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Herein, we report a stereospecific E-olefin dipeptide isostere synthesis that can be used to make gram quantities of the Phe-Phe isostere desired for eliminating a specific backbone H-bond donor and acceptor in the Alzheimer's disease related Abeta peptide. The Phe19-Phe20 E-olefin analogue of Abeta(1-40) was prepared by solid-phase peptide synthesis and was subjected to amyloidogenesis conditions. This analogue can aggregate into spherical morphologies but does not progress on to form protofibrils or fibrils as is the case for the all-amide sequence, providing insight into the structural requirements for amyloidogenesis. 相似文献
24.
Dehydrocyclization of 4-amino-5-arylamidopyrimidines in polyphosphoric acid to 8-arylpurines was further investigated. Various other 8-arylpurines were synthesized by this new procedure in high purity and yield. The ultraviolet absorption spectra of the arylpurines and the corresponding acylamidopyrimidines were measured at pH 1, 7, and 13 at 0.1 molar ionic strength. The spectra of the purines resemble those of the corresponding acylamidopyrimidines and, to a lesser extent, the corresponding 4,5-diaminopyrimidines. The bathochromic shifts caused by different substituting groups in monosubstituted purines, in general, parallel the inductive effect of the substituting groups. The hyperchromicity and the inductive constant of the substituting groups assume a linear relationship, for substitution at either the 2 or 6 position. These changes in the wave length of maximum absorption and in the molar extinction coefficient caused by 2, 6 and 8 substitutions of the purines reported here are in agreement with previous studies by Mason. 相似文献
25.
Spontaneous vesicle formation was observed in aqueous mixtures of partially hydrolyzed polyacrylamide and single-tailed cationic surfactants. The influences of pH, addition of long-chain alcohols, variation of hydrocarbon chain length, and head group size of the cationic surfactants were investigated systematically. It was found that 30-50% hydrolyzed degree of polyacrylamide and cationic surfactants with 10 or 12 methylene groups are suitable for vesicle formation in the mixed aqueous systems at pH 9.2. Addition of long-chain alcohols was found to be helpful for vesicle formation in some cases. The vesicle stability was also investigated at various temperatures and amounts of ethanol addition. Copyright 2001 Academic Press. 相似文献
26.
Sodium phosphate tellurite glasses in the system (NaPO(3))(x)(TeO(2))(1-) (x) were prepared and structurally characterized by thermal analysis, vibrational spectroscopy, X-ray photoelectron spectroscopy (XPS) and a variety of complementary solid-state nuclear magnetic resonance (NMR) techniques. Unlike the situation in other mixed-network-former glasses, the interaction between the two network formers tellurium oxide and phosphorus oxide produces no new structural units, and no sharing of the network modifier Na(2)O takes place. The glass structure can be regarded as a network of interlinked metaphosphate-type P(2) tetrahedral and TeO(4/2) antiprismatic units. The combined interpretation of the O 1s XPS data and the (31)P solid-state NMR spectra presents clear quantitative evidence for a nonstatistical connectivity distribution. Rather, the formation of homoatomic P--O--P and Te--O--Te linkages is favored over mixed P--O--Te connectivities. As a consequence of this chemical segregation effect, the spatial sodium distribution is not random, as also indicated by a detailed analysis of (31)P/(23)Na rotational echo double-resonance (REDOR) experiments. 相似文献
27.
MO theoretical calculations based on the perturbational method of Dewar provide good correlation between predicted and observed structures of products formed during: (1) isomerization of arene oxides to phenols; (2) hydration and nucleophilic addition to arene oxides; and (3) dehydration of arene dihydrodiols. The method is equally applicable to the arene oxides, dihydrodiols, etc. derived from carcinogenic and noncarcinogenic polycyclic hydrocarbons. Extension to the related enzymatic reactions occurring during metabolism of carcinogenic hydrocarbons and to the reactions of the biologically active arene diolepoxides and aryloxirenes suggests the potential utility of this approach in predicting (a) metabolite structure and (b) the structural requirements for carcinogenic and mutagenic activity. 相似文献
28.
S. A. Darke S. E. Long C. J. Pickford J. F. Tyson 《Fresenius' Journal of Analytical Chemistry》1990,337(3):284-289
Summary The performances of two alternative sample introduction methods for use with Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) have been evaluated for the analysis of the same sample material. The laser ablation sample introduction system is based on a Nd:YAG laser to which an x-y-z translational sampling head had been added. A study has been made of a number of parameters which affect the performance of the system to find the optimum operating conditions. The slurry introduction system involved aspirating the slurries into the plasma using a de Galan nebuliser and a Scott-type spray chamber arrangement. A study has been made of the parameters which control the production of stable homogeneous slurries. Initial particle size measurements have been carried out on the slurried samples to show how this affects this method of sample introduction. Results are presented for the analysis of a South African reference material rock sample (SARM 5) by ICP-OES with both laser ablation and slurry nebulisation sample introduction and some preliminary results for the analysis by ICP-MS with laser ablation introduction. Semi-quantitative results are obtained for laser ablation ICP-OES as only one matrix matched standard is used. However, the agreement between the results obtained for slurry nebulisation and the certificate value is poor. It is suggested by comparison with previous studies that this may be due to particle size effects. Encouraging results were obtained for the determination of trace elements by laser ablation ICP-MS. 相似文献
29.
Wang Xiaolin Chen Yinliang Wang Xiuyuan Sun Ying Fu Yibei Guo Gaopin 《Journal of Radioanalytical and Nuclear Chemistry》1991,147(2):377-384
A method of determination of trace gold and iridium in steel is presented. The samples were preconcentrated with an anion exchanger. The chemical recoveries were determined by an isotope tracer technique using198Au and192Ir. 相似文献
30.
The weak hydrogen bonded systems H2CO ?HX (X = F, Cl, Br, and I) have been studied by means of ab initio MO method with pseudopotential approximation. The stabilization energies of these hydrogen bonds are 8.96, 4.12, 3.00, and 2.21 kcal/mol, respectively. The interaction eneraction energies are farther decomposed according to Morokuma's energy decomposing scheme. It is found that the title complexes are mainly electrostatic, although the contribution of charge transfer is also significant. 相似文献