Formation Of Phenoxy Radicals Research Articles

Significant nuclear quantum effect of proton in the reaction of phenol and hydroxyl radical

The reaction between phenol and hydroxyl radical, leading to the formation of phenoxy radicals, is analyzed using DFT calculations at the multi-component M06-2X level that accounts for the nuclear quantum effects (NQEs) of hydrogen nuclei. The NQEs affect not only the relative energy diagram of the reaction but also the geometric parameters of the stationary point structures. In the energy minimum structures, the OH covalent bonds have lengthened, while the OH…O hydrogen bonds have shortened due to the NQEs. Furthermore, the NQEs also affect the transition state structure, where the distance between carbon and oxygen atoms is strongly influenced by the NQEs of hydrogen nuclei through a hydrogen bond network formed by surrounding water molecules. Different NQEs of hydrogen nuclei have been observed in the ipso/ortho and the para stepwise reaction pathways due to differences in the driving force of the reaction.

N-Hydroxysuccinimide-catalyzed facile synthesis of high-phenolic-hydroxyl-content lignin for enhanced antioxidant properties

The formation of phenoxy radicals that scavenge free radicals is critical for improving the antioxidant activity of lignin. Novel modified lignin with high phenolic hydroxyl content and enhanced antioxidant properties was prepared from kraft lignin using a two-step method. First, diethylenetriamine-modified amine lignin (DAL) was synthesized via the Mannich reaction. Second, the DAL was reacted with 3,4-dihydroxyhydrocinnamic acid (DHCA) using N-hydroxysuccinimide as the catalyst. The DHCA-modified DAL (DDAL) was characterized and tested for the removal of 1,1-diphenyl-2-picrylhydrazyl (DPPH) from aqueous solutions. The optimum conditions were as follows: a reaction temperature of 80 °C, a 1:1 mass ratio of DHCA to DAL, and a catalytic time of 2 h. The phenolic content of DDAL increased from 0.81 to 1.51 mmol/g. More importantly, the DPPH free radical scavenging capacity of DDAL reached 79.94% at a concentration of 0.50 mg/mL. This study provides a practical strategy for preparing high-performance bio-based antioxidants.

Differentiation of Pathways of Nitrated Byproduct Formation from Ammonium and Nitrite During Sulfate Radical Oxidation.

Recent studies found that both nitrite (NO2-) and ammonium (NH4+) lead to nitrophenolic byproducts in SO4•- oxidation processes, during which NO2• generated through the oxidation of the inorganic nitrogen by SO4•- is the key nitrating agent. This study demonstrates that the formation of phenoxy radicals to which NO2• can be incorporated immediately is another governing factor. Two types of sites having distinct reactivities in natural organic matter (NOM) molecules can be transformed to phenoxy radicals upon SO4•- oxidation. Fast sites associated with phenolic functionalities are primarily targeted in the reaction sequence involving NO2-, because both are preferentially oxidized. Following the depletion of NO2-, NH4+ becomes the main precursor of NO2• that interacts with slow sites associated with the carboxylic functionalities. Experimental data show that the formation of total organic nitrogen in 24 h reached 6.28 μM during SO4•- oxidation of NOM (4.96 mg/L organic carbon) in the presence of both NO2- (0.1 mM) and NH4+ (1.0 mM), while the sum of those formed in the presence of each alone was only 3.52 μM. Results of this study provide further insights into the mechanisms of nitrated byproduct formation when SO4•- is applied for environmental remediation.

Peroxide-Mediated Oxygenation of Organic Compounds by Fungal Peroxygenases.

Unspecific peroxygenases (UPOs), whose sequences can be found in the genomes of thousands of filamentous fungi, many yeasts and certain fungus-like protists, are fascinating biocatalysts that transfer peroxide-borne oxygen (from H2O2 or R-OOH) with high efficiency to a wide range of organic substrates, including less or unactivated carbons and heteroatoms. A twice-proline-flanked cysteine (PCP motif) typically ligates the heme that forms the heart of the active site of UPOs and enables various types of relevant oxygenation reactions (hydroxylation, epoxidation, subsequent dealkylations, deacylation, or aromatization) together with less specific one-electron oxidations (e.g., phenoxy radical formation). In consequence, the substrate portfolio of a UPO enzyme always combines prototypical monooxygenase and peroxidase activities. Here, we briefly review nearly 20 years of peroxygenase research, considering basic mechanistic, molecular, phylogenetic, and biotechnological aspects.

Guaiacol Nitration in a Simulated Atmospheric Aerosol with an Emphasis on Atmospheric Nitrophenol Formation Mechanisms.

