Initial Degradation Pathways Research Articles

Effects of oxidant molecules on the thermal decomposition of 4,7‐diaminopyridazino[4,5‐c]furanoxane crystal: Ab initio molecular dynamics simulations

AbstractInserting oxidant molecules into the crystal voids to form oxidant cocrystal can greatly enhance the detonation properties of energetic materials. Therefore, oxidant cocrystals are regarded as one type of potential candidates of high energy density materials. The effects of the oxidants (HNO3 and HClO4) on the thermal decomposition mechanisms of two oxidant cocrystals (DPF(4,7‐diaminopyridazino[4,5‐c] furanoxane):HNO3 and DPF:HClO4) were investigated by ab initio molecular dynamics (AIMD). AIMD simulations are performed using a NVT ensemble. The temperature was controlled by a Nosé thermostat. First, we performed 5 ps simulations at 298.15 K to get an equilibrium system. Then, the simulations were performed at 3000 K with a time step of 1 fs and a total time of 15 ps. The initial degradation pathways of the two cocrystals have the same and involve the cleavage of the N–H and N–O bonds. Five different decay pathways were found in the subsequent reactions: intermolecular H transfer, N–O bond cleavage, N–N bond cleavage, and C–C bond formation and cleavage. Both HNO3 and HClO4 can promote the formation of N2, H2O and CO2, but only have a certain effect. Different oxidant molecules hardly affect the initial and subsequent decomposition paths of DPFH.

Thermal and degradation study of candesartan under pyrolysis and oxidizing conditions

Candesartan is a widely used active pharmaceutical ingredient in the treatment of systemic arterial hypertension. This work presents the investigation of the thermal behavior of candesartan, such as its thermal stability, melting temperature, and thermal decomposition, both in pyrolysis (N2) or oxidizing (air) conditions by evolved gas analysis (EGA). The thermal study showed that the drug melts at 189 °C (onset at 184 °C) followed by decomposition in six mass loss steps. The drug presents a similar thermal behavior under pyrolytic and oxidizing conditions up to 360 °C. The main gaseous products released are N2O, NH3, CO, CO2, HCN, and CH3CH2NCO (ethyl isocyanate). Pyrolysis degradation products have been analyzed, and a thermal degradation pathway has been proposed with fifteen different structures based on the HRMS and MS/MS analysis. Furthermore, the characterization of the pyrolytic degradation products of candesartan by UHPLC-HRMS is reported for the first time, showing that the initial degradation pathway occurs through a dimerization reaction.

Biodegradation of organophosphorus pesticide profenofos by the bacterium Bacillus sp. PF1 and elucidation of initial degradation pathway

ABSTRACT Among the organophosphate pesticides, the wide and indiscriminate use of profenofos (PFF) in agricultural and horticultural crops has resulted in serious environmental and animal health concerns and therefore demands an urgent need to develop a biological solution for its effective removal from the environment. For the bioremediation of PFF, a strain PF1, capable of utilizing profenofos as the sole source of carbon and energy was isolated from the soil samples of apple orchards of Shimla region of Himachal Pradesh, India. Based on the biochemical, FAME, and 16S rRNA gene analysis the bacterium PF1 was identified as Bacillus altitudinis (GenBank: MH986176). The strain was able to degrade 50μg mL−1 PFF up to 93% within 30 days of incubation at 28°C, pH 7.0. A linear regression analysis performed on the data-set revealed the statistical significance of the relationship between the growth of the bacterial population and the degradation of pesticides. The compound 4-Bromo-2-chlorophenol (BCP) was detected as one of the pathway metabolites which further were completely degraded to lower pathway metabolites. A probable PFF degradation pathway has been proposed which follows the path from PFF to BCP and ultimately enters into the TCA cycle. To the best of our knowledge, this is the first report of PFF biodegradation by any Bacillus species of western Himalayan origin exhibiting close phylogenetic association with Bacillus altitudinis. This indigenous bacterium can be useful to bio-remediate the PFF contaminated soil as this pesticide is extensively used in the different horticulture fields in Himachal Pradesh, India.

