Journal of Fluorescence - In this study, a sensitive fluorimetric method is proposed for the determination of piroxicam using nitrogen graphene quantum dots (N-GQDs) and gold nanoparticles coated... 相似文献
Eight new guaipyridine sesquiterpene alkaloids, rupestines F-M (1-8) were isolated from the leaves of Artemisia rupestris and their structures were elucidated on the basis of 2D-NMR data. The absolute configurations of 1-8 have been assigned by comparison of their experimental and calculated circular dichroism (CD) spectra. 相似文献
Following an initial clean-up step on a Sephadex LH-20 column, high-speed countercurrent chromatography was successfully applied to the isolation and purification of three flavonoids from a crude sample of Helichrysum arenarium (L.) Moench. HSCCC was performed with a two-phase solvent system composed of ethyl acetate–water (1:1, v/v). Naringenin-7-O-β-d-glycoside (2.3 mg), isoquercitrin (3.5 mg), and astragalin (6.7 mg), with purities of 96.05%, 93.63%, 95.23%, respectively, were separated from 160 mg of crude sample in a one-step separation. The structure identification was by 1H NMR and 13C NMR. 相似文献
A facile synthesis of 1,2‐azidoalcohols from their epoxides using α,α',α”‐N‐hexakis(triethylammoniummethylene chloride)‐melamine as a multi‐site phase‐transfer catalyst in water was developed. By this eco‐friendly and highly atom‐economic method, a variety of 1,2‐azidoalcohols was obtained in high yields with excellent regioselectivity and in short reaction times. Application of this six‐site PTC in the ring opening of epoxides by cyanide, acetate and chloride anions was also studied. The catalyst can be recovered after completion of the reaction and can be recycled without affecting the catalytic property. 相似文献
The original spiral tube assembly for high-speed counter-current chromatography (HSCCC) is further improved by a new tube configuration called “flat-twisted tubing” which was made by extruding the tube (1.6 mm I.D.) through a narrow slot followed by twisting along its axis forming about 1 cm twisted screw pitch. This modification interrupts the laminar flow of the mobile phase through the tube and continuously mixes the two phases through the column. The performance of this spiral tube assembly was tested by three types of two-phase solvent systems with different polarities each with a set of suitable test samples such as DNP-amino acids, dipeptides and proteins at the optimal elution modes. In general all these test samples yielded higher resolution with the lower mobile phase than the upper mobile phase. In the most hydrophobic two-phase solvent system composed of hexane–ethyl acetate–methanol–0.1 M hydrochloric acid (1:1:1:1, v/v/v/v), DNP–amino acids were separated with Rs-a (peak resolution based on the same column capacity adjusted for comparison) at 4.40 and 73% of stationary phase retention at a flow rate of 0.5 ml/min with the lower mobile phase. In the polar solvent system composed of 1-butanol–acetic acid–water (4:1:5, v/v/v), dipeptide samples were resolved with Rs-a at 4.06, compared to 2.79 with the cross-pressed tube assembly at 45% stationary phase retention, each at a flow rate of 1 ml/min. Finally in the aqueous–aqueous polymer phase systems composed of polyethylene glycol 1000 – dibasic potassium phosphate each 12.5% (w/w) in water, protein samples were resolved with Rs-a at 2.53 compared to 1.10 with the cross-pressed tube assembly at 52% of stationary phase retention, each at a flow rate of 1 ml/min. These results indicate that the present system substantially improves the partition efficiency with a satisfactory level of stationary phase retention by the lower mobile phase. 相似文献
Isothermal and dynamic differential scanning calorimetry (DSC) was exploited to study the curing behavior of diglycidyl ether bisphenol-A epoxy resin with various combining ratios of dicyandiamide (DICY) and nadic methyl anhydride (NMA). Curves of prepared samples indicated that the enthalpy of the reaction decreased with increasing the molar ratios (NMA/DICY) up to 40% after which an exothermic peak peculiar to the effect of anhydride appeared at a higher temperature. The curing behavior examination of the samples containing the aforementioned molar ratio of NMA/DICY (= 40%) was carried out using isothermal condition at different temperatures (130–145 °C) and dynamic condition DSC at various heating rates (2.5–20 °C min−1). Under the isothermal condition, by constructing a master curve, the values of activation energy (Ea) and pre-exponential factor (A) were calculated 89.3 kJ mol−1 and 1.2 × 10+9 s−1, respectively. The activation energy of the curing reactions in a dynamic mode was obtained 85.32 kJ mol−1 and 88.02 kJ mol−1 using Kissinger and Ozawa methods, respectively. Likewise, pre-exponential factors were also calculated 3.35 × 10+8 and 7.4 × 10 +8 s−1, respectively. The overall order of reaction for both conditions was found to be a value around 3.
A new fatty acid ester (1) and seven known phenolic compounds, i.e. salfredin B11 (2), nigephenol C (3), nigephenol B (4), acetovanillion (5), p-hydroxybenzoic acid (6), p-hydroxy-acetophenone (7) and p-hydroxybenzaldehyde (8), were isolated from the seeds of Nigella sativa var. hispidula. Among them, compounds 5, 7 and 8 were isolated from Nigella for the first time. Their structures were elucidated with HR-ESI-MS, 1D and 2D NMR spectra. Evaluation of the isolated compounds on protein tyrosine phosphatase (PTP1B) assay indicated that although compounds 2–8 show no promising anti-PTP1B activities, compound 1 possess anti-PTP1B activity with an IC50 value of 7.38 ± 0.14 μM in vitro. 相似文献
Torsion rods are a primary component of many power transmission and other mechanical systems. The behavior of these rods under
elastoplastic torsion is of major concern for designers. Different methods have so far been proposed which deal with the elastoplastic
torsion of rods, most of which assume constant yield stress. This assumption produces rough and inaccurate results when the
rods are heat treated, since in the process of heat treatment the form of yield stress distribution across the rod cross section
changes. We propose a new method for calculating elastoplastic torsion of rods of simply connected cross section which is
based on heat treatment observations. In our method the full plastic stress function is obtained by using the semi-inverse method. Elastoplastic stress function is obtained by generalizing the idea of the membrane analogy and using a piecewise continuous stress function. Since the proposed form of yield stress distribution can not be handled
by the current Finite Element packages, we produce a computer package with a 3D graphical interface capable of calculating
and displaying the 3D elastoplastic stress function, shear stress contours, and torque-angle of rotation per unit length.
We show that our method produces excellent agreement for several known cross sections in comparison to methods which assume
constant yield stress. 相似文献