Atmospheric nitrophenols are pollutants of concern due to their toxicity and light-absorption characteristics and their low reactivity resulting in relatively long residence times in the environment. We investigate multiphase nitrophenol formation from guaiacol in a simulated atmospheric aerosol and support observations with the corresponding chemical mechanisms. The maximal secondary organic aerosol (SOA) yield (42%) is obtained under illumination at 80% relative humidity. Among the identified nitrophenols, 4-nitrocatechol (3.6% yield) is the prevailing species in the particulate phase. The results point to the role of water in catechol and further 4-nitrocatechol formation from guaiacol. In addition, a new pathway of dark nitrophenol formation is suggested, which prevailed in dry air and roughly yielded 1% nitroguaiacols. Furthermore, the proposed mechanism possibly leads to oligomer formation via a phenoxy radical formation by oxidation with HONO.

Improvement of Ferulic Acid Antioxidant Activity by Multiple Emulsions: In Vitro and In Vivo Evaluation

Ferulic acid is a derivative of cinnamic acid showing efficacious anti-oxidant activity. It catalyzes the stable phenoxy radical formation, upon absorption of ultraviolet light, giving the strength to ferulic acid for terminating free radical chain reactions. Ultraviolet rays are one of the most dangerous factors that daily assault the skin, causing excessive generation of reactive oxygen species (ROS), which are regarded to be important contributors to a variety of cutaneous alterations. The skin possesses endogenous antioxidant defense systems, but the excess of ROS leads to an oxidant–antioxidant imbalance. Although ferulic acid is daily introduced in human organism with the diet, its bioavailability after oral administration is poor, particularly in the skin. The aim of this investigation was to evaluate three types of emulsions (W/O/W multiple emulsions and two simple emulsions) as suitable formulations for topical application of the active compound. In vitro studies were performed to investigate the stability and release profiles of these systems. Multiple emulsions showed great stability and the best ability to carry and release ferulic acid. In vivo evaluations highlighted their best capability to treat UV-B-induced erythema. These findings suggested multiple emulsions as an innovative and more efficient vehicle for topical application of ferulic acid.

Comparative Study for Interactions of Sulfate Radical and Hydroxyl Radical with Phenol in the Presence of Nitrite.

Sulfate radical (SO4•-)- and hydroxyl radical (HO•)-based advanced oxidation processes (AOPs) are effective for the removal of organic pollutants in water treatment. This study compared the interactions of SO4•- and HO• for the transformation of phenol in UV/peroxydisulfate (PDS) and UV/H2O2 with the presence of NO2-, which is widely present in aquatic environments and transforms SO4•- and HO• to •NO2. By using laser flash photolysis, the products of phenol reacting with SO4•- and HO• were demonstrated to be phenoxy radical and phenol-HO-adduct radical, respectively. This result, along with density functional theory (DFT) calculations, indicate that the predominant reaction mechanisms of phenol with SO4•- and HO• with phenol are electron transfer and addition, respectively. The different mechanisms induced the much higher formation of nitrophenols by SO4•- than HO• in the presence of NO2- through the fast combination of phenoxy radicals and •NO2. The conversion yields of phenol to nitrophenols (including 2-nitrophenol and 4-nitrophenol), were 47.5% by SO4•- versus 5.3% by HO• at the experimental conditions. Increasing PDS/H2O2 dosages from 0.2 to 1 mM resulted in a 61.9% increase of nitrophenol conversion yield in UV/PDS/NO2- but a 35.4% decrease of that in UV/H2O2/NO2-. In addition, the significant formation of phenoxy radicals by SO4•- also induced many nitrated polymers in UV/PDS/NO2-, while those induced in UV/H2O2/NO2- were negligible. The significant formation of nitrophenols and nitrated polymers increased the mutagenicity by 860.5% when the removal rate of phenol was 98% by UV/PDS/NO2-. This is the first study to demonstrate the different mechanisms of phenol transformation by SO4•- and HO• in the presence of NO2-.

Electronic Conductivity in a Р+-Ion Implanted Positive Photoresist

FP-9120 positive photoresist films 1.8 μm thick implanted with boron and phosphorus ions deposited onto the surface of KDB-10 single-crystal (111) silicon wafers by centrifugation are investigated by their conductivity and electron spin resonance measurements. It is shown that the Р+ ion implantation leads to the formation of a layer with an electronic conductivity of about 10−9 Ω−1 cm−1. At a phosphorous implantation dose of 6 × 1015 cm−2, the electron spin resonance spectrum contains a narrow isotropic line with a g factor of 2.00654 and a width of 3.83 G, which is most likely related to the formation of phenoxy radicals. As the implantation dose increases to 1.2 × 1016 cm−2, a line with a g factor of 2.00264 and a width of 3.96 G is detected in the electron spin resonance spectrum, which is caused by unpaired electrons delocalized according to the π-polyconjugated system.