Degradation of Cytosine Radical Cations in 2'-Deoxycytidine and in i-Motif DNA: Hydrogen-Bonding Guided Pathways.

Radical cations of nucleobases are key intermediates causing genome mutation, among which cytosine C•+ is of growing importance because the ensuing cytosine oxidation causes GC → AT transversions in DNA replication. Although the chemistry and biology of steady-state C oxidation products have been characterized, time-resolved study of initial degradation pathways of C•+ is still at the preliminary stage. Herein, we choose i-motif, a unique C-quadruplex structure composed of hemiprotonated base pairs C(H)+:C, to examine C•+ degradation in a DNA surrounding without interference of G bases. Comprehensive time-resolved spectroscopy were performed to track C•+ dynamics in i-motif and in free base dC. The competing pathways of deprotonation (1.4 × 107 s-1), tautomerization (8.8 × 104 s-1), and hydration (5.3 × 103 s-1) are differentiated, and their rate constants are determined for the first time, underlining the strong reactivity of C•+. Distinct pathway is observed in i-motif compared with dC, showing the prominent features of C•+ hydration forming C(5OH)• and C(6OH)•. By further experiments of pH-dependence, comparison with single strand, and with Ag+ mediated i-motif, the mechanisms of C•+ degradation in i-motif are disclosed. The hydrogen-bonding within C(H)+:C plays a significant role in guiding the reaction flux, by blocking the tautomerization of C(-H)• and reversing the equilibrium from C(-H)• to C•+. The C radicals in i-motif thus retain more cation character, and are mainly subject to hydration leading to lesion products that can induce disruption of i-motif structure and affect its critical roles in gene-regulation.

Photo-electro-Fenton process applied to the degradation of valsartan: Effect of parameters, identification of degradation routes and mineralization in combination with a biological system

In this work, the oxidation of the antihypertensive drug valsartan by the Electro-Fenton (EF) and photo-electro-Fenton (PEF) processes was studied using a Ti/IrO2 doped with SnO2 as anode, and a carbon felt air diffusion electrode as cathode. Initially, the influence of variables such as supporting electrolyte type, current density, and pH on EF and/or PEF processes was evaluated. The processes were carried out in batch mode, in an open and undivided cell of 200 cm3. The efficiency of the systems was evaluated in terms of the removal of the initial contaminant and rate of mineralization. When NaCl was used as a supporting electrolyte at pH 3.0 and current density 3.46 mA/cm2 adding 3.6 × 10−5 mol/L of Fe2+, total valsartan (20 mg/L) degradation was observed after 45 min. After 120 min, even if total removal of valsartan was reached, only 25% of mineralization was obtained. Thus, valsartan degradation tests at near neutral pH in presence of oxalic acid (4.6 × 10−5 mol/L) lead to comparable results with those obtained at pH 3.0. Primary aromatic intermediates were identified by high resolution mass spectrometry (HRMS) using hybrid quadrupole- time-of-flight (QTOF) MS, from which an initial degradation pathway was proposed. At the end of the PEF system, several aliphatic acids were accumulated and observed, which were effectively removed in a subsequent aerobic biological system. The results demonstrate the feasibility of PEF and biological coupling process to completely mineralize emerging pharmaceutical pollutants, such as valsartan, at natural pH.

Degradation kinetic of phthalate esters and the formation of brominated byproducts in heat-activated persulfate system

