THE PROBLEM OF LIMIT CYCLES FOR A LIENARD TYPE CUBIC SYSTEM

The purpose of this paper is to study a general Lienard type cubic system with one antisaddle and two saddles. We give some results of the existence and uniqueness of limit cycles as well as the evolution of limit cycles around the antisaddle for system (2) in the following when parameter a1 changes.     

Comparison of dispersive liquid–liquid microextraction and hollow fiber liquid–liquid–liquid microextraction for the determination of fentanyl,alfentanil, and sufentanil in water and biological fluids by high-performance liquid chromatography

Dispersive liquid–liquid microextraction (DLLME) and hollow fiber liquid–liquid–liquid microextraction (HF-LLLME) combined with HPLC–DAD have been applied for the determination of three narcotic drugs (alfentanil, fentanyl, and sufentanil) in biological samples (human plasma and urine). Different DLLME parameters influencing the extraction efficiency such as type and volume of the extraction solvent and the disperser solvent, concentration of NaOH, and salt addition were investigated. In the HF-LLLME, the effects of important parameters including organic solvent type, concentration of NaOH as donor solution, concentration of H₂SO₄ as acceptor phase, salt addition, stirring rate, temperature, and extraction time were investigated and optimized. The results showed that both extraction methods exhibited good linearity, precision, enrichment factor, and detection limit. Under optimal condition, the limits of detection ranged from 0.4 to 1.9 μg/L and from 1.1 to 2.3 μg/L for DLLME and HF-LLLME, respectively. For DLLME, the intra- and inter-day precisions were 1.7–6.4% and 14.2–15.9%, respectively; and for HF-LLLME were 0.7–5.2% and 3.3–10.1%, respectively. The enrichment factors were from 275 to 325 and 190 to 237 for DLLME and HF-LLLME, respectively. The applicability of the proposed methods was investigated by analyzing biological samples. For analysis of human plasma and urine samples, HF-LLLME showed higher precision, more effective sample clean-up, higher extraction efficiency, lower organic solvent consumption than DLLME.     

One‐Step Synthesis of Hybrid Core–Shell Metal–Organic Frameworks

Epitaxial growth of MOF‐on‐MOF composite is an evolving research topic in the quest for multifunctional materials. In previously reported methods, the core–shell MOFs were synthesized via a stepwise strategy that involved growing the shell‐MOFs on top of the preformed core‐MOFs with matched lattice parameters. However, the inconvenient stepwise synthesis and the strict lattice‐matching requirement have limited the preparation of core–shell MOFs. Herein, we demonstrate that hybrid core–shell MOFs with mismatching lattices can be synthesized under the guidance of nucleation kinetic analysis. A series of MOF composites with mesoporous core and microporous shell were constructed and characterized by optical microscopy, powder X‐ray diffraction, gas sorption measurement, and scanning electron microscopy. Isoreticular expansion of microporous shells and orthogonal modification of the core was realized to produce multifunctional MOF composites, which acted as size selective catalysts for olefin epoxidation with high activity and selectivity.     

Microbiological Transformation of L-Tyrosine to L-Dopa from Methanol Pretreated Biomass of a Novel Coriolus versicolor under Submerged Culture

The present study is concerned with the microbiological transformation of L-tyrosine to L-dopa by a newly isolated turkey tail mushroom Coriolus versicolor DOB-4. As tyrosinase (catechol oxidase, EC 1.10.3.1) is an extracellular enzyme, therefore biomass was used as an enzyme source in the reaction mixture. Biomass particles were pretreated with methanol and oven dried at 105 °C for 2 h. The optimal L-dopa production was achieved when 1.5 mg/ml L-tyrosine was used as the basal substrate. Thin layer chromatography and high-performance liquid chromatography analysis depicted that citric acid supports higher substrate conversion and product formation rates. A noticeable enhancement was observed when process parameters viz. L-tyrosine concentration (1.5 mg/ml), citric acid (1.5 mg/ml), time of incubation (50 min), and reaction temperature (60 °C) were optimized using Plackett–Burman design. The maximum production of L-dopa was found to be 0.872 mg/ml with L-tyrosine consumption of 1.002 mg/ml. The model terms were found highly significant (HS, p?≤?0.05), suggesting the potential commercial utility of the culture (df?=?3, LSD?=?0.342).     

