Structural Determination of Bis-histidinopeptide Zinc Complexes by 15N NMR (HMBC) Spectra

Polynitrogen receptors such as bis-histidine peptides possess strong ability to bind metals, which play much important roles in medicinal, bioinorganic, bioorganic, biomimetic and supramolecular chemistry. In order to investigate the interaction of these hosts with a variety of neutral, cationic and anionic guests, several techniques, for example, NMR,potentiometric tirations and monocrystal X-ray diffraction have been employed. Among them NMR is a powerful technique for unraveling the structure of polynitrogen receptors as long as they are in solution where the rapid tumbling of molecules averages out the anisotropies such as chemical shift and dipole-dipole interactions. General 1H NMR approach has been widely used for the study of host-guest interaction, but it is difficult for the accurate measurement in complexes structures, particularly metal complexes structures in which how the polynitrogen receptors bind metal, and which nitrogen binds metal and so on.     

溶剂对固相反应法制备H7PW12O42掺杂聚苯胺的影响

利用固相反应法,分别以微量的水和乙腈作为溶剂,制备了磷钨酸(H7PW12O42)掺杂聚苯胺,并以红外光谱(FTIR),电子扫描显微镜(SEM),X射线衍射(XRD),循环伏安(cyclic voltamogram)等测试方法对聚苯胺进行了表征。结果表明,固相反应法合成的聚苯胺分子链排列有序,晶化率较好,并且表现出有较好的电化学稳定性。而以微量的乙腈作为溶剂通过固相反应法得到的磷钨酸掺杂聚苯胺在颗粒形貌、结晶性、导电率等方面均优于相同条件微量的水作为溶剂时的掺杂聚苯胺。     

A study of low level selenium determination by hydride generation atomic fluorescence spectrometry in water soluble protein and peptide fractions

Development of a method for very low level selenium determination in water soluble protein and peptide fractions, obtained after various separation procedures, is presented. A hydride generation atomic fluorescence spectrometry (HG-AFS) detection system was optimised and the influence of Cu(II), Sb(V), As(III) and HNO₃ interferences in the measurement of Se by HG-AFS was investigated. A destruction procedure using HNO₃ and H₂O₂ was also optimised and the average recovery of the digestion of a solution of selenomethioneine was 92 ± 4% (n=14). Combination of this digestion with the detection system gave reliable results. Accuracy was tested by comparison with two independent methods. A very low detection limit (DL) of 0.2 ng/g of measuring solution was achieved. The whole procedure from weighing to measuring was performed in the same Teflon tube. The addition of HNO₃ to the fractions before long term storage at -20°C was necessary to prevent adsorption on the test tubes.Selenium was measured in water soluble protein and peptide fractions obtained after extraction, and Sephadex G-75 chromatography performed on liver samples from: i) hens exposed to As₂O₃, ii) hens fed with a high fat feed and iii) the certified reference material dogfish liver (CRM DOLT-2). Because of the very low DL we were able to observe the Se distribution in chromatographic fractions of samples of organisms which were not exposed to excess amounts of Se. The presence of selenium associated with metallothioneins was observed.     

Conformations of trimethoxymethylsilane: matrix isolation infrared and ab initio studies

Conformations of trimethoxymethylsilane were studied using matrix isolation infrared spectroscopy and ab initio computations. Trimethoxymethylsilane was trapped in both argon and nitrogen matrixes using heated nozzle effusive sources and a supersonic jet source, in an effort to alter the conformational population in the matrix. Ab initio calculations were carried out at the HF and B3LYP level using 6-31++G basis set to support our experimental observations. The frequencies computed at the B3LYP level was found to fit well with our experimental data. A conformer with a C1(g(+/-)g(+/-)t) structure was predicted by our computations to be the ground state conformer.     

A mechanism from quantum chemical studies for methane formation in methanogenesis

