Immunosuppressive ent‐Kaurene Diterpenoids from Isodon serra

Three new enmein‐type ent‐kaurenoids, i.e., the two pairs 1 and 2 of 20‐epimers and the (20R)‐isomer 3 , besides the seven known diterpenoids 4 – 10 , were isolated from the aerial parts of Isodon serra. Their structures were elucidated by spectroscopic techniques and X‐ray diffraction. The immunosuppressive effect for T‐lymphocytes proliferation induced by Con A in BALB/c mouse was evaluated for the isolates 1 – 10 . They all displayed a remarkable inhibitory effect, with multi‐glycosides of Tripterygium wilfordii as positive reference substance (Table 3).     

Addition of grignard reagents to aryl acid chlorides: an efficient synthesis of aryl ketones

[chemical reaction: see text]. Direct addition of Grignard reagents to acid chlorides in the presence of bis[2-(N,N-dimethylamino)ethyl] ether proceeds selectively to provide aryl ketones in high yields. A possible tridentate interaction between Grignard reagents and bis[2-(N,N-dimethylamino)ethyl] ether moderates the reactivity of Grignard reagents, preventing the newly formed ketones from nucleophilic addition by Grignard reagents.     

A zwitterion of 1,5‐bis­(2‐hydroxy­benzamido)‐3‐aza­pentane: a two‐dimensional hydrogen‐bonding network involving dimers

The title compound, 2‐{N‐[2‐(2‐hydroxy­benzamido)ethyl­ammonio­ethyl]amino­carbon­yl}phenolate, C₁₈H₂₁N₃O₄, crystallizes in a zwitterionic form as a result of inter­molecular proton transfer and possesses a negatively charged phenolate group and a protonated amino group. The 2‐hydroxy­benzamide and 2‐(amino­carbonyl)­phenolate moieties attached to the two ends of the C—C—N—C—C backbone adopt a cis conformation in relation to this backbone. All N‐ and O‐bound H atoms are involved in hydrogen‐bond formation; the zwitterions are first linked into head‐to‐tail dimers, which are further organized into a two‐dimensional network parallel to the crystallographic bc plane.     

Use of the tubulin bound paclitaxel conformation for structure-based rational drug design

A new computational docking protocol has been developed and used in combination with conformational information inferred from REDOR-NMR experiments on microtubule bound 2-(p-fluorobenzoyl)paclitaxel to delineate a unique tubulin binding structure of paclitaxel. A conformationally constrained macrocyclic taxoid bearing a linker between the C-14 and C-3'N positions has been designed and synthesized to enforce this "REDOR-taxol" conformation. The novel taxoid SB-T-2053 inhibits the growth of MCF-7 and LCC-6 human breast cancer cells (wild-type and drug resistant) on the same order of magnitude as paclitaxel. Moreover, SB-T-2053 induces in vitro tubulin polymerization at least as well as paclitaxel, which directly validates our drug design process. These results open a new avenue for drug design of next generation taxoids and other microtubule-stabilizing agents based on the refined structural information of drug-tubulin complexes, in accordance with typical enzyme-inhibitor medicinal chemistry precepts.     

Adsorptive stripping measurements of germanium(IV) in the presence of pyrogallol

The adsorptive stripping voltammetric determination of germanium(IV) based on the adsorptive accumulation of the germanium(IV)-pyrogallol complex on a hanging mercury drop electrode is reported. The reduction current of the adsorbed germanium complex is measured by differential-pulse cathodic stripping voltammetry. The peak potential is at -0.42 V vs. Ag AgCl (saturated KCL). The effects of various parameters (ligand concentration, supporting electrolytic composition and concentration, accumulation potential and collection time) on the response are discussed. With controlled accumulation for 3 min, the detection limit is 1.2 x 10(-9) M germanium. The relative standard deviation (at 1.2 x 10(-8) M germanium) is 3.6%. Possible interferences are evaluated. The applicability of the method to the determination of germanium(IV) in ore samples was also successfully carried out.     

Reactions of hydrazoic acid on TiO2 nanoparticles: an experimental and computational study

