Formation Of Phenoxyl Radicals Research Articles

Proton Domino Reactions at an Imidazole Relay Control the Oxidation of a TyrZ-His190 Artificial Mimic of Photosystem II.

A close mimic of P680 and the TyrosineZ-Histidine190 pair in photosystem II (PS II) has been synthesized using a ruthenium chromophore and imidazole-phenol ligands. The intramolecular oxidation of the ligands by the photoproduced Ru(III) species is characterized by a small driving force, very similar to PS II where the complexity of kinetics was attributed to the reversibility of electron transfer steps. Laser flash photolysis revealed biphasic kinetics for ligand oxidation. The fast phase (τ < 50 ns) corresponds to partial oxidation of the imidazole-phenol ligand, proton transfer within the hydrogen bond, and formation of a neutral phenoxyl radical. The slow phase (5 - 9 µs) corresponds to full oxidation of the ligand which is kinetically controlled by deprotonation of the distant 1-nitrogen of the imidazolium. These results show that imidazole with its two protonatable sites plays a special role as a proton relay in a 'proton domino' reaction.

Theoretical study on the influence of three different surfactants on the binding of laccase with bisphenol A

Laccase catalyzes the formation of phenoxyl radicals as an intermediate in Bisphenol A (BPA) degradation, and certain surfactants can affect the effectiveness of laccase in removing BPA from various effluents, yet the specific molecular details remain unclear. In this study, based on laccase and BPA complex, three surfactants, cetyltrimethylammonium bromide (CTAB), sodium dodecylbenzene sulfonate (SDBS), and TritonX-100 (TX-100), were introduced to construct four systems. We applied MD simulation along with MM-PBSA method to investigate the effects of different surfactants on the binding of laccase with BPA. During the MD simulations, these three surfactants enveloped the enzyme surface to varying degrees, causing the solvent to separate from parts of the laccase surface. The addition of CTAB and SDBS suppressed BPA degradation mainly by influencing the polar solvation free energy and van der Waals (vdW) interactions between laccase and BPA. In contrast, TX-100 promoted the degradation of BPA by reducing polar solvation free energy. This explains why in the laccase-BPA complex system the addition of a certain concentration of TX-100 resulted in a higher degradation rate compared to CTAB or SDBS. Our work clarifies the molecular mechanisms of the interaction between laccase and BPA under the influence of CTAB, SDBS, and TX-100 surfactants, and provides some guidance for laccase degradation of BPA.

Oxygen bond cleavage and its migration mechanisms during pyrolysis of naomaohu sub-bituminous coal

Investigating the oxygen bond cleavage and its migration mechanisms during the pyrolysis process of sub-bituminous coals is essential for their classification utilization. The present study employed a fixed-bed pyrolysis technique (temperature range: 350–600 °C) to explore the cleavage of oxygen-containing bonds and subsequent behavior of oxygen migration and transformation during the pyrolysis process of Naomaohu sub-bituminous coal (NSBC). X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy results reveal that the organic oxygen in NSBC predominantly exists as the form of C–O bonds, accounting for 22.81 wt% of the total composition, mainly encompassing hydroxyl, carbonyl, ester, and ether groups. The organic oxygen in the light tar is mainly in the form of phenols, reaching 1.71 wt%, of which methyl substituted phenols is predominant at a final pyrolysis temperature of 600 °C. This can be attributed to the presence of more aryl ether bonds and hydroxyl groups in NSBC, which are prone to cleavage at higher temperatures, resulting in the formation of phenoxyl radicals. Additionally, the higher temperature facilitates the weakening of hydrogen bonds between phenolic hydroxyl groups, thereby enhancing the yield of phenolic compounds. The ester compounds present a decreasing trend with increasing temperature, primarily associated with the cleavage temperature of ester linkers being 350 °C, suggesting that an increase in temperature favors the cleavage of ester bonds. In addition, the cleavage of C–O bonds begins to accelerate significantly during the pyrolysis temperature range of 450–500 °C, subsequently accounting for the pronounced proliferation of phenols within this specific thermal interval. The oxygen content in semi-coke decreased with the increase of temperature, from 85.27 wt% at 350 °C to 37.17 wt% at 600 °C, while the oxygen content in the pyrolysis gas increases from 11.63 wt% at 350 °C to 47.53 wt% at 600 °C, indicating that a significant migration of organic oxygen to the pyrolysis gas during the pyrolysis of NSBC. This study can deepen the understanding of the transformation mechanism of oxygen-containing structure in NSBC pyrolysis process, and provide a theoretical basis for high-grade utilization of low-rank coals.

