnetic resonance active, but is subsequently oxidized to the stable nitrone adduct, which can be detected and visualized by immuno-spin trapping and has the potential to be further characterized by other analytical techniques. The radical was found to be located on the 2′-deoxyadenosine (dAdo) moiety of DNA. The nitrone adduct was repaired on a time scale consistent with DNA repair. In vivo experiments for the purpose of detecting DMPO–DNA nitrone adducts should be conducted over a range of time in order to avoid missing adducts due to the repair processes.

Photo-ribonucleotide reductase β2 by selective cysteine labeling with a radical phototrigger

Photochemical radical initiation is a powerful tool for studying radical initiation and transport in biology. Ribonucleotide reductases (RNRs), which catalyze the conversion of nucleotides to deoxynucleotides in all organisms, are an exemplar of radical mediated transformations in biology. Class Ia RNRs are composed of two subunits: α2 and β2. As a method to initiate radical formation photochemically within β2, a single surface-exposed cysteine of the β2 subunit of Escherichia coli Class Ia RNR has been labeled (98%) with a photooxidant ([Re]=tricarbonyl(1,10-phenanthroline)(methylpyridyl)rhenium(I)). The labeling was achieved by incubation of S355C-β2 with the 4-(bromomethyl)pyridyl derivative of [Re] to yield the labeled species, [Re]-S355C-β2. Steady-state and time-resolved emission experiments reveal that the metal-to-ligand charge transfer (MLCT) excited-state (3)[Re](∗) is not significantly perturbed after bioconjugation and is available as a phototrigger of tyrosine radical at position 356 in the β2 subunit; transient absorption spectroscopy reveals that the radical lives for microseconds. The work described herein provides a platform for photochemical radical initiation and study of proton-coupled electron transfer (PCET) in the β2 subunit of RNR, from which radical initiation and transport for this enzyme originates.

Principal Limitations in Homogeneous Gas Phase Chemistry in Non-Thermal Plasmas

This study highlights the oxidation of H2, CH4, and HCl present in the range of some volume percent in a homogeneous O2 or air phase in a flow through glass barrier discharge reactor. The oxidation of all three compounds is highly exothermic and exergonic at ambient temperature and proceeds at sufficiently high temperatures as radical chain reaction. The conversion of each compound was below 10% in a non-thermal oxygen plasma under various reaction conditions. Increasing concentrations of H2 and CH4 above the lower explosion limit did not lead to higher conversion degrees. It is assumed that only initial radical formation by electron impact dissociation and exothermic steps within the chain process run in a sufficiently fast manner at ambient temperature. For endothermic steps within the radical chain, the necessary activation energy is not available and the chain reaction aborts, most likely, after formation of peroxyl (hydro- or methyl-peroxyl) radicals.

Thioxanthone-ethylcarbazole as a soluble visible light photoinitiator for free radical and free radical promoted cationic polymerizations

Thioxanthone-ethylcarbazole (TX-EC), 7-ethylthiochromeno[2,3-b]carbazol-13(7H)-one, as a novel visible light absorbing one-component Type II photoinitiator with high solubility, was synthesized and characterized. Its ability to initiate free radical photopolymerization of methyl methacrylate (MMA) was demonstrated and compared with that of the parent thioxanthone-carbazole (TX-C) both in the presence and absence of a co-initiator. It is found that TX-EC is a more effective photoinitiator in all cases. The mechanism of initiating radical formation for free radical polymerization in the absence of an added co-initiator involves photoexcitation of TX-EC and hydrogen abstraction of triplet states of TX-EC from the ethyl group attached to the carbazole moiety. In the presence of triethyl amine, the corresponding hydrogen abstraction reaction occurs between a triplet photoinitiator and the amine compound. The possibility of visible light photoinitiation of free radical promoted cationic polymerization of several monomers, namely cyclohexene oxide (CHO), n-butyl vinylether (BVE), N-vinyl carbazole (NVC) and 3,4-epoxycyclohexyl-3′,4′-epoxycyclohexene carboxylate (EEC) using TX-EC and Ph2I+PF6− combination was also demonstrated.