The degradation kinetics of two phthalate esters (PAEs) (i.e., diethyl phthalate (DEP) and dibutyl phthalate (DBP)) in heat-activated persulfate (PS) system were studied and followed pseudo-first-order kinetics. Results demonstrated that both sulfate radical (SO4-·) and hydroxyl radical (HO) were responsible for degrading DEP and DBP. The pseudo-first-order rate constant (kobs) enhanced remarkably as reaction temperature and PS dosages increased, while it decreased as the initial PAEs concentrations increased. Lower pH values promoted the degradation over the pH range chosen here (5.0–8.0). Natural organic matter (NOM) and HCO3- showed inhibitory effects on PAEs degradation by quenching SO4-· and HO. At any pH value studied here, Cl− enhanced PAEs degradation at lower concentrations while it suppressed degradation as Cl− concentration further increased. The coexisting Br− hindered the degradation of PAEs mainly by suppressing SO4-·. Significant formation of dibromoacetonitrile (DBAN) and bromoform (TBM) was verified with Br− and NOM in the degradation system, while only TBM was detected without NOM. Bromate was also detected in the Br− containing degradation system. Generally, NOM facilitated the formation of brominated disinfection byproducts (Br-DBPs), while it suppressed the generation of bromate. Additionally, it is noteworthy that the degradation efficiency of DBP was higher than that of DEP probably owing to the longer alkyl side chain of DBP. The DBP degradation system was observed to form slightly more Br-DBPs than DEP, while bromate had an opposite trend. Besides, the potential initial degradation pathways of DEP and DBP were discussed by the computational analysis.

Mechanical reinforcement and thermal properties of PVA tricomponent nanocomposites with chitin nanofibers and cellulose nanocrystals

Investigating tricomponent materials containing poly(vinyl alcohol) (PVA), cellulose nanocrystals (CNCs), and chitin nanofibers (ChNFs) provides potential for understanding the modulation of material properties. CNCs or ChNFs in polymers have been documented previously; however, composites containing both in combination has been underexplored. The results show that at certain CNC/ChNF ratios, the modulus and tensile strength of the composites are enhanced beyond that possible with either nanofiller alone. However, composites containing CNC/ChNF ratios that corresponded to balanced nanofiller surface charges showed reduced mechanical properties, potentially due to charge-driven association of the nanofillers. Furthermore, changes in thermal degradation behavior suggested that these nanofillers could hinder initial degradation pathways, providing additional benefits to material performance. Overall, these initial results suggest that tricomponent nanocomposites utilizing PVA, CNCs, and ChNFs open additional possibilities for tuning the mechanical and thermal properties of creating high-performance materials with reduced environmental impact.

Efficient mineralization of aqueous antibiotics by simultaneous catalytic ozonation and photocatalysis using MgMnO3 as a bifunctional catalyst

Herein a novel coupling process of simultaneous catalytic ozonation and photocatalysis (SCOP) using a bifunctional catalyst for the antibiotics wastewater treatment was developed. MgMnO3, as the first example of bifunctional catalyst, was successfully prepared by a simple sol-gel method. In the MgMnO3-mediated SCOP process, the TOC removal of sulfamethoxazole, tetracycline, ciprofloxacin, and trimethoprim within 80 min reached 94.7 ± 0.9%, 88.4 ± 0.9%, 97.8 ± 1.0%, and 76.3 ± 0.9%, respectively, much higher than that in ozonation or photocatalysis (less than 20%). The first order kinetics reaction constant of TOC removal in the case of tetracycline degradation by SCOP is 2.58 × 10−2 min−1, which is 2.7, 23.0, and 6.2 times that by catalytic ozonation, photocatalysis, and photo ozonation, respectively. ESR spectra show that much more ·OH radicals are generated in SCOP than the sum in photocatalysis and catalytic ozonation processes, indicating the synergism between two processes and explaining the high mineralization efficiency in SCOP. Finally, eight intermediates were identified by LC-MS analysis and the initial degradation pathway of tetracycline in SCOP was proposed. Excellent treatment efficiency, high ozonation utilization (>93%) and low energy consumption (14.9 KJ/mg TOC) suggest that SCOP is a promising choice for antibiotics-containing wastewater treatment.