Comparative DNA-/BSA-binding, DNA cleavage, and cytotoxic properties of copper(II) amino/salicyl-phenolate Schiff bases (phen) complexes that induce generation of phenoxyl radicals

A series of mixed-ligand Cu(II) complexes of the type [Cu(L1-5)(diimine)(H2O)](ClO4) (1–5) and [Cu(L6)(diimine)](ClO4) (6), where HL1-HL4 are aminophenolate Schiff base ligands viz., 2-(benzylideneamino)phenol (HL1, 1), 2-(4-fluorobenzylideneamino)phenol (HL2, 2), 2-(4-chlorobenzylideneamino)phenol (HL3, 3), and 2-(4-nitrobenzylideneamino)phenol (HL4, 4) whereas HL4 and HL5 are salicyl-phenolate Schiff base ligands viz.,2-((phenylimino)methyl)phenol (HL5, 5) and (E)-2-(((2-(dimethylamino)ethyl)imino)methyl)phenol (HL6, 6) as primary ligands and the diimine 1,10-phenanthroline (phen) as co-ligand have been isolated. TheSchiffbaseligandsand their mixedligandchelates were characterizedby spectral techniques (UV–Visible, IR, mass, 1H, and 13C NMR), elemental analysis, cyclic voltammetry (CV), and differential pulse voltammetric (DPV) techniques. The molecular structure of 6, which alone formed appreciable crystals, was determined by single-crystal X-ray studies, and two crystallographically independent molecules (a and b) existed in square pyramidal distorted trigonal bipyramidal (SPDTBP) geometry. The complexes 1–4 showed a reversible (ΔEp for 3; 64), quasi-reversible (2; 105 and 4; 115), and irreversible (1; 181 mV) oxidation waves discernible at E1/2 ranging from + 0.035 V to −0.120 V (DPV) which are attributed to the formation of copper(II)-phenoxyl radical species. Among them, 2 and 3 showed a well-defined reversible phenoxyl radical formation in CV and DPV. Also, these two complexes exhibited more stable phenoxyl radical in the presence of CAN (cerium ammonium nitrate). The DNA binding studies revealed that the fluoro- and chloro-substituted complexes 2 and 3 exhibited higher DNA binding propensity than the other complexes. All the complexes displayed moderate to high oxidative DNA cleavage property in the presence of H2O2, wherein complexes 2 and 3 exhibited higher DNA cleavage than the others. Interestingly, complex 3 exhibited less AC50 value (18.26 µM) than complex 2 (43.3 µM). The mechanistic study led to the inference that the DNA cleavage is possibly mediated via copper(II)-bound phenoxyl-radical. The cytotoxicity of complexes 1–6 was investigated in A549 non-small cell lung carcinoma cell. The amino-phenolate complexes 2 (IC50 = 3.52 µM) and 3 (IC50 = 3.4 µM) brought about higher cytotoxicity and ≈ 3.8-times more potency than cisplatin. The complex 3 showed to be 47-times less toxic to non-cancerous peripheral blood mononuclear cells (PBMC). Fluorescent staining assay revealed that the complexes 1–6 induced apoptotic cell death with some degree of necrosis. JC-1 staining assay revealed that complex 3 efficiently depolarized the mitochondrial membrane potential. Put together, the results substantiate that amino-phenolate-containing copper(II) mixed ligand complexes are more cytotoxic than salicyl-phenolate-containing copper(II) complexes.