The CBS values of ΔfHo298.15 and So298.15 of the phenoxy radicals, formed by abstraction of H atom from the components of surrogate bio-oil

In the present work, the standard thermochemical properties of the most thermochemically stable radicals, produced by abstraction of the hydrogen atom from 2,4-dimethylphenol (2,4-xylenol), 2-methoxy-4-methylphenol, 3-methoxy-4-formylphenol (vanillin), are determined using the results of RO/CBS-4M and RO/CBS-QB3 calculations. It is found that the formation of phenoxy radicals is the most thermochemically favorable pathway for the H atom abstraction from the considered compounds.The consistent values of the standard enthalpies of formation of the most thermochemically stable structures are determined using the RO/CBS-4M and RO/CBS-QB3 thermochemistry of the homodesmotic reactions, as well as using the correction dependencies.The values of the standard entropies of these compounds, calculated in the present work, are also reported.

Enzymatically cross-linked arabinoxylan microspheres as oral insulin delivery system

Arabinoxylans (AX) microspheres with different insulin/AX mass ratio were prepared by formation of phenoxy radical issued from the ferulic acid by enzymatic oxidation (entrapped in situ of insulin). Phenolic acid content and FT-IR spectrum of unloaded and insulin-loaded AX microspheres revealed that the phenoxy radical issued from the ferulic acid by enzymatic oxidation did not interact covalently with insulin. The microspheres showed a spherical shape, smooth surface and an average diameter of particles of 320 μm. In vitro control release found that AX microspheres minimized the insulin loss in the upper GI tract, retaining high percentage (~75%) of insulin in its matrix. The stability of the secondary structure of insulin was studied by dichroism circular (CD). The CD spectra of insulin released from AX microspheres did not change according to the insulin/AX mass ratio of the microsphere. Significant hypoglycemic effects with improved insulin-relative bioavailability tested on an in vivo murine model revealed the efficacy of these enzymatically cross-linked arabinoxylans microspheres as a new oral insulin carrier.

Spore cells from BPA degrading bacteria Bacillus sp. GZB displaying high laccase activity and stability for BPA degradation

Laccase has been applied extensively as a biocatalyst to remove different organic pollutants. This study characterized a spore-laccase from the bisphenol A (BPA)-degrading strain Bacillus sp. GZB. The spore-laccase was encoded with 513 amino acids, containing spore coat protein A (CotA). It showed optimal activity at 70 °C and pH = 7.2 in presence of 2, 6-dimethoxyphenol. At 60 °C, optimal activity was also seen at pH = 3.0 and pH = 6.8 with 2, 2′-azino-bis (3-ethylbenzothiazoline-6-sulfonate) and syringaldazine, respectively. The spore-laccase was stable at high temperature, at acidic to alkaline pH values, and in the presence of different organic solvents. Spore-laccase activity was increased by introducing Cu2+, Mg2+, and Na+, but was strongly inhibited by Fe2+, Ag+, l-cysteine, dithiothreitol, and NaN3. The cotA gene was cloned and expressed in E. coli BL21 (DE3); the purified protein was estimated as having a molecular weight of ~63 kDa. Different synthetic dyes and BPA were effectively decolorized or degraded both by the spore laccase and recombinant laccase. When BPA oxidation was catalyzed using laccase, there was an initial formation of phenoxy radicals and further oxidation or CC bond cleavage of the radicals produced different organic acids. Detailed reaction pathways were developed based on nine identified intermediates. The acute toxicity decreased gradually during BPA degradation by laccase. This study is the first report about a genus of Bacillus that can produce a highly active and stable laccase to degrade BPA.

Peroxidative Oxidation of Lignin and a Lignin Model Compound by a Manganese SALEN Derivative

The manganese catalyst, (1R,2R)-(−)-[1,2-cyclohexanediamino-N,N′-bis(3,5-di-t-butylsalicylidene)]manganese(III) chloride, was used to activate H2O2 to oxidize organosolv lignin and a lignin model compound. Oxidation of the β-O-4 lignin model substrate 1-(4-hydroxy-3-methoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol (320.3 m/z) and poplar organosolv lignin resulted in both fragmentation and polymerization processes, likely via phenoxy radical formation. Matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS) of the reaction products from the β-O-4 model substrate showed oligomers of the substrate with masses of 661.192, 979.355, and 1297.466 m/z that correspond to a dimer, trimer, and tetramer of the β-O-4 model substrate, respectively. Nuclear magnetic resonance (NMR) shows the formation of 5–5 diphenyl and 4-O-5 linkages in the β-O-4 model substrate oxidation products. Gel permeation chromatography (GPC) detected three peaks, corresponding to the β-O-4 model subs...