The mechanism for methane formation in methyl-coenzyme M reductase (MCR) has been investigated using the B3LYP hybrid density functional method and chemical models consisting of 107 atoms. The experimental X-ray crystal structure of the enzyme in the inactive MCR(ox1)(-)(silent) state was used to set up the initial model structure. The calculations suggest a mechanism not previously proposed, in which the most remarkable feature is the formation of an essentially free methyl radical at the transition state. The reaction cycle suggested starts from a Michaelis complex with CoB and methyl-CoM coenzymes bound and with a squareplanar coordination of the Ni(I) center in the tetrapyrrole F(430) prosthetic group. In the rate-limiting step the methyl radical is released from methyl-CoM, induced by the attack of Ni(I) on the methyl-CoM thioether sulfur. In this step, the metal center is oxidized from Ni(I) to Ni(II). The resulting methyl radical is rapidly quenched by hydrogen-atom transfer from the CoB thiol group, yielding the methane molecule and the CoB radical. The estimated activation energy is around 20 kcal/mol, which includes a significant contribution from entropy due to the formation of the free methyl. The mechanism implies an inversion of configuration at the reactive carbon. The size of the inversion barrier is used to explain the fact that CF(3)-S-CoM is an inactive substrate. Heterodisulfide CoB-S-S-CoM formation is proposed in the final step in which nickel is reduced back to Ni(I). The suggested mechanism agrees well with experimental observations.     

On-line coupling of packed capillary electrochromatography with coordination ion spray-mass spectrometry for the separation of enantiomers

Pressure-supported packed capillary electrochromatography (CEC) and packed capillary high-performance liquid chromatography (pHPLC) have been coupled on-line to electrospray ionization-mass spectrometry (ESI-MS) and coordination ion spray-mass spectrometry (CIS-MS). Separation of enantiomers of barbiturates and chlorinated alkyl phenoxypropanoates were performed on a permethylated beta-cyclodextrin stationary phase by pressure-supported CEC. For on-line detection with ESI- and CIS-MS, a modified sheath-liquid interface was used. CIS-MS is a universal, novel ionization technique which improves the selectivity as well as the sensitivity. Charged complexes were formed through the addition of central complexing ions such as silver(I), cobalt(II), copper(II), and lithium(I) to the sheath flow. Advantages of CIS-MS detection compared to the ESI-MS mode are discussed. In the CIS-MS mode, increased sensitivity and high selectivity was attained through different possibilities of complexation. The superiority of pressure-supported CEC compared to pHPLC in the hyphenation with CIS-MS is demonstrated.     

New synthetic routes to cationic rhenium tricarbonyl bipyridine complexes with labile ligands

Triflate abstraction from the complex [Re(OTf)(CO)(3)(bipy)] (1) using the salt NaBAr'(4) (Ar' = 3,5-bis(trifluoromethyl)phenyl) in dichloromethane solution in the presence of L = PPh(3), NCMe, NCPh, imines, ketones, Et(2)O, THF, MeOH, and MeI affords cationic complexes [Re(L)(CO)(3)(bipy)](+) as their BAr'(4)(-) salts. The new complexes have been characterized spectroscopically and, for [Re(eta(1)-O=C(Me)R)(CO)(3)(bipy)]BAr'(4) (R = CH(3), 6a; R = Ph, 6b), and [Re(THF)(CO)(3)(bipy)]BAr'(4) (9), also by single-crystal X-ray diffraction. Compared with conventional methodologies, the route reported here allows the coordination of a broader range of weakly coordinating ligands and requires considerably milder conditions. On the other hand, the reactions of lithium acetylides with [Re(THF)(CO)(3)(bipy)]BAr'(4) (9) can be used for the high-yield syntheses of rhenium alkynyls [Re(Ctbd1;CR)(CO)(3)(bipy)] (R = Ph, 12; R = SiMe(3), 13). Complex 9 was found to catalyze the aziridination of benzylideneaniline with ethyl diazoacetate.     

Analysis of metabolites of glucose pathways in human erythrocytes by analytical isotachophoresis

A method for the quantitative determination of the major anionic constituents of fountain solutions, typically mono-, di- and hydroxycarboxylates, alkylbenzenesulfonates, and inorganic anions, including orthophosphate and polyphosphates, is presented here for the first time. The analytical problems arising from extensive co-elution of many of these analytes on an ion-exchange column have been resolved through a combination of (i) careful selection of the concentration gradient of the sodium hydroxide eluent; (ii) parallel analysis by ion-exclusion chromatography; and (iii) determination of total phosphorus by inductively coupled plasma atomic emission spectrometry.     

A new three-carbon synthon for efficient synthesis of benzannelated and 1-(2-arylethenyl) heterocycles

The novel three-carbon synthon 1-(1H-1,2, 3-benzotriazol-1-yl)-3-chloroacetone for the synthesis of benzothiazoles, pyrido[1,2-a]indoles, and styryl-substituted indolizines and imidazo[1,2-a]pyridines is reported. The proposed routes are a general and efficient approach for heterocyclizations followed by benzannelations or attachment of arylethenyl pharmacophores.