Synthesis, Structure and Catechol Oxidase Activity of Mono Nuclear Cu(II) Complex with Phenol-Based Chelating Agent with N, N, O Donor Sites

A square-planar Cu(II) complex [Cu(L)Cl], 1, with sterically constrained tridentate phenol-based ligand (HL= N,N,N′-trimethyl-N′-(2-hydroxy-3,5-di-tert-butylbenzyl)-ethylenediamine) with N, N, O donor sites has been synthesized. The complex is characterized by single crystal X-ray diffraction study as well as other spectroscopic techniques. The reported complex crystallizes in monoclinic space group C2/c with a = 30.248(6), b = 13.750(3) and c = 11.410(2) Å with β = 110.232(2)°. The Cu(II) ion adopts a square planar environment in this complex. Electrochemical study of the complex 1 gives quasi-reversible reductive response at E1/2 ≈ −0.5 V due to the reduction of the Cu(II) center along with a reversible oxidation peak at E1/2 ≈ 0.75 V. The oxidation peak arises due to the ligand-based oxidation of phenolate group to phenoxyl radical in the complex. The Cu(II) complex exhibits catechol oxidase activity in methanol as observed by the UV–vis spectroscopy of the aerial oxidation of 3,5-DTBC to 3,5-DTBQ and the reaction proceeds via the formation of ligand phenoxyl radical. The turnover number for complex 1 is 2560 h−1.

Oxidative polymerization of bisphenol A (BPA) via H-abstraction by Bi2.15WO6 and persulfate: Importance of the surface complexes

Bi2+xWO6 are a group of reactive photocatalysts in the photooxidation of various contaminants. However, their catalytic reactivity in the persulfate (PS)-based oxidation processes in the absence of light is still unknown. In this study, Bi2.15WO6 shows a high reactivity in the catalytic oxidation of bisphenol A (BPA) by PS in the absence of light under alkaline conditions, and 20 mg L-1 BPA can be rapidly removed within 30 min at pH 10.0 with the low stoichiometry ratio of 1.79 between consumed PS and removed BPA. Characterizations of the oxidation products indicate that 98.4% of soluble BPA has been oxidatively polymerized to BPA polymers, instead of being oxidized to CO2 and H2O. FTIR, Raman, XPS and electrochemical analyses confirm that surface Bi(III) is the reactive site for the adsorption of BPA and PS, which then initiates reactions between BPA and PS. DFT calculations reveal that the O-O bond in PS is moderately activated via the nonradical pathway after adsorbing on Bi(III), instead of being decomposed to SO4•−, followed by H-abstraction from BPA by the activated PS with the formation of phenoxyl radicals and the subsequent polymerization to BPA polymers. The apparently lower energy barrier from adsorbed BPA (ΔG = 0.056 eV) than that from dissolved BPA (ΔG = 0.263 eV) further reveals that H-abstraction from adsorbed BPA to the activated PS on the surface of Bi2.15WO6 is more favorable than that from dissolved BPA. This study gives new insight on the efficient removal of recalcitrant contaminants via the H-abstraction pathway.

In Situ-Formed Phenoxyl Radical on the CuO Surface Triggers Efficient Persulfate Activation for Phenol Degradation.

Transition-metal oxide (MxOy)-based persulfate (PDS) activation processes have demonstrated enormous potential for pollutant degradation in water purification. However, the mechanistic insight of PDS activation by a MxOy catalyst concerning the mediate role of the organic substrate remains obscure. Here, we demonstrated that the in situ-formed phenoxyl radical on the CuO surface can trigger efficient persulfate activation for phenol degradation. The formation of the phenoxyl radical was an inner-sphere process, which involved the successive steps of chemisorption through surface hydroxyl group substitution and the subsequent spontaneous electron transfer reaction from adsorbed phenol to CuO. The organic substrate phenol can be oxidized by the PDS molecule and surface-bound SO4•- through the nonradical and free-radical pathways, respectively. Such a unique "half-radical" mechanism resulted in an extraordinarily high PDS utilization efficiency of 188.9%. More importantly, a general rule for phenoxyl radical formation was concluded; it can be formed in the cases of organic substrates with a Hammett constant σ+ lower than -0.02 and metal ion of a 3d subshell between half-filled and fully filled. This study clarifies the mediate role of the organic substrate for interfacial PDS activation on MxOy and also gives new insights into the rational design of a highly efficient MxOy catalyst for selective phenolic/aniline pollutant degradation in wastewater.