Photocatalytic Properties of Tin Oxide and Antimony-Doped Tin Oxide Nanoparticles

For the first time it is shown that N-doped SnO2nanoparticles photocatalyze directly the polymerization of the C=C bonds of (meth)acrylates under visible light illumination. These radical polymerizations also occur when these particles are doped with Sb and when the surfaces of these particles are grafted with methacrylate (MPS) groups. During irradiation with visible or UV light the position and/or intensity of the plasmon band absorption of these nanoparticles are always changed, suggesting that the polymerization starts by the transfer of an electron from the conduction band of the particle to the (meth)acrylate C=C bond. By using illumination wavelengths with a very narrow band width we determined the influence of the incident wavelength of light, the Sb- and N-doping, and the methacrylate (MPS) surface grafting on the quantum efficiencies for the initiating radical formation (Φ) and on the polymer and particle network formation. The results are explained by describing the effects of Sb-doping, N-doping, and/or methacrylate surface grafting on the band gaps, energy level distributions, and surface group reactivities of these nanoparticles. N-doped (MPS grafted) SnO2(Sb ≥ 0%) nanoparticles are new attractive photocatalysts under visible as well as UV illumination.

Histidine Residue Mediates Radical-induced Hinge Cleavage of Human IgG1

Hydroxyl radicals induce hinge cleavage in a human IgG1 molecule via initial radical formation at the first hinge Cys(231) followed by electron transfer to the upper hinge residues. To enable engineering of a stable monoclonal antibody hinge, we investigated the role of the hinge His(229) residue using structure modeling and site-directed mutagenesis. Direct involvement of His(229) in the reaction mechanism is suggested by a 75-85% reduction of the hinge cleavage for variants in which His(229) was substituted with either Gln, Ser, or Ala. In contrast, mutation of Lys(227) to Gln, Ser, or Ala increased hinge cleavage. However, the H229S/K227S double mutant shows hinge cleavage levels similar to that of the single H229S variant, further revealing the importance of His(229). Examination of the hinge structure shows that His(229) is capable of forming hydrogen bonds with surrounding residues. These observations led us to hypothesize that the imidazole ring of His(229) may function to facilitate the cleavage by forming a transient radical center that is capable of extracting a proton from neighboring residues. The work presented here suggests the feasibility of engineering a new generation of monoclonal antibodies capable of resisting hinge cleavage to improve product stability and efficacy.

Mechanism of 3,4-dihydroxybenzaldehyde electropolymerization at carbon paste electrodes: catalytic detection of NADH

Cyclic voltammetry was used to study 3,4-dihydroxybenzaldehyde (3,4-DHB) electropolymerization processes on carbon paste electrodes. The characteristics of the electropolymerized films were highly dependent on pH, anodic switching potential, scan rate, 3,4-DHB concentrations and number of cycles. Film stability was determined in citrate/phosphate buffer solutions at the same pH used during the electropolymerization process. The best conditions to prepare carbon paste modified electrodes were pH 7.8; 0.0 <= Eapl <= 0.25 V; 10 mV s-1; 0.25 mmol L-1 3,4-DHB and 10 scans. These carbon paste modified electrodes were used for NADH catalytic detection at 0.23 V in the range 0.015 <= [NADH] <= 0.21 mmol L-1. Experimental data were used to propose a mechanism for the 3,4--DHB electropolymerization processes, which involves initial phenoxyl radical formation.

Generation of nitroxyl radicals in reactions of nitrogen dioxide with p-benzoquinones

The formation of oxynitroxyl radicals has been observed by EPR in the course of exposure of p-benzoquinones to nitrogen dioxide. This process involves addition of nitrogen dioxide to the double bond system of quinones. Thermal conversion of oxynitroxyl radicals lead to the formation of acyl(alkyl)nitroxyl and iminoxyl radicals along with molecular nitration products. Kinetics of the oxynitroxyl radical formation depending on the nitrogen dioxide pressure has been defined. The temperature dependence of stationary concentrations of oxynitroxyl radicals at 230–310 K is determined by the equilibrium between the monomer and dimer forms of nitrogen dioxide. Nitrogen tetraoxide was found to initiate radical formation in p-benzoquinones.