Removal of Benzothiazole from Contaminated Waters by Ozonation: The Role of Direct and Indirect Ozone Reactions

AbstractBenzothiazoles are emerging chemical pollutants mainly coming from leather, paper and rubber industries; due to their use as: herbicides, corrosion inhibitors, anti-freezers, and vulcanisation accelerators. This article presents experimental data on ozone treatment of benzothiazole contaminated waters. The effect of the initial concentration of benzothiazole, ozone dosage, temperature (10-30 °C), and pH (2-9), on ozonation removal rate were assessed at bench scale. Experimental results show that reaction between ozone and benzothiazole could be approximated to a second-order kinetic law. Kinetic parameters for direct and indirect ozone reactions are estimated and temperature dependence of rate parameters is evaluated. Moreover, an initial degradation pathway of benzothiazole ozonation is proposed.

Degradation of Crystal Violet by catalytic ozonation using Fe/activated carbon catalyst

An Fe/activated carbon catalyst was prepared and used in catalytic ozonation of Crystal Violet. The catalyst was characterized by SEM and XRD, and the results showed that the iron loaded on activated carbon existed in α-FeOOH phase. An improved effect was achieved in the decolorization of Crystal Violet via catalytic ozonation in the presence of Fe/activated carbon compared with ozonation alone. Almost complete decolorization (>96%) of 400mg/L Crystal Violet was achieved after 30min reaction under the conditions of 4.44mg/min ozone dosage, 300mL/min gas flow rate, 2.5g/L catalyst dosage and pH 7, while 57% COD removal efficiency was obtained when the reaction time was prolonged to 90min. The acute toxicity to Daphnia magna decreased during 90min of catalytic ozonation. The main intermediates were separated and identified by gas chromatography–mass spectrometry (GC–MS) technique and an initial degradation pathway of Crystal Violet was proposed.

Microwave-assisted photocatalytic degradation of dimethyl phthalate using a microwave discharged electrodeless lamp

The microwave-assisted photocatalytic degradation (MW/PC) of dimethyl phthalate (DMP) was investigated, using a microwave discharged electrodeless lamp (MDEL) as the UV source. DMP could be rapidly and completely degraded by the MW/PC process. The effects of photocatalyst dosage, UV light intensity, initial pH and concentration of DMP, as well as irradiation time, were studied in detail. Synergistic effects were found to exist between microwave irradiation and TiO2 photocatalysis, and the greater efficiency of the MW/PC process might be ascribed to non-thermal effects of microwave irradiation. Detection results confirmed that microwave irradiation could give rise to the generation of hydroxyl radicals. The major intermediates of DMP in the MW/PC process were deduced to be phthalic acid monomethyl ester, phthalic acid, o-phenyl diethyl ketone and o-phthalic anhydride. Based on the results, two initial degradation pathways of DMP were proposed.

Photosonolysis of TCA, TCE, and PCE in Flow-Through Reactor System

Photosonolysis of 1,1,1-trichloroethane (TCA), trichloroethylene (TCE), and tetrachloroethylene (PCE) in water was investigated using a cup-horn, flow-through reactor system. Water containing titanium dioxide was deliberately contaminated with a mixture of volatile organic compound (VOC)—TCA, TCE, and PCE—and individual VOC. These solutions were irradiated with ultraviolet light (UV) and ultrasonic waves (US), independently and concurrently (UVUS). The values of the first-order degradation rate constant and the removal efficiency for the VOC were evaluated for the UV, US, and UVUS treatments. The results showed that the concurrent use of UV and US increased the VOC degradation rate and removal efficiency beyond the additive effect of UV and US, suggesting that the UVUS effect on the decomposition of the VOC is synergistic under certain conditions. TCE and PCE were more readily degraded than TCA, suggesting that double-bond cleavage is one of the initial degradation pathways. With the reactor specification...