A mechanistic approach towards the formation of bityrosine in proteins by ionizing radiation – GYG model peptide

One of the methods of protein crosslinking used in the synthesis of protein-based nanoparticles is the formation of bityrosine bridges, which may be achieved by the recombination of phenoxyl-type radicals in irradiated protein solutions. Radiation-induced formation of phenoxyl radicals in tyrosine is promoted in presence of H+. In this work, kinetics of this process and the influence of pH were studied by pulse radiolysis measurements of a model peptide H-Gly-Tyr-Gly-OH (GYG), which may help to solve questions raised for bigger, more complex systems in comparable conditions. The main route of phenoxyl radicals formation consists of the addition reaction of hydroxyl radical to the phenol ring on the tyrosine side-chain and proton catalyzed water molecule elimination. A similar effect was observed in studies of tripeptide solutions containing phosphate buffer. The presented data include time-resolved optical spectroscopy of transient species formed under pulse electron beam irradiation and a comparison of the kinetics of the phenoxyl radicals formation in samples at various pH and in presence of phosphate buffer. The postulated mechanism and obtained values of rate constants of the formation and decay of transient species were additionally checked by simple probabilistic simulations.

Screen-Printed Electrode Modified with 3-D Nanoporous Nickel for the Determination of Narirutin in Wastewater from Citrus Industry