Poly(amidoamine) dendrimer‐coated magnetic nanoparticles for the fast purification and selective enrichment of glycopeptides and glycans

Glycosylated proteins modulate various important functions of organisms. To reveal the functions of glycoproteins, in‐depth characterization studies are necessary. Although mass spectrometry is a very efficient tool for glycoproteomic and glycomic studies, efficient sample preparation methods are required prior to analyses. In the study, poly(amidoamine) dendrimer‐coated magnetic nanoparticles were presented for the specific enrichment and fast purification of glycopeptides and glycans. The enrichment and purification performance of the developed method was evaluated both at the glycopeptide, and the glycan level using several standard glycoprotein digests and released glycan samples. The poly(amidoamine) dendrimer‐coated magnetic nanoparticles not only showed selective affinity (Immunoglobulin G/Bovine Serum Albumin, 1/10 by weight) to glycopeptides and released glycans but also good sensitivity (0.4 ng/µL for Immunoglobulin G) for glycoproteomic and glycomic applications. Thirty‐five glycopeptides of Immunoglobulin G were detected after enrichment with poly(amidoamine) dendrimer‐coated magnetic nanoparticles. In addition, 55 ¹⁸O tagged deamidated glycopeptides belonging to human plasma glycoproteome were confirmed. Finally, fifty 2‐aminobenzoic acid, and 30 procainamide‐labelled human plasma N‐glycans released from human plasma glycoproteins were determined after purifications. The results indicate that the proposed enrichment and purification method using poly(amidoamine) dendrimer‐coated magnetic nanoparticles could be simply adjusted to sample preparation methods.     

Spectral,thermal, molecular modeling and biological studies on mono- and binuclear complexes derived from oxalo bis(2,3-butanedionehydrazone)

Background

Hydrazones and their metal complexes were heavily studied due to their pharmacological applications such as antimicrobial, anticonvulsant analgesic, anti-inflammatory and anti-cancer agents. This work aims to synthesize and characterize novel complexes of VO²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Zr⁴⁺and Pd²⁺ ions with oxalo bis(2,3-butanedione-hydrazone). Single crystals of the ligand have been grown and analyzed.

Results

Oxalo bis(2,3-butanedionehydrazone) [OBH] has a monoclinic crystal with P 1 21/n 1 space group. The VO²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Zr⁴⁺ and Pd²⁺ complexes have the formulas: [VO(OBH–H)₂]·H₂O, [Co(OBH)₂Cl]Cl·½EtOH, [Ni₂(OBH)Cl₄]·H₂O·EtOH, [Cu(OBH)₂Cl₂]·2H₂O, [Zn(OBH–H)₂], [Zr(OBH)Cl₄]·2H₂O, and [Pd₂(OBH)(H₂O)₂Cl₄]·2H₂O. All complexes are nonelectrolytes except [Co(OBH)₂Cl]Cl·½EtOH. OBH ligates as: neutral tetradentate (NNOO) in the Ni²⁺ and Pd²⁺ complexes; neutral bidentate (OO) in [Co(OBH)₂Cl]Cl·½EtOH, [Zr(OBH)Cl₄]·2H₂O and [Cu(OBH)₂Cl₂]·2H₂O and monobasic bidentate (OO) in the Zn²⁺ and VO²⁺ complexes. The NMR (¹H and ¹³C) spectra support these data. The results proved a tetrahedral for the Zn²⁺ complex; square-planar for Pd²⁺; mixed stereochemistry for Ni²⁺; square-pyramid for Co²⁺ and VO²⁺ and octahedral for Cu²⁺ and Zr⁴⁺ complexes. The TGA revealed the outer and inner solvents as well as the residual part. The molecular modeling of [Ni₂(OBH)Cl₄]·H₂O·EtOH and [Co(OBH)₂Cl]Cl·½EtOH are drawn and their molecular parameters proved that the presence of two metals stabilized the complex more than the mono metal. The complexes have variable activities against some bacteria and fungi. [Zr(OBH)Cl₄]·2H₂O has the highest activity. [Co(OBH)₂Cl]Cl·½EtOH has more activity against Fusarium.

Conclusion

Oxalo bis(2,3-butanedionehydrazone) structure was proved by X-ray crystallography. It coordinates with some transition metal ions as neutral bidentate; mononegative bidentate and neutral tetradentate. The complexes have tetrahedral, square-planar and/or octahedral structures. The VO²⁺ and Co²⁺ complexes have square-pyramid structure. [Cu(OBH)₂Cl₂]·2H₂O and [Ni₂(OBH)Cl₄]·H₂O·EtOH decomposed to their oxides while [VO(OBH–H)₂]·H₂O to vanadium. The energies obtained from molecular modeling calculation for [Ni₂(OBH)Cl₄]·H₂O·EtOH are less than those for [Co(OBH)₂Cl]Cl·½EtOH indicating the two metals stabilized the complex more than mono metal. The Co(II) complex is polar molecule while the Ni(II) is non-polar.