The role of CuCl on the mechanism of dibenzo-p-dioxin formation from poly-chlorophenol precursors: A computational study.

A computational study is performed for the elucidation of the role played by CuCl in the condensation of two polychlorophenol molecules to yield PCDDs. The mechanism found consists of six sequential steps, which allow the final recuperation of the CuCl molecule, and applies for phenol molecules with an ortho chlorine. In the temperature range of 453-473 K (previously reported as adequate to diminish PCDDs formation in the post-combustion area), CuCl is able to softly retain chlorophenol molecules, mainly those less chlorinated. After a first HCl release, Cu(I) remains bonded to phenol oxygen atom, thus avoiding the formation of phenoxy radicals and the subsequent radical processes. A temperature raise up to 1200 K destabilizes the initial CuCl-chlorophenol complexes and causes that the rate limiting step change from the formation of the first oxygen bridge to HCl elimination. It has been checked that tetra and penta-chlorophenols undergo essentially the same reaction process of 2-chlorophenol. In view of our results and trying to arrive at a practical way to diminish the rate of formation of PCDDs, we propose that an extra addition of powdered CuCl to the post-combustion zone, cooled down to temperatures lower than 473 K, could act as an inhibitor in the formation of these pollutants.

Computational assessment of the fluorescence emission of phenol oligomers: A possible insight into the fluorescence properties of humic-like substances (HULIS)

Abstract Compounds with fluorescence in the humic-like substances (HULIS) region are known to be formed under conditions where the oxidation (which could lead to the oligomerization) of phenolic compounds is operational, because of the formation of phenoxy radicals. However, there was no evidence to date that such fluorescence emission was really due to phenol oligomers. In this work, the fluorescence of phenol and some of its oligomers was studied by computational methods and it was compared with experimental data, when allowed by the availability of commercial standards. The oligomer fluorescence depends on the contribution of different stable conformers, which differ from one another for the dihedral angles between the aromatic rings. Differences in the dihedral angles are also observed between the ground states and the corresponding excited singlet states. The predicted wavelengths of fluorescence emission increase with increasing the number of aromatic rings, up to a plateau at around 450 nm. Compounds with more than three-four aromatic rings are not expected to show emission in a different range, because the transitions causing fluorescence involve only three consecutive rings. These results support the hypothesis that oligomers account for the fluorescence emission in the HULIS region, which has been observed under the photochemical and photosensitized transformation of phenolic compounds.

Properties of formaldehyde-free environmentally friendly lignocellulosic composites made from poplar fibres and oxygen-plasma-treated enzymatic hydrolysis lignin

The objective of this paper was to evaluate the properties of formaldehyde-free, environmentally friendly lignocellulosic composites made from poplar fibres and plasma-treated enzymatic hydrolysis lignin (EHL) as a function of plasma processing time and power. The modulus of rupture (MOR), modulus of elasticity (MOE), internal bond strength (IB), and thickness swelling (TS) of the boards were determined. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) spectroscopy were employed to characterise the chemical changes and free radial formation on the surface of oxygen-plasma-treated EHL. The MOR, MOE, and IB of the samples increased with increasing plasma processing time and power, whereas the TS of the boards decreased. A plasma treatment of 200W for 5min was sufficient for the use of EHL as a natural binder in the production of high-performance lignocellulosic composites. The bonding improvement was most likely due to the generation of oxygen-related functional groups and the formation of phenoxy radicals on the surface of EHL after the oxygen plasma treatment.

Redox properties of novel pyrrolidine derivatives containing sterically hindered phenol fragment

The method of cyclic voltammetry on a platinum electrode in acetonitrile is used to study the redox properties of a number of novel pyrrolidine derivatives containing a sterically hindered phenol fragment. It is found that the obtained derivatives of (3,5-di-tert-butyl-4-hydroxyphenyl)pyrrolidine-2-carboxylic acid are irreversibly oxidized in two stages with formation of a phenoxy radical. The structural analogue containing no phenyl fragment is irreversibly oxidized under similar conditions in a single stage. Formation of a stable phenoxy radical from synthesized (3,5-di-tert-butyl-4-hydroxyphenyl)pyrrolidines under PbO2 oxidation is confirmed by ESR spectroscopy. Application of the synthesized compounds as antioxidant agents has been suggested.