Yellow Water (YW) is the term given to liquid residues produced from the washing of orange fruits, distilling activities of distilleries, and the production of essential oil from orange; the handling and disposal of these residues are a matter of great concern to orange industries worldwide. An average size orange plant produces about 100,000 liters of YW daily. This wastewater has a considerable amount of organic compounds with high polluting power. Citrus processing industries are commercially interested in recovering significant quantities of some usefully relevant compounds present in YW. Among the compounds that have high added value, citrus flavonoids show an important biological activities, highlighting the molecule narirutin - a flavanone glucoside, which has been shown to present interesting therapeutic properties against cancer, Alzheimer's disease, and more recently, effects against the coronavirus disease 2019 (COVID-19). So, this work reports for the first time the electrochemical determination of narirutin using three-dimensional nanostructured porous nickel with high surface area on screen-printed electrode (3DnpNi/SPE), since the literature reports only chromatographic methods for the detection and quantification of narirutin. The modified electrode was successfully developed by the dynamic hydrogen bubble template method. This method involves the evolution of hydrogen bubbles together with the metal deposition process. Thereby, these bubbles become a dynamic template for the electrochemical deposition. Compared to the non-modified SPE, the 3DnpNi/SPE presented better electrochemical performance in terms of the oxidation of narirutin. The 3DnpNi/SPE was characterized by scanning electron microscopy (SEM), proving the formation of nickel nanopores with homogeneous distribution on the electrode surface, with a diameter varying between 70 - 120 nm. The energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) showed the presence of nickel and nickel oxide in the nanostructure. The EIS technique was applied for the electrochemical characterization of the 3DnpNi/SPE. The electron-transfer resistance was analyzed using a solution of 5.0×10−3 mol L-1 [Fe(CN)6]3-/4- in 0.1 mol L-1 KCl. The results obtained were discussed in terms of Rct (charge-transfer resistance) from the equivalent circuits. It was can observe the presence of a semicircle for the unmodified SPE with Rct of 2.3 KΩ. When the SPE was modified with nanoporous nickel, the electrochemical response appeared to be close to a straight line, and the Rct value was notably lower, 39 Ω; this shows that the modification of the electrode facilitated the transfer of electrons between the redox probe and the electrode surface. The cyclic voltammetry (CV) results showed that the 3DnpNi/SPE exhibited a peak current 3.3 times greater than that of bare SPE. The oxidation process involved 1 electron and 1 proton in a single irreversible step, with the formation of phenoxyl radical, which was adsorbed on the electrode surface due to the irreversible reaction (dimerization), to form dimer products. The quantification of narirutin was conducted by differential pulse voltammetry (DPV), which showed a wide concentration range (1.0×10-7 – 1.0×10-5 mol L-1), with low detection limit (3.9×10-8 mol L-1), and excellent sensitivity (0.31 A L mol-1). The good electrochemical performance of the proposed electrode was attributed to the high density of the nanostructured porous nickel on the SPE. The inter-electrode repeatability (n=5) was investigated by CV using the concentration of 5.0×10-6 mol L-1 of narirutin in 0.1 mol L-1 citrate buffer solution (pH 4.0). The average oxidation current obtained was 2.2 µA, with RSD of 5.9 %. In addition, the 3DnpNi/SPE was stored for 10 days in contact with air at room temperature (25 °C). The current response relative to narirutin detection was found to be 94.2 % of its initial peak current value. These results show that the 3DnpNi/SPE has good repeatability and long-term stability. The standard addition method was adopted for determining the concentration of narirutin in the sample. The sample was spiked with known concentrations of standard narirutin in the range of 2.0×10-6 mol L-1 to 6.0×10-6 mol L-1. A linear relationship was observed between the current peak and the added concentration of narirutin. The regression equation was as follows: y = 0.56 x + 2.3×10-6 (R = 0.999). From the standard addition curve, and based on the extrapolation of the straight line and the sample dilution (20-fold), the concentrations of narirutin found in the yellow water sample was (8.3 ± 0.39)×10-5 mol L-1 (n=3). To determine the analytical accuracy of the proposed method, recovery experiments were performed in standard solutions. The recovery rates obtained ranged from 97.1% to 100.9%, with RSD lower than 7.0 %. The concentration of narirutin in the YW sample was also determined by HPLC and the concentration value obtained was (8.6 ± 0.25)×10-5 mol L-1. Thereby, the RSD obtained between the two techniques was 0.16 %. These results demonstrate a good agreement between the proposed method and the comparative method (HPLC); essentially, this points to the reliability of the method proposed in this work. Thus, the findings of the present study show that the proposed 3DnpNi/SPE-based electroanalytical method can certainly be used for the determination of narirutin in yellow water. Figure 1

Dinuclear Zn Complex: Phenoxyl Radical Formation Driven by Superoxide Coordination

AbstractThe dinuclear zinc complex Zn2L2 (HL=2‐{[[di(2‐pyridyl)methyl](methyl)amino]methyl}phenol) has been synthesized and isolated as colorless crystals. The interaction of the compound with superoxide in anhydrous organic solvents has been evaluated by CV, stopped‐flow Uv‐vis, EPR and ESI‐MS suggesting the binding and the activation of the coordinated superoxide, thanks to the Lewis acidity of the Zn(II) centers. The results obtained in this study highlight the formation of a Zn2L2−O2.− intermediate and a metastable phenoxyl‐radical driven by the coordinated superoxide.

Insight to pyrolysis behavior of three aromatic ethers by pyrolysis coupled with single-photon ionization molecular-beam mass spectrometry

The pyrolytic behaviors of three aromatic ethers were investigated by a self-developed pyrolysis coupled with single-photon ionization molecular-beam mass spectrometry at pressure of pyrolysis chamber 3–6 Pa and temperature from 200 °C to 1100 °C. The results showed that the homolysis of C-O/C-C bridged bonds results into the formation of phenyl, benzyl, phenethyl and phenoxyl radicals. The cracking of benzyl and phenoxyl radicals generates cyclopentadienyl radical around 800 °C. The reaction route of cyclopentadienyl radical to methylcyclopentadiene to benzene existed in all three aromatic ethers pyrolysis process, and two reaction paths for the formation of styrene were also observed. Furthermore, a lot of new polycyclic compounds, such as phthalan, naphthalene, biphenylene, fluorene, dibenzofuran, phenanthrene, 9,10-dihydro-phenanthrene and 9-ethylidenefluorene, are formed through dehydrocyclization and rearrangement of partial thermal cracking species. A reasonable thermal cracking pathway was proposed. Partial experimental results were supported by the density functional theory calculation at the B3LYP level.