Degradation Pathways of a Peptide Boronic Acid Derivative, 2‐Pyz‐(CO)‐Phe‐Leu‐B(OH)2

The peptide boronic acid derivative 2‐Pyz‐(CO)‐Phe‐Leu‐B(OH)2 is a potent inhibitor of 20S proteasome and a proposed anticancer agent. During preformulation studies, the compound presented erratic stability behavior. Efforts were made to isolate and identify the degradation products, thereby helping to identify possible mechanisms for the degradation. The reaction of 2‐Pyz‐(CO)‐Phe‐Leu‐B(OH)2 with hydrogen peroxide not only provided a convenient way to isolate the initial degradation products seen from hydrolysis in aqueous buffers but also showed that the major, initial degradation pathway was probably oxidative in nature. The isolated degradation products were characterized by nuclear magnetic resonance spectroscopy, mass spectrometry, and optical rotation dispersion. In the presence of hydrogen peroxide, the boronic acid group was cleaved from 2‐Pyz‐(CO)‐Phe‐Leu‐B(OH)2 to give an alcohol with an apparent retention of the original stereochemistry. Subsequent isomerization and further hydrolysis were then seen. Surprisingly, added ascorbate and EDTA accelerated rather than inhibited degradation. Degradation of 2‐Pyz‐(CO)‐Phe‐Leu‐B(OH)2 under acidic and basic conditions seemed to be mediated by an initial oxidative degradation pathway similar to that seen with the peroxide. © 2000 Wiley‐Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 89: 758–765, 2000

Formation of sugar-specific reactive intermediates from (13)C-labeled L-serines.

Analysis of the pyrolysis products of [1-(13)C], [2-(13)C], and [3-(13)C]-labeled L-serines has indicated the presence of three initial degradation pathways. Decarboxylation followed by deamination produces aminoethanol and acetaldehyde, respectively; a retro-aldol pathway generates formaldehyde and glycine. Dehydration of L-serine can lead to the formation of pyruvic acid, which eventually can be converted into the amino acid alanine. Formation of alanine and glycine was confirmed due to the detection of 2, 5-diketo-3,6-dimethylpiperazine and cycloglycylalanine. Most of the advanced decomposition products of L-serine can be rationalized on the basis of these initial degradation products. Label incorporation studies have elucidated the origin of carbonyl precursors of methyl- and 2,3-dimethylpyrazines formed in the thermal decomposition mixture of L-serine. Three mechanistic pathways were identified for the formation of carbonyl precursors of methyl- and 2, 3-dimethylpyrazines. The major pathway (70%) for the formation of the precursor of methylpyrazine involved aldol addition of formaldehyde to glycolaldehyde to form glyceraldehyde. On the other hand, the major pathway (60%) for the formation of the precursor of 2,3-dimethylpyrazine involved an aldol condensation of acetaldehyde with glycolaldehyde to form 2,3-butanedione.

Cyanobacteria as agents for the control of pollution by pesticides and chlorinated organic compounds

Cyanobacteria are phototrophic aquatic micro-organisms that are found in a variety of environments, including polluted ones. Fifteen strains of cyanobacteria that belong to three taxonomic groups are able to degrade lindane (λ-hexachlorocyclohexane, a recalcitrant pesticide). The initial degradation pathway has been studied in two filamentous nitrogen-fixing strains of Anabaena sp. PCC 7120 and Nostoc ellipsosporum. These cyanobacteria dechlorinated loindane first to pentachlorocyclohexene and then to a mixture of trichlorobenzenes, and possibly further. Lindane dechlorination by these organisms occurred only in the presence of nitrate in the medium. Both ammonium and darkness inhibited the process. This combination of observations led us to the hypothesis that the nitrate-reduction system of cyanobacteria may be involved in dechlorination. The hypothesis was proven by the analysis of Anabaena sp. transpositional mutants in four genes of the nir operon. The mutants were unable to dechlorinate lindane. However, there was no correlation between lindane dechlorination and activities of individual proteins encoded by this operon. Genetic engineering of Anabaena sp. and N. ellipsosporum that introduced linA lindane dechlorination operon from Psuedomonas paucimobilis allowed us to uncouple dechlorination from nitrate requirement. Introduction, by genetic engineering, of fcABC (the 4-chlorobenzoate dechlorination system from Arthrobacter globiformis) to Anabaena sp and N. ellipsosporum rendered these strains newly capable of 4-chlorobenzoate dechlorination both constitutively and inducible by an environmental factor.