The catalytic role of uranyl in formation of polycatechol complexes

To better understand the association of contaminant uranium with natural organic matter (NOM) and the fate of uranium in ground water, spectroscopic studies of uranium complexation with catechol were conducted. Catechol provides a model for ubiquitous functional groups present in NOM. Liquid samples were analyzed using Raman, FTIR, and UV-Vis spectroscopy. Catechol was found to polymerize in presence of uranyl ions. Polymerization in presence of uranyl was compared to reactions in the presence of molybdate, another oxyion, and self polymerization of catechol at high pH. The effect of time and dissolved oxygen were also studied. It was found that oxygen was required for self-polymerization at elevated pH. The potential formation of phenoxy radicals as well as quinones was monitored. The benzene ring was found to be intact after polymerization. No evidence for formation of ether bonds was found, suggesting polymerization was due to formation of C-C bonds between catechol ligands. Uranyl was found to form outer sphere complexes with catechol at initial stages but over time (six months) polycatechol complexes were formed and precipitated from solution (forming humic-like material) while uranyl ions remained in solution. Our studies show that uranyl acts as a catalyst in catechol-polymerization.

Effects of the antioxidants in pomegranate juice on DNA‐protein crosslinking via guanine oxidation

DNA‐protein cross‐linking is a direct result of oxidative damage. Here we investigated effects of antioxidants from pomegranate juice on DNA‐protein crosslinking via guanine oxidation. Cross‐linking was detected by gel shift assay. After flash quench treatment, pUC19 exhibited a drop in mobility and highly cross‐linked DNA remained in the wells. With addition of POM brand pomegranate juice to samples, the plasmid runs as a single band even after oxidation via the flash‐quench technique. Significant inhibition of cross‐linking is evident at a 1:500 dilution of the juice, a concentration that has physiological relevance. Individual compounds found in the juice (gallic acid, quercetin, catechin, cyanin, ellagic acid, and p‐coumaric acid ) were investigated. These compounds were active down to concentrations of 1μM. Transient absorption spectroscopy results showed a decay of guanine radical concomitant with the formation of phenoxy radical, suggesting that the phenolic components repair guanine radical by reduction.

Glassy carbon electrodes modified with mixed covalent monolayers of choline, glycine, and glutamic acid for the determination of phenolic compounds

A series of mixed covalent monolayer carbon glassy electrodes (choline/amino acid/GCE) was prepared using choline and amino acids, and the properties were investigated by cyclic voltammetry, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. The oxidations of phenolic compounds including dopamine hydrochloride, epinephrine and phenol have been studied. Electrooxidation of phenols, first leads to the formation of phenoxy radical, which reduces to phenols and hydroquinone; the reaction of polyoxyphenylene is effectively restrained at this electrode, unlike only at chlorine or amino acids modified electrode or bare electrode, which rapidly forms polyoxyphenylene and results in electrode passivation. Owing to significant improvement of electrode passivation shaped insulating polymerization, these mixed covalent monolayer carbon glassy electrodes show themselves excellent resistance ability for pollution, could be successfully used as amperometric sensor for phenolic compounds.

Oxidation of milled wood lignin with laccase, tyrosinase and horseradish peroxidase

In this paper the oxidation of milled wood lignin (MWL), catalysed by three enzymes, i.e. laccase, tyrosinase and horseradish peroxidase (HRP) was studied. The oxidation was followed by measuring the consumption of O(2) during laccase and tyrosinase treatment and of H(2)O(2) during HRP treatment. Both laccase and HRP were found to oxidise lignin effectively, whereas the effect of tyrosinase was negligible. The changes in MWL molecular-weight distributions caused in the reactions were analysed by gel permeation chromatography. Both laccase and HRP treatments were found to polymerise MWL. Peroxidase treatment was found to decrease the amount of phenolic hydroxyls in MWL, whereas no such effect could be detected in the laccase-treated sample. Both laccase and HRP treatments were, however, found to increase the amount of conjugated structures in MWL. The formation of phenoxy radicals during the treatments was studied by electron paramagnetic resonance spectroscopy. Phenoxy radicals were detected in both laccase and HRP-treated samples. The amount of the formed phenoxy radicals was found to be essentially constant during the detected time (i.e. 20-120 min after the addition of enzyme).