Phthalocyanine as a Bioinspired Model for Chlorophyll f-Containing Photosystem II Drives Photosynthesis into the Far-Red Region.

The textbook explanation that P680 pigments are the red limit to drive oxygenic photosynthesis must be reconsidered by the recent discovery that chlorophyll f (Chlf)-containing Photosystem II (PSII) absorbing at 727 nm can drive water oxidation. Two different families of unsymmetrically substituted Zn phthalocyanines (Pc) absorbing in the 700-800 nm spectral window and containing a fused imidazole-phenyl substituent or a fused imidazole-hydroxyphenyl group have been synthetized and characterized as a bioinspired model of the Chlf/TyrosineZ /Histidine190 cofactors of PSII. Transient absorption studies in the presence of an electron acceptor and irradiating in the far-red region evidenced an intramolecular electron transfer process. Visible and FT-IR signatures indicate the formation of a hydrogen-bonded phenoxyl radical in ZnPc II-OH. This study sets the foundation for the utilization of a broader spectral window for multi-electronic catalytic processes with one of the most robust and efficient dyes.

Long-chain phenols oxidation using a flow electrochemical reactor assembled with a TiO2-RuO2-IrO2 DSA electrode

The electrochemical degradation of long-chain phenols present in toxic residues from the cashew-nut processing industry was systematically investigated for the first time, using a flow electrochemical reactor assembled with a Ti/TiO2-RuO2-IrO2 DSA anode. After optimizing the current density (40 mA cm−2) and chloride concentration (0.028 mol L−1) in 0.5 mol L−1 Na2SO4 supporting electrolyte, it was achieved expressive total organic carbon (TOC) removal percentage for (un)saturated cardanol and cardol, either alone (85–92%) or in association/real industrial CNSL sample (82%), over 4 h of electrolysis. Evidently, the removal rate for the mixture changes (56–88% TOC removal) according to its starting concentration (0.005–0.100% residue), showing that the system performance can improve by increasing treatment time. Chromatography coupled to mass spectrometry (GC–MS) analysis showed that the first stage of the electrooxidation process was driven towards the formation of phenoxyl radicals, followed by the formation of polymeric dimers. These intermediates were continuously oxidized by different free and chemisorbed reactive species until obtaining short-chain organic acids, CO2 and H2O. The proposed system proved to be efficient, eco-friendly, and viable to treat industrial waste containing refractory phenolic compounds.

Photochemical reaction kinetics and mechanism of bisphenol A with K2S2O8 in aqueous solution: a laser flash photolysis study

The photochemical reaction kinetics and mechanism of bisphenol A (BPA) with potassium persulfate (K2S2O8) were investigated by using 266 nm laser flash photolysis and gas chromatography mass spectrum (GC-MS) technique. Sulfate radical (SO4•−), generated upon K2S2O8 photolysis, reacted with BPA with the overall rate constant of (1.61 ± 0.15) × 109 L mol−1 s−1, and two main reaction mechanisms were involved. One was addition channel to generate BPA–SO4•− adduct with a specific second-order rate constant of (1.09 ± 0.15) × 109 L mol−1 s−1. Molecular oxygen was involved in the decay of the BPA–SO4•− adduct with a rate constant of (1.28 ± 0.14) × 108 L mol−1 s−1. Another channel was the formation of BPA’s phenoxyl radical, likely derived from a deprotonation of the cation radical (BPA•+) generated from single electron transfer reactions. The specific rate constant of BPA’s phenoxyl radical formation was determined to be (6.16 ± 0.08) × 108 L mol−1 s−1. The overall rate constant was in line with the sum of aforementioned two specific rate constants for two main reaction channels. By comparing these rate constants, it was indicated that SO4•− addition channel accounted for ∼65% (1.09/1.61) to the overall reaction, and phenoxyl radical formation accounted for only ∼35% (0.62/1.61). The transformation products of BPA were identified by using GC-MS including 4-isopropylphenol, 4-isopropenylphenol, and 2,4-di-tert-butylphenol, and the reaction mechanism was proposed. These results may provide microscopic kinetics and mechanism information on BPA degradation using SO4•−-based advanced oxidation processes.