Graphical abstract

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Electromembrane extraction of gonadotropin‐releasing hormone agonists from plasma and wastewater samples

In the present study, for the first time electromembrane extraction followed by high performance liquid chromatography coupled with ultraviolet detection was optimized and validated for quantification of four gonadotropin‐releasing hormone agonist anticancer peptides (alarelin, leuprolide, buserelin and triptorelin) in biological and aqueous samples. The parameters influencing electromigration were investigated and optimized. The membrane consists 95% of 1‐octanol and 5% di‐(2‐ethylhexyl)‐phosphate immobilized in the pores of a hollow fiber. A 20 V electrical field was applied to make the analytes migrate from sample solution with pH 7.0, through the supported liquid membrane into an acidic acceptor solution with pH 1.0 which was located inside the lumen of hollow fiber. Extraction recoveries in the range of 49 and 71% within 15 min extraction time were obtained in different biological matrices which resulted in preconcentration factors in the range of 82–118 and satisfactory repeatability (7.1 < RSD% < 19.8). The method offers good linearity (2.0–1000 ng/mL) with estimation of regression coefficient higher than 0.998. The procedure allows very low detection and quantitation limits of 0.2 and 0.6 ng/mL, respectively. Finally, it was applied to determination and quantification of peptides in human plasma and wastewater samples and satisfactory results were yielded.     

Imidomethylation of C-nucleophiles using O-phthalimidomethyl trichloroacetimidate and catalytic amounts of TMSOTf

The O-phthalimidomethyl trichloroacetimidate (1), as a latent aminomethylating agent, exhibits high electrophilicity towards a variety of C-nucleophiles in the presence of catalytic amounts of TMSOTf and mild reaction conditions. The nucleophiles include aromatics, alkenes and active methylene compounds 2-11 whereby a phthalimidomethyl group could be introduced to give compounds 12-22. Removal of the phthaloyl group gave the respective amines, β-amino ketones, and β-amino acids. The O-(trichloroacetamido)methyl trichloroacetimidate (35) was also found to be a good amidomethylating agent.     

Separation and determination of amitriptyline and nortriptyline by dispersive liquid-liquid microextraction combined with gas chromatography flame ionization detection

Dispersive liquid–liquid microextraction (DLLME) coupled with gas chromatography–flame ionization detection (GC–FID) was applied for the determination of two tricyclic antidepressant drugs (TCAs), amitriptyline and nortriptyline, from water samples. This method is a very simple and rapid method for the extraction and preconcentration of these drugs from environmental sample solutions. In this method, the appropriate mixture of extraction solvent (18 μL Carbon tetrachloride) and disperser solvent (1 mL methanol) are injected rapidly into the aqueous sample (5.0 mL) by syringe. Therefore, cloudy solution is formed. In fact, it is consisted of fine particles of extraction solvent which is dispersed entirely into aqueous phase. The mixture was centrifuged and the extraction solvent is sedimented on the bottom of the conical test tube. 2.0 μL of the sedimented phase is injected into the GC for separation and determination of TCAs. Some important parameters, such as kind of extraction and disperser solvent and volume of them, extraction time, pH and ionic strength of the aqueous feed solution were optimized. Under the optimal conditions, the enrichment factors and extraction recoveries were between 740.04–1000.25 and 54.76–74.02%, respectively. The linear range was (0.005–16 μg mL⁻¹) and limits of detection were between 0.005 and 0.01 μg mL⁻¹ for each of the analytes. The relative standard deviations (R.S.D.) for 4 μg mL⁻¹ of TCAs in water were in the range of 5.6–6.4 (n = 6). The performance of the proposed technique was evaluated for determination of TCAs in blood plasma.     

Investigation of the effect of SSP in stabilizing the structure of condensation polymer blends via rheological measurements

We investigated the influence of solid-state polymerization (SSP) process on the reactions that could be taken place at the interphase of polycondensation polymer blends to stabilize the structure obtained after melt mixing. Polyethylene terephthalate (PET) and polyamide 66 (PA66) were melt blended in a mixer, and subsequent SSP process was performed for each sample. FTIR spectra indicated reactions between two polymers. Viscosity behavior and interfacial slip were investigated by measuring shear viscosity of components and blends before and after SSP and then compared with the viscosity calculated from the log-additivity model. The results showed that after SSP, there was no sign of interfacial slip, the slope of viscosity reduction with increasing shear rate became smaller, and the viscosity of blends showed positive deviation at all examined shear rate from the log-additivity model while this deviation was negative at higher shear rate before SSP. SEM micrographs, which were taken after shear stress was imposed on the samples, also indicated the morphological stability after SSP. Furthermore, we studied the effect of functional groups concentration on the reactions at the interphase by using hydrolyzed PET as a precursor for blends. The results showed that slip at the interface would decrease with increasing functional groups of the precursors. These results are particularly valuable for using recycled polymers.