Evidence for β Scission in the Oxidation of Amino Acids

Time-resolved ESR and steady-state spin trapping were used to identify the radicals and their kinetics following the radiolytic oxidation by the •OH radical of the anion of α-methylalanine, H2N−C(CH3)2−CO2-. Spectra of two new radicals were identified in aqueous solution, the aminyl radical, HN•−C(CH3)2−CO2-, and the decarboxylated radical, H2N−•C(CH3)2. Time-resolved ESR spectra of the two new radicals were obtained, and the hyperfine couplings and g factors were determined. The radical formation times were consistent with both of the radicals being formed in the initial attack of the •OH radical on α-methylalanine. Measurements of the radical yields indicated that the aminyl radical was the major radical product from the initial •OH attack. The observations (a) that the aminyl radical decays via a first-order process (lifetime about 14 μs) and (b) that there is a large presence of •CO2- spin adducts with CH2NO2- are consistent with β scission being a prominent reaction of the aminyl radical from α-methylalanine. Aspects of the radiolytic oxidations by the •OH of α-methylalanine and glycine are contrasted with respect to initial radical formation and the subsequent reactions of the resulting radicals. These experimental findings corroborate predictions from density functional theory calculations of prompt decarboxylation following electron loss from the α-methylalanine anion and preferential formation of the N-centered radical via H abstraction.

Autoxidation of substituted phenols catalyzed by cobalt Schiff base complexes in supercritical carbon dioxide.

This first study of O(2) oxidation (autoxidation) of substituted phenols catalyzed by a dioxygen carrier in supercritical carbon dioxide (scCO(2)) provides additional insights into the established mechanism of reactions that have been much studied in conventional solvents. As has been long believed, the cobalt(II) dioxygen carriers of the class represented by [[N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminato(2-)]cobalt(II)], Co(salen), show both oxidase and oxygenase activities during oxygenation of substituted phenols in scCO(2). The catalytic autoxidation of 2,6-di-tert-butylphenol (DTBP) and 3,5-di-tert-butylphenol (35-DTBP) in scCO(2) was studied by analysis of products in batch reactions with carefully controlled variables, in the presence of a large excess of O(2), at 207 bar of total pressure and a reaction temperature of 70 degrees C. The oxidation of 35-DTBP yielded only traces of products under the same experimental conditions that converted DTBP totally to a mixture of the oxygenation product 2,6-di-tert-butyl-1,4-benzoquinone (DTBQ) and the related product of radical coupling 3,5,3',5'-tetra-tert-butyl-4,4'-diphenoquinone (TTDBQ). The effects on conversion of DTBP to products and on selectivity between the two products were studied for variations in temperature and the concentrations of catalyst, oxygen, and methylimidazole. Selectivity in favor of the O-transfer product DTBQ over the self-coupling of the phenoxy radical was observed upon changing the oxygen concentration. In contrast, selectivity remained unaffected over a wide range of temperatures and catalyst concentrations. The oxygen dependence of both the conversion and selectivity showed saturation effects identifying the dioxygen complex as the effective oxidant in both the initial radical formation step and the oxygenation of that radical. No direct reaction is observed between the electrophilic phenoxy radical and O(2).

Studies on radical intermediates in the early stage of the nonenzymatic browning reaction of carbohydrates and amino acids.

Determination of the color intensity of heated mixtures of L-alanine and carbohydrate degradation products revealed furan-2-carboxaldehyde and glycolaldehyde as by far the most effective color precursors. EPR studies demonstrated that furan-2-carboxaldehyde generated colored compounds exclusively via ionic mechanisms, whereas glycolaldehyde led to color development accompanied by intense radical formation. In agreement with literature data, these radicals were also detected in heated mixtures of L-alanine and pentoses or hexoses, respectively, and were identified as 1,4-dialkylpyrazinium radical cations by EPR as well as LC/MS measurements. Studies on the mechanisms of radical formation revealed that under the reaction conditions applied, glyoxal is formed as an early product in hexose/L-alanine mixtures prior to radical formation. Reductones then initiate radical formation upon reduction of glyoxal and/or glyoxal imines, formed upon reaction with the amino acid, into glycolaldehyde, which was found as the most effective radical precursor. LC/MS measurements gave evidence that these pyrazinium radicals cations are not stable but are easily transformed into hydroxylated 1,4-dialkyl-1, 4-dihydropyrazines upon oxidation and hydrolysis of intermediate diquarternary pyrazinium ions. Besides other types of color precursors, these intermediates might be involved in the formation of colored compounds in the Maillard reaction.