Electrochemical Oxidation of Phenolic Analogues Towards Formation of New C-C Bonds

The electrochemical oxidation of selected phenols 2,6-di-tertbutylphenol (1), 2,6-diphenylphenol (2), and 2,2'-dyhydroxybiphenyl (3) was studied using cyclic voltammetry, chronoamperometry, UV-vis spectroscopy and fluorescence spectroscopy. All compounds exhibited oxidation of the corresponding phenol using cyclic voltammetry and chronoamperometry. Electrochemical oxidation of phenols resulted in the electrochromic reaction and formation of yellow-coloured and fluorescent products. The photophysical properties of products were characterized by spectroscopy. Data indicates that electrochemical oxidation of phenols, via phenoxyl radical formation, lead to new carbon-carbon bond formation and synthesis of products with extended conjugation, absorbance in the visible range and enhanced fluorescence.

Unexpected light emission from tyrosyl radicals as a probe for tyrosine oxidation

Tyrosine residues (Tyr) on proteins are a favoured site of one-electron oxidation due to their low one-electron reduction potentials. In this work, light-induced oxidation of Tyr residues was investigated using direct ionisation (via 266 nm light excitation) and sensitized photo-oxidation (by 3-carboxybenzophenone as sensitizer and 355 nm). Light emission (fluorescence) was observed at 410–440 nm as a result of Tyr oxidation. This novel light emission process is shown to be dependent on the solvent and aromatic ring substituents, however it does not depend on pH. It is proposed, that after initial formation of tyrosine phenoxyl radicals (TyrO●) by one electron-oxidation, the TyrO● absorbs a second photon to give an excited state species that undergoes subsequent light emission. The intensity of this emission depends on the Tyr concentration, and the detection of this emission can be used to identify and quantify one-electron formation of oxidized Tyr residues on proteins.

Evidence for the Prerequisite Formation of Phenoxyl Radicals in Radical-Mediated Peptide Tyrosine Nitration In Vacuo.

The elementary mechanism of radical-mediated peptide tyrosine nitration, which is a hallmark of post-translational modification of proteins under nitrative stress in vivo, has been elucidated in detail by using an integrated approach that combines the gas-phase synthesis of prototypical molecular tyrosine-containing peptide radical cations, ion-molecule reactions, and isotopic labeling experiments with DFT calculations. This reaction first involves the radical recombination of . NO2 towards the prerequisite phenoxyl radical tautomer of a tyrosine residue, followed by proton rearrangements, finally yielding the stable and regioselective 3-nitrotyrosyl residue product. In contrast, nitration with the π-phenolic radical cation tautomer is inefficient. This first direct experimental evidence for the elementary steps of the radical-mediated tyrosine nitration mechanism in the gas phase provides a fundamental insight into the regioselectivity of biological tyrosine ortho-nitration.

