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101.
The natural abundance 13C Fourier transform magnetic resonance spectra of rifamycin S and some of its derivatives have been studied. A combination of five different approaches has made unambiguous assignments for most of the resonances possible: (1) comparative study of the non- decoupled and noise-decoupled spectra; (2) 13C spectral characteristics; (3) spectral comparison between derivatives; (4) selective proton decoupling; (5) biogenetic evidence. Pulse and Fourier transform 13C NMR spectroscopy provides a more complete picture of these complex molecules than was previously obtained by 1H NMR spectroscopy. 相似文献
102.
Krishna Bisetty Francesc J. Corcho Josep Canto Hendrik G. Kruger Juan J. Perez 《Journal of Molecular Structure》2006,770(1-3):221-228
In this paper, we report on the conformational profile of the pentacyclo-undecane (PCU) cage tripeptide carried out by molecular dynamics (MD) simulation using water as an explicit solvent. The MD solution phase studies carried on the model peptide analogues (A)=Ac–Ala–Ala–Ala–NHMe; (B)=Ac–Cage–Cage–Cage–NHMe; (C)=Ac–Ala–Cage–Ala–NHMe and (D)=Ac–Ala–Pro–Ala–NHMe, are used as a complimentary technique to the corresponding gas phase simulated annealing (SA) study previously carried out in our laboratory. No significant structural changes were observed over the MD trajectories. However, the results reported here provide further evidence that the (PCU) cage amino acid exhibits C7eq, C7aq, R and L conformations, and the theoretical results suggest that the PCU cage amino acid is a strong β-turn inducer. These results support the prediction that when the PCU cage residues are in the (i) and (i+2) positions, the β-turn can be extended in either direction to form anti-parallel β-pleated sheets, thereby forming the basis of the mechanism for the folding back of the chain in a cross-β-turn structure. 相似文献
103.
A selective cleavage of acetals of 1,2- and 1,3-diols has been achieved under mild conditions using ZnBr2 in dichloromethane at room temperature. Acetal types cleavable by this procedure include benzylidene, isopropylidene and cyclohexylidene acetals. This method is compatible with several other types of hydroxyl protecting groups such as Bn, Bz, TBDPS, TIPS and TBDMS. 相似文献
104.
HUANG Ru-dan ** LI Juan ZHANG Jing . Department of Chemistry School of Science Beijing Institute of Technology Beijing P. R. China . Key Laboratory of Rare Earth Chemistry Physics Changchun Institute of Applied Chemistry Chinese Academy of Sciences Changchun P. R. China . Department of Chemistry Normal College Beihua University Jilin P. R. China 《高等学校化学研究》2005,(1)
IntroductionChirality is ubiquitous in the nature.Chiralcompounds have many applications in the areas ofmedicine,catalysis and materials,etc. Polyox-ometallates are inorganic polymers,which have at-tracted much interest since they have electri-cal[1_ 3 ] ,magnetic[4] ,optical properties[5_ 7] ,andsome characters as liquid crystals[8,9] .In this workwe report theα- Keggin structural polyoxometallatenanoparticles with optical activities.The opticalactivities of the samples are [α]13 D =+ 1 7… 相似文献
105.
García F García JM García-Acosta B Martínez-Máñez R Sancenón F Soto J 《Chemical communications (Cambridge, England)》2005,(22):2790-2792
A sensory polymeric material for the colorimetric sensing of cyanide in water has been developed based on the reactivity of this anion with the pyrylium cation. 相似文献
106.
It is well known that porphyrin derivatives play a key role in the primary process of photo-synthesis[1], in which porphyrins directly absorb the sunlight or indirectly acquire excitation en-ergy from light-harvesting antenna system to reach their excited state, and then donate electrons to quinone acceptors to yield a series of charge-separated species. In general, only first singlet ex-cited state of porphyrins is involved in energy transfer process[2]. However, highly excited state (S2 stat… 相似文献
107.
108.
A versatile process for the preparation of composite films consisting of magnetite (Fe3O4) nanoparticles embedded in a polyaniline (PANI) matrix is reported. Spectroscopic properties of polyaniline matrix (PANI-EB), polyaniline protonated with camphor sulfonic acid (PANI-CSA0.5) PANI-ES and PANI/Fe3O4-CSA0.5 composites were studied, both in the state of the solutions of m-cresol and in thin films processed from the same solvents. The results of these studies indicate that m-cresol can be used for PANI/Fe3O4 composite preparation. Such films show both reasonably high electrical conductivity and magnetic permeability. A controlled application of a magnetic field during the casting process resulted in the formation of the materials with an unusual combination of magnetic and transport properties. The obtained films show the behavior that can be explained by the presence of both ferromagnetic and paramagnetic phases. The superparamagnetic contribution, if any, is very small. Application of the external magnetic field during fabrication of the composites stimulates creation of the aggregates of magnetic particles which, although keeps conductivity at a relatively high level, leads to a small decrease of the conductivity value. 相似文献
109.
Fe3Al and Cr18-Ni8 stainless steel were diffusion-bonded in vacuum and a Fe3Al/Cr18-Ni8 interface with reaction layer was formed. Microstructure in the reaction layer at Fe3Al/Cr18-Ni8 interface was analyzed by means of scanning electron microscope (SEM) and electron probe micro-analyzer (EPMA). The growth of reaction layer with heating temperature (T) and holding time (t) was researched. The results indicate that FeAl, Fe3Al, Ni3Al, and alpha-Fe (Al) solid solution are formed in the reaction layer. These phases are favorable to promote the element diffusion and to accelerate the formation of the reaction layer at Fe3Al/Cr18-Ni8 interface. The growth of reaction layer obeys the parabolic law and its thickness (X) is expressed by X2 = 7.5 x 10(-4)exp(-83.59/RT)(t - t0). 相似文献
110.
The thermal decompositions of azodicarbamide (AZDICA), 2,2′-azobisisobutyronitrile (AIBN), benzoyl peroxide (Bz2O2), dicumyl peroxide (DICUP) andα, α′-bis(t.-butylperoxy)-m/p-diisopropylbenzene (PEROXIMON F) in binary and ternary mixtures containing low density polyethylene (LDPE) have been studied by means of DSC alone. Binary mixtures including 2% by weight of Bz2O2 or DICUP develop a decomposition heat of 64.2 and 59.1 J/g mixture, respectively. These values are higher than those measured for the decomposition of the pure peroxides. In all the ternary mixtures studied, containing LDPE, a peroxide and an azoderivative, the absolute enthalpic values attributed to the peroxide are lower than those obtained from the LDPE-peroxide mixtures. The enthalpy changes observed have been interpreted on the basis of interactions of the peroxide radicals with the polymer support and with the azo derivative. 相似文献