This article reports the results of a computational and experimental study on the reaction of hydrazoic acid, HN3, adsorbed on 15-20 nm TiO2 particle films. Experimentally, FTIR spectra of HN3(a) have been measured by varying HN3 dosage, UV irradiation time and surface annealing temperature. Three sharp peaks, related to v(a)(NNN) of HN3(a) and N3(a) with different configurations in the 2000-2200 cm(-1) region, and a broad band absorption, related to associated and isolated HN(a) and HO(a) adsorptions in the 3000-3800 cm(-1) region, have been detected. Computationally, molecular structures, vibrational frequencies and adsorption energies of possible adsorbates including HN3 and its derivatives, N3, N2, NH, and H, have been predicted by first-principles calculations based on the density functional theory (DFT) and the pseudopotential method. On the basis of the experimental and computational results, the peak appeared at 2075 cm(-1), which increases at a faster rate with HN3 exposure time, is attributed to a stable adsorbate, N3-Ti(a), with the predicted adsorption energy, E(ads) = 13 kcal/mol. The peak at 2118 cm(-1), which survives at the highest surface temperature in the heating experiment, is attributable to the most stable adsorbate, Ti-N2N(H)-O(a) with E(ads) = 36 kcal/mol. The peak at 2170 cm(-1), which vanishes most readily in all of the aforementioned experiments, is related to less stable molecular adsorbates, end-on HN3-Ti(a) with E(ads) = 5 kcal/mol and side-on HN(N2)-Ti(a) with E(ads) = 8 kcal/mol. A potential energy diagram for the formation of various absorbates with their transition states has been established for the HN3/TiO2 system. On the basis of the predicted desorption energies, the four most stable products of the HN3 reaction on TiO2 are H-O(a), 118 kcal/mol; HN-O(a), 85 kcal/mol; Ti-N2N(H)-O(a), 36 kcal/mol; and N3-O(a), 19 kcal/mol.     

Analysis of Equilibrium and Kinetics of Chromium-Fluoride Complexation from Spectroscopic Data via Chemometrics Methods

IntroductionStudies on reversible kinetic systems are consi-dered as a hotspot of chemical and biochemical kineticresearches[1,2]. Of late, some researches have been fo-cused on the simultaneous optimization of the obverseand reverse rate constants[3,4].H…     

The determination of trace amounts of tin by flow-injection hydride generation atomic-absorption spectrometry with on-line ion-exchange separation and preconcentration

A hydride generation atomic-absorption spectrometric (AAS) method with flow-injection (FI), aimed at developing a practical routine assay for the determination of tin in food digests, is described. In order to modify the sample matrix and to achieve optimized and reproducible conditions for the hydride generation reaction, the analyte is initially converted into its chlorostannate-complex thereby allowing it to be separated and preconcentrated on-line on an incorporated micro-column packed with a strongly basic anion exchanger and subsequently to be eluted by diluted nitric acid under strictly controlled conditions. Optimum acidic conditions for the FI hydride generation AAS system was found to be 0.01-0.05M nitric acid. At a consumption of 2.7-ml sample volume, aspirated by time-based injection, the procedure resulted in an enrichment factor of 3.5 and yielded a detection limit of 0.08 microg/l. (3sigma) at a sampling frequency of 72/hr. The precision was 2.5% rsd at the 10 microg/l. level. Potential interferents, such as Ni(II), Co(II), Zn(II) and Fe(III) could, at a Sn level of 10 microg/l., be tolerated at an excess of 1000 times without impairing the assay, while a 100-1000-fold excess of Cu(II) decreased the signal by 10-15%. Recoveries in the range 94-102% were obtained for canned food sample digests spiked with 10 microg/l. Sn.     

Studies of the electrochemical behavior of epinephrine at a homocysteine self-assembled electrode

The self-assembled electrode with the homocysteine monolayer (Hcy/Au) has been characterized by infrared spectroscopy and ac impedance spectroscopy in electrolyte. The Hcy/Au electrode is demonstrated to promote the electrochemical response of epinephrine (E) by cyclic voltammetry. A pair of well-defined redox waves was obtained and the calculated standard rate constant (k_s) is 2.1×10⁻² cm s⁻¹ at the self-assembled electrode. The reduction peak of E can be used to determine the concentration of E in presence of ascorbic acid (AA) owing to the Hcy/Au also promoting the electrochemical oxidation of AA.     

Application of the slotted quartz tube in flow-injection flame atomic-absorption spectrometry

The slotted quartz has been applied to flow-injection flame atomic-absorption spectrometry (FI-FAAS) showing several important advantages. The tube life was improved by a factor of 5-6 compared to conventional continuous aspiration. Flow impact systems were found not to be necessary in the applications so that larger enhancement factors may be achieved without sacrifice in precision. For 1.0 mg/l. copper, 1.0 mg/l. lead, 0.1 mg/l. cadmium and 1.0 mg/l. gold sensitivity enhancement factors of 3.1, 5.5, 5.3 and 4.0 were obtained with precisions of 1.3%, 1.1%, 1.6% and 1.7% RSD (n = 11) respectively. Application of the slotted quartz tube FI-FAAS method to the determination of heavy metals in urine has shown improved tolerance to interfering matrices. Recoveries obtained by spiking undiluted urine samples with 0.1 mg/l. copper and lead, and 0.01 mg/l. cadmium were in the range 100-102%.