Enzymatic synthesis of highly flexible lignin cross-linked succinyl-chitosan hydrogels reinforced with reed cellulose fibres

This study reports on a novel strategy for laccase mediated synthesis of highly flexible succinyl chitosan (SucCTS) hydrogels cross-linked with lignosulfonates whose mechanical properties were further enhanced by incorporating glycerol as a plasticizer and reed cellulose fibres as reinforcement agent. Laccase enzyme was used to oxidize lignosulfonates resulting in the formation of phenoxyl radicals that reacted with NH2 groups SucCTS either through radical coupling or Michaeĺs addition. The oxidation of lignosulfonates resulted in 51 decrease in lignosulfonate OH groups and 74% SucCTS-NH2 groups, and concomitant gradual increase in molecular weight of the copolymer from an average 15 kDa to above 600 kDa. Incorporation of glycerol as plasticizer prevented stiffness, shrinking and random breaking of the hydrogels, as evidenced by the remarkable increase in elongation at break (up to160%) of the hydrogels. The laccase remained active and completely reduced oxygen to water in a closed system, making it suitable for application as oxygen barrier system. This study therefore demonstrate an environmentally friendly process for synthesizing 100% biobased (chitosan based hydrogels) using naturally occurring lignin as cross-linking agent and laccase as biocatalyst.

INFLUENCE OF DIHYDROQUERCETIN ON FRICTION OF SUPER-HIGH-MOLECULAR POLYETHYLENE BY STEEL AND ALLOY Ti6Al

The main reasons reducing the lifetime and failure of implants are tribo-oxidative processes. Stabilizing additives are used to inhibit tribo-oxidative activity and increase the lifetime of the implant. The most common stabilizer is α-tocopherol. The biocompatible stabilizer of a number of flavonoids, dihydroquercetin, was selected as the thermo- and tribo-oxidative stabilizer of UHMWPE. Dihydroquercetin (DHQ) stabilizes peroxide radicals formed as a result of the tribological process (friction of UHMWPE) by detaching a hydrogen atom from OH groups with the formation of a non-reactive phenoxyl radical. A comparative analysis of the samples, stabilized with DHQ and α-tocopherol, was carried out. The samples studied were prepared by direct compression molding at a temperature of 190ºС. The comparative study of thermo- and tribo-oxidative stability of UHMWPE samples modified with DHQ and α-tocopherol evidences that the thermo-oxidative stability induced with DHQ is higher that of α-tocopherol. In the first case the thermal-oxidative stability of the composition is maintained up to a temperature of 230 °C. Evolution of friction compositions at temperatures higher than that of the human body was investigated. Such tests were conducted using a gradual increase in friction velocity. During the tests, an increase in weight of initial UHMWPE and the composition with α-tocopherol induced by tribo-oxidative processes was observed. The introduction of dihydroquercetin, a highly effective biostabilizer of thermal oxidation, leads to the conservation of the original weight of the samples. When selecting a counterbody for the developed composition with DHQ, a study was conducted of friction on steel 3X13 and Ti6Al alloy used in medicine and in endoprostheses of artificial joints. It was found that the best results are obtained in the case of using a steel counterbody.

MODELING OF THE BIOSYNTHESIS OF LIGNIN AS A PROCESS OF ENZYMATIC DEHYDROPOLYMERIZATION MONOLIGNOLS

The results of theoretical modeling of lignin biosynthesis in the system of peroxidase-hydrogen peroxide-monolignol are presented. The process under study is presented as a sequence of enzymatic and chemical reactions occurring in an open system. The growth of the lignin molecule is due to successive stages of the formation of phenoxyl radicals of different generation numbers and acts of recombination of these radicals. The mathematical model of the process in the form of a system of 14 differential equations is proposed for the first time. For the numerical solution of the system, the eighth order Runge-Kutta-Fehlberg method with an automatic integration step was applied. The results of numerical integration are presented, which indicate a significant dependence of the process dynamics on the initial conditions. Various modes of the process, including oscillatory modes, are found. Features of the system dynamics are due to the choice of the initial concentration of monolignol, hydrogen peroxide and the enzyme, as well as the nonlinearity of differential equations and the presence of feedbacks. The results of numerical experiments within the framework of the first proposed mathematical concept on dehydropolymerization of monolignols allow us to give a theoretical justification of the phenomena that are currently treated as the processes of dynamic self-organization in the biosynthesis of this polymer.